Tweak: The Dimensions on the case necks from the casts are:

Gun / Near Case Shoulder / Near Case Mouth
S-04 / .2576 / .2560 (Sig 551 - 5.56 NATO)
A-08 / .2568 / .2558 (RRA - .223 Wylde)
A-04 / .2586 / .2572 (Armalite)
Spare #1 / .2585 / .2564 (")
Spare #2 / .2586 / .2560 (")

As far as contact with the ammo maker, funny you should ask! I just received a letter back from ATK / Federal just this week. They also report a similar problem with the San Antonio, Texas PD where they discovered their Armalite guns had .223 chambers so their cure was to return all the ammo. They also indicate that blown primers in commercial .223 is usually a combination of a weaker case (head & web), dirty-hot gun and high ambient temperature. Knowing that, their LE line of ammo uses a mil-quality case instead of a standard case and don't load to 5.56 levels. They also point out that "XM-193 is a machine gun compatible full mil-spec load." I would expect it to be hot! The Ammo-oracle states M-193 should give you about 3150 FPS out of a 16" barrel. That was what I got out of the 16" Styer-AUG from the get go and RRA guns were below that. I can't imagine there is any real specific difference between these two versions.

Since Armalite is apparently going to actually fix the guns and in the process cut the chrome in the barrel throat, what is the long-run effect of doing that? How much is that going to cut down my overall barrel life? Right now I'd have to say these guns probably have 7,500 to 10,000 round through them.

wt,

did you cast the entire chamber or just the neck and throat?

I'm wondering if the chamber diameter is undersized.

the design concern was MV, IIRC gas port pressure wasn't quantified until the rifle had already entered service, that was part of the testing brought about by the troubles in RVN.

I don't have my refs here so I can't comment on the chamber dims but if we use the SIG as a baseline A-08 Spare #2 are tight near the neck and Spare #1 dupes the SIG numbers, A-04 is larger than the SIG. I've lost track of which rifle is doing what, do you see a pattern of behaviour related to the chamber dims?

Originally Posted By wrecktech: Do you know of someplace I can find this information? The Black Rifle is the reference I'm thinking of, unfortunately my copy is currently in another county. Spare #2 was the original problem child with popping primers. A-04 has been one of the better behaved ones of the group. Spare #1 had its share of breakdowns to get moved off line but never blew a primer that I recall but did have very short distance from case mouth to rifling on the casts.

So Spare #1 and A-04 are the better behaved ones (regarding pressure signs) and their chamber dims mimic (or exceed) those of the SIG control?

a tight chamber will lead to FtEx and FtEj and sometimes what appears to be a short stroke but very rarely blown primers. The tight, steep, short throats will lead to higher chamber pressures and can create the same malfs as those listed above.

those look familiar. funny how things get around online.

not sure what you're asking?

You will find variations in dimensions for the "same" chamber among the various reamer manufacturers. Then ya have variations in the reamer manufacturing process. Then you have the reamer cutting a smaller chamber every time it is used due to wear. Then you have (sometimes large) variations in the actual chamber due to reamer wobble caused by poor equipment and/or lack of skill of the operator.

Bottom line is that 2 chambers with the same name probably won't be identical on all the dimensions. This can drive you cracy if you are looking for a chamber to match the print.

None of this is to say that you can't get some prints and chamber casts and figure out what is going on. You are doing a very good job of it. Just don't expect any chamber to match any other or the print that you are referring to in every dimension.

Right now I am using the chamber casts to try to determine the throat angle (If I can. Maybe the better question is: Do I need to?).

Among the dimensions Billski & I am trying to pick up are the land (Bore) & groove diameter, the throat diameter (freebore), throat length and throat taper length. However I must admit that once again I am confused. Am I correct that the throat cylinder diameter would be the dimension just off the end of the case neck? I think I am correct in that this size should NOT necessarily be the same as the groove diameter?

The bore diameter is the original dimension of the hole drilled the length of the barrel and the grooves are cut during the rifling process. The throat cylinder may be cut in during the chambering process. Right?

In order to determine the throat length, I will have to carefully check the smaller bore dimension for changes between where they start on the cast and where they finally "bottom" out.

On another point, my former Rangemaster knowing about these trouble rifles called last night and left a message that there is an article in the September issue of "Guns" magazine relating to this issue. Has anyone else seen it? My copy hasn't arrived yet and now I'm getting impatient waiting.

wrecktech,
here's what I just posted on the Competitions forum;

I just read through the parallel thread on the "Troubleshooting" forum. I'll throw out a couple of random thoughts and experiences for you.

1) The velocities recorded for the Armalite barrels and XM193 are WAAAAY high for a 16" barrel. That alone is indicative of excessive pressure for me (remember there's no free lunch). Swollen/blown primer pockets are just extra nails in the coffin. Normal velocities and pressure signs in the AUG? Sounds like an Armalite barrel problem.

2) I've played with the 223/AR and slow powders in a pressure measuring upper (strain gage). Most of my experiences are with heavy bullets 69, 75, 77, 80 and 90's. But I do shoot LC M855 (62gr) as a reference round before each session. No pressure problems there. ...and I did a workup with 52's and N540 which in the context of the situation and theories being floated is pretty slow. I got up to 3300+fps with a 20" 6.5 twist Pac-Nor Wylde Chamber. Surprising pressures did not appear to be excessive (strain and case/primer appearance). Typically what happens when you're too slow on the powder burn rate is you run out of room in the case before you get to max pressure.



3) Powder too slow being blamed for primer blowing because port pressure too high/carrier moving back too early/too fast? (...or 16" barrel with a port closer in for higher port pressures). I've seen the Carrier Weight System (to slow down carrier velocity) help with case and even primer appearances but the benefit wasn't nearly as drastic as comparing a normal case to your blown primers.

4) The last time I had to go through a similar academic exercise, it involved Federal Ammo (CMP 77gr Loading) and a theory about tight bores (Krieger). In the end, that was inconclusive. Ammo showed fine in my Pac-Nor barrels. Fine in my Kriegers as well. I ended up suspecting chamber/throat, but never followed that to a conclusion. (oops, that pic is of Fed XM223A which is an 80gr loading. Similar pressure situation, similar conclusion though).


5) If you'd like, I can run some of your ammo through my pressure Upper. I suspect it won't show much through since the AUG's were fine. E-mail me via my profile on this site if you'd like to send some over. There are some practical challenges to work through, but I'm willing if you're interested.

6) I saw the chamber drawings from JGS and some unanswered questions about dimension ie leade angle. Here's a comparison of chambers from the Clymer prints if it helps;

Tag for a problem I seem to be having again using the new wolf m193: Found this thread while searching for this old one about my previous issue!
archive.ar15.com/forums/topic.html?b=3&f=66&t=243133

I haven't posted anything on this topic in a very long time. The reason for this was I was directed by my agency administration to stop after a threat of legal action for libel. The end result was that all the uppers on the ten questionable rifles were replaced with brand new units from another maker. Since that time, there hasn't been a single ammo related failure. I also haven't had any more fractured extractors either. The other uppers are sitting on a shelf in a closet gathering dust.