I have a rifle that I've been asked to look at that is blowing primers using Federal XM-193 ammo. About 1 time per magazine full. The case heads of other rounds are showing some raised primers and bright breech face marks. All of which is saying to me, "Over Pressure Problem." Switching ammo is not a real choice as that is the only training ammo he has available to him. The rifle is now about 3 years old and had not previously been having this problem. This is something that has popped up (pun intended) in the last few months along with a short stroke / bolt-over-base feed problem that was fixed with new gas rings.

Tomorrow, I am going to run 100 rounds of .223 through it and see if anything occurs and will post back. Any other suggestions? Edited to add: Sorry, the 100 round test will end up having to wait several days but will include a chronograph test of the ammo and twin rifle for similar signs of pressure.

Okay here goes the latest details:

Headspace Check:
Forster .223 Go Gauge 1.4636 - Closes with no problem
Forster .223 No-Go Gauge 1.4666 - Closes with no problem
Forster .223 Field Gauge 1.4696 - Resisted closing but then popped closed and opened without any problem with charging handle.
?????? .223 (Max-Field) 1.4736 - Would not close
IIRC, All the gauges were purchased from Brownell's

Suspect Gun: Armalite M15A2 5.56 Cal.

.223 Ammo Test (Win. USA223R1) 70 Deg. / 15' from muzzle /
Lo: 3011.93
Hi: 3043.74
Avg: 3024.37
No malfunctions of any kind in all rounds fired! No evidence of pressure marks on the case bases / primers and no raised primers visible.

XM193 (Lot 39)
Lo: 3342.99
Hi: 3371.86 (Blew primer on this shot)
Avg: 3356.66

Comparison Gun: Styer AUG 16" ".223"

XM-193 (Lot 39)
Lo: 3112.62
Hi: 3153.16
Avg: 3132.81
(This AUG is known to take 5.56 without a problem.)

My testing partner and I pretty much agree now that the M15A2 is probably a .223 chamber even though the barrel was replaced by the manufacturer shortly after a batch purchase resulted in troubles with every gun bought in the group. Why the gun has not had problems in the past two years and is now acting up remains a mystery.

An idea has been suggested to me that perhaps a way to cure potential chamber problems is to have a reamer made that will not cut the shoulder area of the chamber but will modify only the forward area of the chamber to the 5.56 spec. This makes me wonder what effect a chrome barrel would have on such a modification. Will it cut through the chrome and cause it to start to peel? Would excess wear be a factor? Of course such a modification would almost certainly void a warranty and not be acceptable in a Department owned gun. Any other thoughts?

Tweek, you must get tired of this stuff. I am sure that most of this is in the FAQ's.

Short answer is either do not use the ammo that gives trouble or put in a new good barrel chambered with a 5.56NATO reamer.

I keep seeing the same things in these threads, so let’s get some things straight:

Excess pressure is generally caused by three things (in combination) - powder in the case , throat/barrel dimensions, throat/barrel condition;

Inadequate headspace and too small chamber body diameters cause rounds to not chamber and may have a puny impact upon pressures. The case just does not expand as  much as normal, but this effect is really small ;

Excess headspace can result in flattened primers (the case is shoved forward by the firing pin strike, leaving a space between case and bolt, then the primer backs out  under its presssure to fill that space, then the case slides back under powder gas pressure flattening the primer), but usually means that case life (if you reload) will be short and you might get a head seperation, which is potentially dangerous! Pressures will actually be lower than standard, but the problem here is not pressure, it is that the case is streached too far and breaks.

Two chamber dimensions contribute to big chamber pressures - Neck diameter and neck length, and these only when they are the same size or smaller than the neck of the cartridge. They contribute to pressure by restricting the departure of the bullet, slowing the expansion of the powder gases. How to check? Simplest way is to check that a new bullet will drop through the neck of the fired case. It should, and if it does not, either the neck walls of the case are thick or the chamber is tight there. Get out a dial indicator and measure the fired inside and outside neck diameters and compare them against chamber drawings and bullet diameter.

The other chamber dimensions, like headspace and body diameters have puny effect on pressures because they do not change the volume enough to matter. So, no more citing headspace when diagnosing excess pressure! Got that? Measure neck diameters and report those.

Next, throats matter a bunch. GI/NATO throats are large in diameter and long, and should be what you find in a barrel marked 5.56 NATO. They will develop less pressure with a given batch of ammo than just about any other throat because the bullet travels more freely early in the firing cycle. Commercial rifles marked 223 Rem will usually have a SAAMI throat, which is shorter and the cylindrical portion is usually the same size as the bullet, which is generally more accurate and gives more pressure with given ammo. After that, you get into match throats, and there are both long and short. What is present in this rifle, in terms of chamber, neck, and throat dimensions is best found by taking a chamber cast (Brownell's and Midway sell chamber casting alloy for this purpose) and measuring it.  

