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.

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.

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

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?

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 ronemus: This rifle has excessive headspace.

the HS is within milspec.

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).

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

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