Originally Posted By Billski: While I firmly believe Dano and Tweak know these rifles and are absolutely correct about slow powders aggravating things for 16" shorties,

Hold up hoss, I never said a thing about the powder. I'm just seeing pressure signs. I already discussed how the cases drop primers.

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.

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....

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

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....

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!

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!

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.

Originally Posted By Billski: Wrecktech and I went over the neck diameter issue early on,

Cool, I had completely forgotten about it but had seen it be an influence previously. Anyone been in contact with the ammo maker?

I'm sure Armalite (at least an employee there) has seen this, most of the makers (or their spawn) lurk in this forum.