A really rough and/or contaminated throat (LOTS of jacket material and primer ash)can raise pressures. Copper solvents and JB bore cleaner will take that stuff out and should be used every few hundred rounds no matter what the duty on the rifle...

Last is the rifled bore. If it is small, it can cause higher pressures, but it usually is a small actor.

Now, on to the rest of the issue on this thread... If only one batch of ammo is giving trouble, and only in this rifle, "Doctor, doctor, it hurts when I do this" "Well, don't do that. That'll be five dollars"  The ammo is too hot for your gun. Either send the ammo to the maker and complain, or sell it to someone who has guns that like that ammo. It really is that simple.

Now if not just that batch is hot in this rifle, but other batches of commercial ammo too, you have a gun problem, most likely the throat or the neck. If it is a steel or stainless steel barrel with no chrome, a gunsmith can apply the correct reamer to the barrel and your problem should be solved. In an AR15 that you are feeding whatever you happen to find, that should be the 5.56 NATO reamer. If you are willing to chack ammo and reject any that acts up, but still want miltary grade ammo to be acceptable, the Wylde chamber should be OK, and you can still go back and put the 5.56 NATO reamer in if the Wylde chamber is not enough.

If the bore and chamber have a coating of hard chrome, you are back to having to find out what ammo the gun likes, and only use that. Hard chrome is quite a bit harder than chamber reamer steel, so you will likely ruin a $50 reamer rather than fix the chamber. Perhaps there are some titanium nitride or cubic boron nitride coated reamers, and then you could theoretically fix it.  You could pull the barrel, take a chamber drawing and the barrel to a tool shop and have them center the barrel and CNC grind the chamber to dimensions, but that will cost way more than a new correct barrel will.

Many people are concerned about the chrome layer peeling, but that is not actually the problem it is made out to be. Chrome and steel have very high affinity for each other, so if it was done right, it will staty put pretty well. Eventually the chrome is flame blasted away at the throat anyway, and then the throat wears normally until accuracy falls off and the barrel is replaced. If you find a way to fix a chrome chamber that takes off chrome, this process will proceed anyway. Remember, a chrome moly or stainless barrel without chrome that is cleaned regularly (without damaging the throat or crown) will give good match level accuracy to between 4000 and 7000 rounds anyway.

So, find a customer for ammo the rifle does not like and use only ammo that it does like, or replace the barrel with a good GI spec barrel. Clean it. The other stuff is academic and interesting, but otherwise not of much use. Good Luck!

Billski

I must admit that I got caught up in the moment of thinking about reaming the chamber but then woke up to the consequences. Billski and I have been exchanging some off-board ideas and I am still working on the easist of them right now. That will be to fire every Agency rifle with both the XM-193 and .223 55 Gr. SP across the chronograph and chart any differences. Since there is at least one other rifle that is acting up the group numbers should be instructive. The possibility of a chamber cast may follow but I/we will have to see the numbers first.

Okay, it has taken a while but I have the chorograph tests with thirteen rifles complete. I have an Excel spreadsheet with all the values broken down by individual rifle but I will put what I think are the most telling stats here.

In summary, the rifles were all fired with the same Lot numbers of Federal XM-193 and Winchester RA223R (55 gr. SP in .223 Rem.) Ten rifles are in Group "A" and three in Group "B" (three more still to go in this group). All of the rifles have 16" barrels (17-1/4" OAL w/ flash hider).

With the Winchester .223 ammo, the Group "A" Minimum Velocity was 2909 FPS and Max was 3067 FPS and an average of 2994 FPS. The Group "B" Minimum was 2920 FPS, Max. was 3005 FPS and Average was 2961 FPS.

With the Federal 5.56 ammo, the Group "A" Minimum Velocity was 3111FPS and Max was 3321 FPS and an average of 3250 FPS. The Group "B" Minimum was 3035 FPS, Max. was 3150 FPS and Average was 3086 FPS.

Without doing a chamber cast, I think I have enough information to go to the Group "A" manufacturer and start asking serious questions. Thanks to everyone for your help in this endeavour.

(Final Note: I previously mentioned that all the barrels had been replaced by the manufacturer shortly after the initial purchase. I was able to track down the reason. It seems that all the guns were shipped with what was described as ".223 Match" barrels instead of 5.56 barrels.

On the top of the barrel near the delta ring Armalite usually stamps an "N" on there showing it to be a NATO chamber.

Originally Posted By kingcleophus: On the top of the barrel near the delta ring Armalite usually stamps an "N" on there showing it to be a NATO chamber.

I pulled a couple sets of handguards last night and there are no markings on the barrels that I could see. Actually I was going to ask if I would have to pull the barrel off the receiver to see any markings.

It's usually on the top of the barrel, very close to the delta ring, very small, capital N

Your symptoms sure sound like an over-pressure situation. like firing a hot 5.56 in a .223 spec'd chamber.

What year were the barrels swapped? By Armalite?

Couple of clues,

First off, M-193 is loaded with slower powder to push the speed.  Because the primer is crimped in, this prevents the primer from blowing out (like the fed gold is doing/having problems threw gas guns since it too uses a slower powder).

Seems that there has been a lot of XM-193 being shipped out with out the primers crimped in.  If this is the case, and they are using the same powder as the M-193, it could exspain the blown primer problem.

Lastly, XM-193PD has been blowing some rifles up over the past few months.  Don't know if it's due to underloads leaving bullets in the barrels (then another round fired), or if the cases are just out of spec, and not allowing the bolt to fully lock up. Best cause if you do have a batch of this is to get a ammo checker to confirm that the ammo is the correct OAL and size (no buldges). Also, watch your impacts.  If you don't see the shot hit, then its time to check the bore before pulling the trigger again.
www.ar15.com/forums/forum.html?b=3&f=16

Originally Posted By kingcleophus: It's usually on the top of the barrel, very close to the delta ring, very small, capital N Your symptoms sure sound like an over-pressure situation. like firing a hot 5.56 in a .223 spec'd chamber. What year were the barrels swapped? By Armalite?

I will be checking under the handguards of all of them as I can but there is NO marking on the ones I have done.

The guns were delivered in fall 2000 and barrels replaced almost immediately. Early spring 2001 at the latest. I have since lerned that it took almost a year for the aleged barrel replacement. IF they were actually replaced that work was done in Fall 2001 to Spring 2002.

Dano, I reported the velocity test results above on 4-Jun-05. The .223 Rem results were almost identical. The XM-193 were high for the one group but not the other.

The rifle manufacturer was contacted this morning......... One week has passed and no response. From the related past history, this does not surpirse me.

Originally Posted By wrecktech: Okay here goes the latest details: Headspace Check: Forster .223 Go Gauge 1.4636 - Closes with no problem Forster .223 No-Go Gauge 1.4666 - Closes with no problem Forster .223 Field Gauge 1.4696 - Resisted closing but then popped closed and opened without any problem with charging handle. ?????? .223 (Max-Field) 1.4736 - Would not close IIRC, All the gauges were purchased from Brownell's

This rifle has excessive headspace.  The cartridge is driven forward by the firing pin and the case  expands to hold it in place, the primer is pushed back to the bolt face, then the case head is pushed back to the bolt face, stretching the case.  If you reload, you will only get a few reloads before the case head separates.  The period where the primer is pushed out of the case head can lead to piercing.  A common cause of cratering and/or blanking is too much clearance between the firing pin and the bolt, resulting in not enough support for the primer.

Originally Posted By wrecktech: Dano, I reported the velocity test results above on 4-Jun-05. The .223 Rem results were almost identical. The XM-193 were high for the one group but not the other.

It's not the speed, but the slower burning gunpowder that is being used causing the problems.  As stated with the Fed Gold 223 ammo, this leaves too much residual pressure in the barrel when the bolt unlocks, and due to the slower bore pressure build up, the primer is not total sealed/pressure flared to the primer case pocket.

Simply put, if you check the primers on the un-blown primer cases, you will notice that the primer faces have not been completely flatted (signs of overpressure).  What is happening is that the bolt goes to unlock, and the pressure is transferred to the primer.  This causes the primer to be blown out of the pocket during the action unlock.

Again, the solution to the problem is to just stake the primers into the case to prevent the separation. M-193 is primer crimped/staked, so I haven't a clue why the XM-193 is not (they used to be).

Note: on some tighter chambered rifles, this causes the rounds to seal off faster, and allows the primer to be sealed off Better.  But on looser chambered rifles (read auto spec chambers), there is just too much case blow-by (read slower spike build) to seal off the primer into the case pocket.

A point of clarification regarding (excess?) headspace.

In ANY rifle using a spring loaded ejection plunger, like the Arf, the unfired case will be pressed fully forward in the chamber by the ejector BEFORE the round is fired.  Any excess headspace present will be at the rear of the chamber, leaving the case head unsupported at the moment of firing.

It sounds like the potentially unsafe combination of .223 chambers and 5.56 pressure ammo may have been demonstrated again.

Paladin

Dano,

If I understand what you are saying correctly, the powder in the XM-193 burns slower than the .223 Rem stuff. Combined with the tighter chamber throat / leade, there is more pressure build up hence more muzzle velocity once the bullet exits. In the mean time, the higher gas pressure starts to vent through the gas tube into the bolt carrier causing the carrier to start to move back and unlock the bolt. With enough movement, the bolt starts to extract the cartridge case before all the pressure is reduced in the system potentially leading to a case rupture and blown primers.

What I don't understand is if the case head is in reasonable proximity of the bolt face, where does the primer get room to blow out to? Is the room made by the ejector moving the case away as part of the ejection process and if so, why hasn't the pressure equalized at that point since the case is so far out of the chamber?

Firing pin hits primer, powder burns.  Bullet is pushed forward, hit the lands, this and the case pressure it's self, shoves the case rearward against the bolt. This is why the shoulder is expanded/ stretched, and not just the back of the case.  

Now the fun, if use faster burning powder (within range for non-staked primers) the case is quickly sealed off on the chamber wall and the primer is expanded in the case pocket.  Now lets burn slower powder in the rifle.  Same happens as above, but instead of the case being quickly expanded and sealing off the chamber, the pressure builds/spikes slower, and you end up with later/ more than faster burning powder case blow by past the chamber walls/case (depending on how slow the powder burns).  If the chamber is a little larger (side walls), or the power burns too slow, often the end result is soot’d cases (read more blow by around the fouling).

Next comes the action unlock.  On faster burning powder, the powder spike takes place before the bullet hits the gas tube. This means as the bolt is unlocking, the powder burn in the downward spike curve of the burn, and once the action opens, the barrel/case pressure is dropping in relative to the before max spike/pressure.

But what if you really want to push the bullet speed? Slower powder will burn longer, and you get higher speeds.  But the logic to this is it can play havoc on gas guns.  The bolt goes to unlock (remember the less than stellar quick case/primer seal), and the powder is still in full burn mode with the action unlocking around the peek spike. When the extractor pulls on the case, it leaves a gap between the bolt face and the primer* (case still sealed in chamber). Now you have a gap between the primer and the bolt face, and the extra barrel residual pressure to blow the primer out of the pocket.

*Note: you will always have this gap on an auto action, it just the difference on how well the primer has been expanded into the primer pocket, and the barrel residual pressure that can make a difference.

To sum it up, is slower powder bad in gas guns, Hell no. They produce more speed, and on some loads, more consentient ammo.  Is slower powder loaded to max loads bad in gas guns, depends, you can achieve the same as the above statement, but you have to keep in mind about the primer problem, and the primer staking to solve it.

Note: XM/M-193 gunpowder is not as slow at say 4895, just a tad slower than BLC-2.  Since mil-spec is to crimp the primers in the case, it allows this tad slower powder to work. Don’t misunderstand me, It’s not like the powder being used is way slower burning, just enough to gain the extra bit of speed over standard used powders. As stated in previous posts, XM-193 used to have the primers crimped in (same as M-193).  As some point, they stopped doing this.  I don’t know if changed the powder/amount used to compensate for the primer problem.  What I can tell you is that Federal ran into the same problems with their 223 Match ammo, and may be still working to solve the problem (not buying the whole “should not be use in a gas gun” as a plausible cover your ass statement to resolve the known issue anyways).

Originally Posted By Dano523: Now the fun, if use faster burning powder (within range for non-staked primers) the case is quickly sealed off on the chamber wall and the primer is expanded in the case pocket.  Now lets burn slower powder in the rifle.  Same happens as above, but instead of the case being quickly expanded and sealing off the chamber, the pressure builds/spikes slower, and you end up with later/ more than faster burning powder case blow by past the chamber walls/case (depending on how slow the powder burns).  If the chamber is a little larger (side walls), or the power burns too slow, often the end result is soot’d cases (read more blow by around the fouling).

So that's why the NATO rounds tend to be sooty and Remington clean?

Next comes the action unlock.  On faster burning powder, the powder spike takes place before the bullet hits the gas tube. This means as the bolt is unlocking, the powder burn in the downward spike curve of the burn, and once the action opens, the barrel/case pressure is dropping in relative to the before max spike/pressure.

Another important point for the distance for the gas tube from the the chamber and it's effect on function, right?

But what if you really want to push the bullet speed? Slower powder will burn longer, and you get higher speeds.  But the logic to this is it can play havoc on gas guns.  The bolt goes to unlock (remember the less than stellar quick case/primer seal), and the powder is still in full burn mode with the action unlocking around the peek spike. When the extractor pulls on the case, it leaves a gap between the bolt face and the primer* (case still sealed in chamber). Now you have a gap between the primer and the bolt face, and the extra barrel residual pressure to blow the primer out of the pocket. *Note: you will always have this gap on an auto action, it just the difference on how well the primer has been expanded into the primer pocket, and the barrel residual pressure that can make a difference. To sum it up, is slower powder bad in gas guns, Hell no. They produce more speed, and on some loads, more consentient ammo.  Is slower powder loaded to max loads bad in gas guns, depends, you can achieve the same as the above statement, but you have to keep in mind about the primer problem, and the primer staking to solve it. Note: XM/M-193 gunpowder is not as slow at say 4895, just a tad slower than BLC-2.  Since mil-spec is to crimp the primers in the case, it allows this tad slower powder to work. Don’t misunderstand me, It’s not like the powder being used is way slower burning, just enough to gain the extra bit of speed over standard used powders. As stated in previous posts, XM-193 used to have the primers crimped in (same as M-193).  As some point, they stopped doing this.  I don’t know if changed the powder/amount used to compensate for the primer problem.  What I can tell you is that Federal ran into the same problems with their 223 Match ammo, and may be still working to solve the problem (not buying the whole “should not be use in a gas gun” as a plausible cover your ass statement to resolve the known issue anyways).

I took pictures of samples of all the shell casings in my big test today. As soon as I get them cropped and uploaded to the Photo Gallery, I'll let you know but I'm not seeing an obvious crimp on the primers of the XM-193. Of course I might be missing something. A fair amount of purple sealant in places but only a few with might be a crimp. I found LC 00, LC 01 & LC 02 headstamps all in one group of ten from the same box.

Damn that was good stuff, and very informative, but now my eyes hurt.  Thanks

Wrecktech,

1.  Some NATO ammo is loaded with triple base gunpowder, and/or gunpowder that are heavy in graphite, or other compounds used to either form the powder or as a flash suppressant.  This added compounds is often what cause some of the rounds to be dirtier than others.


2. The lengths of the barrel port in regards to the distance from chamber plays some what of a role, but you have to remember that during a normal burn rate, the case/primer is well sealed off in the chamber.  Here, the size of the gas port needed to unlock the action is the more important factor in getting the rifle to cycle correctly.  Remember, an even 10.5" barrel runs if tuned correctly.

3. One theory on XM-193 is that it is produced with M-193 lot overrun cases, and not produced at the LC plant. This would explain the different years in the same box, since ammo is run in lots, and it may require a few over run lots of M-193  to get enough brass to start a lot of the XM ammo.

Another theory is that XM-193 is in fact LC plant produced M-193 ammo that has some sort of fault/defect that would not allow it to pass military testing standards, and hence sold off to the civilian market. If you have XM-193 that is all from the same lot, and the features are not consistent through out, it has to make you wonder if someone wasn’t just boxing ammo from the non-compliant pile.

.

All of this stuff is interesting reading, but none of it addresses something that WreckTech and I discussed off line. We have been corresponding and his problem is both interesting and vexing. He has some rifles that will occaisionally spit out a primer when used with one batch of ammo, and a whole batch of other rifles with Wylde chambers in them that like his "bad" ammo just fine.  My suspicion is that he has either tight bores or short throats or both in the rifles that do not like the "bad" ammo.

Big Point - Primers come out of cases because pressure in the case got above the yield strength of the case head,  squishing the case head and growing the outside and the primer pocket.  The reason the case head did not squirt all over is because it took the case head into the plastic zone for only a tiny period of time near peak pressure, and the metal could only move so far in that time... Pressure in the case and inside the primer pocket is very nearly the same, so the primer pocket opens up while the case head enlarges, and the primer no longer has an interference fit. The primer stays in the pocket through extraction because the case and bolt are all around it. Then, when the case is rotated about the extractor by the ejector, the primer is left standing there by the bolt, while the case leaves, and the primer falls into the trigger.

While I firmly believe Dano and Tweak know these rifles and are absolutely correct  about slow powders aggravating things for 16" shorties, I gotta say that if you have cases that spit their primers into your trigger, you should measure the cases. When you do, you will find that the primer pocket is larger than your new primers and that the case head is probably a little bigger than it was when it went into the gun too.

WreckTech, that is something that you could do with your ammo fired in rifles that like the ammo versus rifles that don't. I bet that if you carefully established case diameter just above the extraction groove on each round before and after firing, you would find that the "good" guns don't grow the case head size, and the "bad" ones do enlarge the cases. It will be small, but you should be able to establish some growth. The ones that spit out their primers should be worse than the others too. This is strong evidence that the pressures are high in some of those barrels.

Good luck, and I look forward to seeing the rest of the saga.

Billski

Here are some pictures of case heads fired from the same rifle:


XM-193 Lot 39
MV: 3371 fps

-------------------------------------


XM-193 Lot 39
MV Hi: 3371, Lo: 3342, Av: 3356, StD: 12.01

-------------------------------------


XM-193 Lot 35
MV Hi: 3320, Lo: 3196, Av: 3266, StD: 35.70

-------------------------------------


Winchester RA223R (55 Gr. FMJ)
MV Hi: 3015, Lo: 2976, Av: 2997, StD: 17.31

-------------------------------------

More photos to be added later as needed or requested from 13 additional rifles.

ETA corrected link to new photo server.

Originally Posted By Billski: My suspicion is that he has either tight bores or short throats or both in the rifles that do not like the "bad" ammo. WreckTech, that is something that you could do with your ammo fired in rifles that like the ammo versus rifles that don't. I bet that if you carefully established case diameter just above the extraction groove on each round before and after firing, you would find that the "good" guns don't grow the case head size, and the "bad" ones do enlarge the cases. It will be small, but you should be able to establish some growth. The ones that spit out their primers should be worse than the others too. This is strong evidence that the pressures are high in some of those barrels.

Bill,

If the bores were tight, wouldn't the MV be higher across the board for all ammo? Since all the guns like the .223 and have fairly equal MV's, wouldn't that point more toward the tight throat of a .223 chamber in those that don't like the XM-193?

Will do some additional checking of the bases.

Armalite did in fact call (Suprise!) and even though they are out of warranty, they want to see a couple guns and if the chambers are wrong, they will "work with us to make it right." I'll be driving down to Geneseo next week to save on the shipping costs and see some of these people face to face.

Teak and Dano,

Sorry if my comments wrongly put words in your mouths. Not my intention...

WrectTech,

I e-mailed you under seperate cover... Some of this is thus for the other readers. I am glad to hear that Armalite is working with you. You might take them some ammo and a rifle that likes the ammo too for a comparison.

The LC cased ammo (from Federal?) are really showing big pressure signs. Holey Primer Pocket Growth Batman! The question (as Always) is why? Were those rounds fired through the problem children or does that ammo do that in everything? What do those batches look like when fired in the other brands?

If the bores are tight, they might give higher velocity or they might give lower velocity, depending upon whether the drag of getting the bullet into and down a tighter bore slows things down more than the extra pressure speeds things up. How about that for wishy-washy? Either thing can happen. Anyway, looking at the data that the other forum readers can not see, it looks like we have three Armalites that are slower than the other seven Armalites, and that is consistent across both ammo types. Now if one group pops primers and the other does not, we have a clue.

Originally Posted By Billski: I am glad to hear that Armalite is working with you. You might take them some ammo and a rifle that likes the ammo too for a comparison.

That sounds like a plan just to make sure they have some of the same ammo.

The LC cased ammo (from Federal?) are really showing big pressure signs.

Yes, all Federal XM-193

Were those rounds fired through the problem children or does that ammo do that in everything?

This was Spare #2 in our discussions. The "Eddie Haskel" of the bunch that started everything off by blowing a primer in nearly every magaznine for a while.

What do those batches look like when fired in the other brands?

I'll have to post a few more pictures as soon as I get them reduced in size for the Photo Gallery.

If the bores are tight, they might give higher velocity or they might give lower velocity, depending upon whether the drag of getting the bullet into and down a tighter bore slows things down more than the extra pressure speeds things up. How about that for wishy-washy?

You ever sell cars? I'm trying to protect my wallet right now and I carry a gun for a living!

Either thing can happen. Anyway, looking at the data that the other forum readers can not see, it looks like we have three Armalites that are slower than the other seven Armalites, and that is consistent across both ammo types. Now if one group pops primers and the other does not, we have a clue.

The chronograph testing data:

Federal XM-193 (Lot #35)
Armalite
Rifle #        Lo MV       Hi MV       Av MV     Spread     StdD
A-1           3259.28     3301.05     3275.11     41.77     14.05
A-2           3166.35     3193.92     3180.75     27.57     14.35
A-4           3111.02     3214.28     3159.51    103.26     32.32
A-5*          3149.87     3223.69     3196.18     73.82     28.30
A-6           3255.77     3312.80     3283.67     57.03     21.36
A-7           3270.73     3321.89     3286.80     51.16     18.38
A-9           3254.02     3307.37     3280.27     53.35     17.42
A-10          3236.62     3270.73     3255.40     34.11     16.30
Spare #1      3257.53     3311.89     3285.78     54.36     20.16
Spare #2*
(Orig. A-5)   3196.45     3320.07     3266.55    123.62     35.70
Lot #39       3342.99     3371.87     3356.67     28.88     12.01

Rock River
A-17          3036.87     3150.69     3089.77    113.82     38.54
A-8           3035.35     3105.43     3070.39     70.08     49.55


Winchester RA223R (Lot# ITH30)
Armalite
Rifle #        Lo MV       Hi MV       Av MV     Spread      StdD
A-1           2971.27     3036.11     3014.96     64.84     20.31
A-2           2910.55     2969.81     2943.14     59.26     23.94
A-4           2909.85     2969.81     2951.97     59.96     17.22
A-5*          2929.56     2945.25     2939.78     15.69      8.86
A-6           2994.80     3046.81     3019.33     52.01     22.23
A-7           2977.85     3026.24     3009.64     48.39     17.01
A-9           2999.99     3041.45     3021.69     41.46     15.74
A-10          2974.19     3012.68     2999.29     38.49     12.54
Spare #1      2990.36     3067.65     3021.95     77.29     22.45
Spare #2*     2976.39     3015.69     2997.69     39.30     17.31

Rock River
A-17          2920.38     2984.46     2954.20     64.08     20.71
A-8           2920.38     3005.20     2967.35     84.82     25.80


Overall Results
XM-193          Lo MV       Hi MV       Av MV      Spread      StdD
Armalite       3111.02     3321.89     3250.19     210.87     51.22
Rock River     3035.35     3150.69     3086.54     115.34     38.67

RA223R          Lo MV       Hi MV       Av MV      Spread      StdD
Armalite       2909.85     3067.65     2994.46     157.80     35.31
Rock River     2920.38     3005.20     2961.60      84.82     23.91

NOTE: Rifle Spare #2 was originally assigned as #A-5 and was rotated out when the problems started with it. This was the one that started all the most recent trouble and prompted the testing.
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Now that it's here, people can copy the table and paste into Word and change font to Courier and it should be readable and make more sense. The extra spaces that I put in don't show up on the web page and don't seem to carry over the word processor, so I'd recommend using the "Replace" command (CTRL-H) to search for the space (" ") chracter and convert to Tabs ("^t" in the dialog box). Then search and replace through the document. It's not perfect, but better than nothing.
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They don't let me play on the computer at work as much as I want, they actually want me on the street if you believe that! Then I have to compete for computer time at home....

Here are the comparison pictures to the Rock River and other guns:
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Rifle A-17 (RRA)

Federal XM-193 Lot 35
Hi: 3150 Lo: 3036 Av: 3089 StD: 20.71
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Rifle A-17 (RRA)

Winchester .223
Hi; 2984 Lo: 2920 Av: 2954 StD: 20.71
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Rifle A-08 (RRA)

Federal XM-193 Lot 35
Hi: 3105 Lo: 3035 Av: 3070 StD: 49.55
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Rifle A-08 (RRA)

Winchester .223
Hi; 3005 Lo: 2920 Av: 2967 StD: 25.80
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Rifle Sig 551 S-04 (14.5" Barrel)

Federal XM-193 Lot 35
Hi: 2922 Lo: 2821 Av: 2866 StD: 33.54
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Rifle Sig 551 S-04 (14.5" Barrel)

Winchester .223
Hi: 2847 Lo: 2790 Av: 2831 StD: 17.51
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Styer AUG 16"

Federal XM-193 Lot 39
Hi: 3153 Lo: 3112 Av: 3132 StD: 14.35
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That's all for this session.

ETA Pictures relinked to new server.

Pattern that I get is that Spare #2 (Armalite) and A17 (RRA) both give high pressures with the Federal (LC02 cases) ammo, with Spare #2 being particularly bad. The Fed ammo looks OK in the others we have seen the photos of, and the WIn stuff looks good in all of them.

Looks like Ammo and Barrel effects both exist.

Spare #2 definitely runs hot, and A17 is hotter than SiG 551 and the AUG, but the Win stuff is always OK.

The Fed ammo is definitely hot, with several carbines at nearly 3300 ft/s, for being shot from a 16" barrel. Another sign of the pressure being too high in that ammo.

So, besides going to Armalite, I would also go to Federal with your data and ask to replace the remaining ammo with another batch.

I wonder how the rest of the case heads look...

Oh, I have sold three cars in the last ten years, have a lousy poker face, and do not drive hard bargains.  So, nothing to worry about there.

Billski

Okay here are the pictures of the chamber casts of five different guns. Dimensions added on 2 July 05. Scale is .020 per line.
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Rifle #A-04 Armalite

Diameter just off case neck .2257, Barrel Lands & Grooves: .2250 & .2219
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Rifle #A-08 RRA

Diameter just off case neck .2270, Barrel Lands & Grooves: .2258 & .2223
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Rifle #S-04 SIG 551 (NATO)

Diameter just off case neck .2254, Barrel Lands & Grooves: .2252 & .2234
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Rifle: Spare #01 Arm.

Diameter just off case neck .2253, Barrel Lands & Grooves: .2253 & .2219
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Rifle: Spare #02 Arm.

Diameter just off case neck .2251, Barrel Lands & Grooves: .2252 & .2212
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Note the rifling starting within .040 of the case neck on the two spares.

ETA Picture link to new server restored.

www.clymertool.com/cgi-bin/reamer.cgi

Do a search under rifle, rimless, 5.56 Nato for chamber dimensions.

Free bore after end of neck cut should be .2270 for 5.56, and .2245 for 223 rem.

Since you have casting of the chambers, you can check the rest of the dimensions as well.

Originally Posted By Tweak: Are the problem bores chromed? Have you mic'd the bores and are they clean?

I believe the bores are supposed to chrome. I haven't gotten around to detailed measurements yet, but will post ASAP.

Originally Posted By Tweak: you know how to check from chrome?

I saw another post you listed that said cold blue won't take. I am borrowing some from a friend...

The guns were dropped off this morning with 100 rounds of Lot 39. They promised an analysis result by the end of the week.

Stay tuned....

Originally Posted By wrecktech: They promised an analysis result by the end of the week. Stay tuned....

Phone message left for me late on Friday by Armalite Repair person: "They are NATO chmabers but cut too low. They have been rechambered to fix the problem." Now I'll have to wait until Tuesday to find out what "fix" means.

The NATO chamber has more room in the throat area.  That lets the gas expand more and reduces pressure a surprising amount.  It's not uncommon for mil spec ammo to blow primers in really tight SAAMI chambers.

Sounds like they A. Used the wrong reamer. B.  Used a very worn reamer.  C.  Didn't ream deep enough, although this should cause headspace problems, as well.

Their cure is likely to be making a few spins with the proper reamer.  I'm not a fan of going back in a chamber with a different reamer.  I guess it will work ok.

Well I finally got time to use a Stoney Point Overall Length Gauge to check the guns for free-travel out of the case. Several of the guns are listed above with photos of the chamber casts. A few others are not.

S-04 (Sig 551) +.140"

A-08 (RRA) +.080"

Armalites:
A-01 +.003"
A-02 +.010"
A-04 +.018"
A-05 -.003"
A-06 -.002"
A-07 -.009"
A-09 -.007"
A-10 -.017"

Note the "-" in front of several measurements. I discovered that the mere fact of chambering a few rounds put the unfired bullet in contact with the rifling. I actually took a live round and chambered it in gun #A-10. I found very slight markings on the bullet after I pulled it out.

Methinks that we may have finally stumbled onto a very important detail that is a major contributing factor to the over-pressure and high velocities that prompted all of this investigation. I regret that I could not have checked Spare #1 & #2 before delivering them to the Armalite factory and their subsequent rechambering. (When I get them back I'll post pictures of the new chamber casts and free-travel measurements.)

A number of sources throughout this ordeal have advised me to verify the NATO v. .223 Rem. chamber, but I have yet to find a simple one-stop test. It's a lot of work with velocity testing, chamber casts and measurements and also the free-travel distance. In the long run all are required to do the job right. Somehow I imagine that the manufacturers may have big GO / NO-GO gauges that test the entire chamber (headspace, various diameters and throat) but seeing as these guns got past Quality Control in the first place, maybe they don't.

Thank you to everyone who offered opinions and assistance!

Those are the kind of dimensions I would expect on a match chamber designed to shoot 55g ball (assuming someone actually wanted to do that).  It wouldn't be very pretty if you used ammo with a longer ogive - say the Hornady 75g OTM.

I think all of those Armalites need to go back and get checked.

Originally Posted By olephart: Those are the kind of dimensions I would expect on a match chamber designed to shoot 55g ball (assuming someone actually wanted to do that).  It wouldn't be very pretty if you used ammo with a longer ogive - say the Hornady 75g OTM. I think all of those Armalites need to go back and get checked.

One of the original claims when the problems started were that the rifles had match chambers but supposedly all the barrels had been changed when they went back to the factory the first time. They are all definitely going back this time and are going to be checked when they come back!

This probably deserves a new thread but since I just read some of the most proffesional and scientific advice and reasoning every seen on the interent in this thread I thought I'd ask here.

Has anyone done any scientific testing to determine the diferences in unlocking speeds based on weight of bolt carriers and buffers? All things being equal, would a M16 carrier be less likely to have suffered from this specific issue (blown primers) vs a cutt-away AR15 carrier?

Thanks

The primer doesn't really blow out.  It is held against the bolt face until the bolt unlocks.  The primer pocket expands to a size too big to hild the primer.  When the case seperates from the bolt face, the primer falls out into the trigger group :).

Originally Posted By _RAGNAR_: This probably deserves a new thread but since I just read some of the most proffesional and scientific advice and reasoning every seen on the interent in this thread I thought I'd ask here. Has anyone done any scientific testing to determine the diferences in unlocking speeds based on weight of bolt carriers and buffers? All things being equal, would a M16 carrier be less likely to have suffered from this specific issue (blown primers) vs a cutt-away AR15 carrier?

Thanks for your kind comments. As far as the testing goes somebody someplace has probably done it either in the military or one of the major manufacturers BUT it's probably restricted release information. I certainly have never seen anything like it. You raise an interesting point about the heavier bolt carrier but the effect might be marginal considering the pressures involved.

Wrecktech did a nice job on documenting the throats. I had previously missed his neck diameters. They look to be OK. Good job Wrecktech. I apologize for not noticing those diameters earlier, and I shall quit pestering you about them.

Wrecktech and I went over the neck diameter issue early on, although it may not have made it into this thread. His fired case necks are running about 0.255" which is right in there on SAAMI and NATO chambers, so I would suspect his chambers would reflect that.

The thing that we have not been covering so far is that even his "good" rifles (SIG with an identifiable NATO chamber and his RRA's with Wylde chamber) are producing some signs of excess pressure with the LC cased ammo AND his MV are rather high for a 16" carbine. The signs I am speaking of are bent rims, marks from ejectors and marks from extractors on the case head face. This case head damage makes me suspicious that he might have a batch of ammo that is hot as well.

So, what do the rest of you think of the pressure signs on the LC ammo fired in the "good" rifles? Is this still too much deformation of the case heads and extraction rims. If this much is normal in military ammo, say so and I shall quit pestering Wrecktech about it.

Either way, it looks like he has gotten to the bottom of this - short throats. Now he only has to get Armalite to fix the rifles... Has anyone sent the Armalite folks a pick to this thread yet?

Billski