AR disease strikes again

[stagpanther]

I called the fine folks at Faxon this morning to see what I could find out about the barrel. The tech guy I talked to said their barrels were tested with the factory ammo available--which is 8 cartridges; 2 by hornady, 4 (they've added a subsonic) by winchester and 2 by federal. Understandably, he said they don't account for hand-loads in their development. I told him about my anomalies and he was at a loss to suggest why they might be happening--though he did say he personally didn't have their test pressure data available when I asked about that. I then asked about the rational for the gas system chosen--and was told that is the one they settled on as being most reliable to successfully cycle across the range of ammo from subsonic to supersonic--and also work with suppresors. I then asked if they tried a longer system (less dwell time) and he said yes, they did, but could not get reliable cycling even with maximum gas port diameters. He said the timing issues tended to be more problematic the larger the bullet, which makes sense.

The conversation ended with him offering to take the barrel back if I was unsatisfied with it--and I told him I couldn't find anything out of spec with it, so I didn't think that would be fair--plus I'm obsessed with figuring out why/what the problem is.

[P Flados]

Stag,

Take some of your hotter "as fired" rounds and roll them across a flat surface looking for wobble on the base.

If the base is out of true, you could be having a problem with either the bolt or the barrel extension.

If there is no wobble, the base is square and the lockup of the gun is probably good. If the lockup is good, bad brass would be the only explanation for uneven compression in the groove region.

I am really scratching my head looking for something that we missed.

While loading, as there any chance at all that you could have either gotten a wrong powder (Lilgun, H110), or gotten some "wrong powder" mixed in with your 1680?

[stagpanther]

Already thought about the roll thing and tried it--bit hard to be certain since a) it's a rebated rim; and b) the taper of the case imparts a bit of a turn. I took a much closer look at the fired American Whitetail--and while the signs are not as nearly pronounced as they are in my handloads using starline brass, they are still there, a noticeable flattening of the primer and rim face and a clear incipient separation line on some of the cases at the top of the web. One thing I'm pretty sure--between the headspace slack and the depth of the radiused chamfer at the the chamber face--it is definitely possible to move chamber support of the case past the top of the head of the case, at least in my set-up. I talked to Faxon today and they indicated they are interested and willing to work with me on resolving my issue; despite the fact they (very understandably) have no warranty for proper functioning with hand-loads, just like everyone else. I told them I was interested in exploring a bit less clearance in bolt face to the barrel's extension, and after doing some math over the phone it seems that their own bolts are made to tighter tolerances than "your average everyday 5.56 bolt." Besides that, they guarantee their bolts to work with their barrels, so I went ahead and bought one of their BCG's.

As for mixing the powders up--I have done that very thing in the past and had a REAL big Kaboom when when I touched off a cartridge releasing over 150,000 psi! Since then I have designed a "preflight" procedure for checking out one keg at a time, checking it off on the load chart before loading begins and then checking it again before loading into the powder hopper. Not foolproof, but pretty close.

[stagpanther]

Some more interesting results/comparisons I did on fired brass.
Because I have nearly a thousand virgin cases--I've simply been grabbing new stuff and using it.

I decided to take some fired brass from both my handloads and the hornady factory American Whitetail and do some handloads with both of them. The starilne stuff, except for the asymmetric compression of the rim in my higher power loads, for the most part retained it's length and head dimensions. The hornady factory brass, did not show any signs of asymmetric compression in the rim. However, two things did seem unusual, the case lengths varied widely from roughly 7.04 all the way out to a full 7.10(!). I assume this reflects case body flow from firing. Also, Hornady appears to use some kind of primer crimp, but I haven't verified that with them. Just like with NATO 5.56, I had to use a primer pocket swager in order to get the small rifle primer to seat.

[mehavey]

Stag,

I would actually expect case stretch, datum-to-boltface, in high-pressure cartridges like the 350/5.56: firing pin strike drives the case inward to max seating, pressure glues the forward/thinner sidewalls, and the case head stretches back to boltface.

That the stretch and/or case movement back to the bolt could then produce "statistically-feasible" uncovering of a weak enough portion of the case web to allow gas breakthrough into the action... is troubling to say the least.

FWIW: I ran QL for the 180 Speer at 42ksi for both Lil'Gun (incipient failure/"belt" produced) and Norma-200 (totally normal). Lil'Gun's rise-time is peaks earlier relative to bullet movement, and remaining pressure as the bullet moves past the port is lower -- yet still that powder produced the anomaly.

Truth in lending: The Lil'Gun anomaly occurred using the LEE 358-200 cast at 42ksi indicated -- and measured velocity was waaaay outta whack* to where I couldn't even jigger Lil'Gun's burn parameters to match it at all -- but the incipient failure mode was the same as yours.

... bothersome that it could be at luck-of-the-draw bolt/chamber/case design (stacking) tolerance levels..... Yeah, cast seals better/a bit higher pressure resultant, but not totally-off-the-charts higher* as indicated by the LABRADAR on two successive shots -- only one case of which "belted" on me.




* I still can't explain why I (and you) saw those velocity/pressure spikes

[stagpanther]

Not that I doubt your results--it's very hard for me to believe that a lead cast by itself could seal too tight to the bore to spike the pressure that much--especially when powered by a higher pressure charge.

[mehavey]

You & I are in violent agreement there.

[mehavey]

Interesting Failure mode vice case-wall/chamber-chamfer interplay:
Almost no "web" at all.



Note also that the "belt" also encompasses the flashhole location.

I'm thinking about it.....

[stagpanther]

Your "top of the belt" is above the flash hole and web, which is where I think the support might be ending if the case moves back towards the bolt face. Have you "asked around" if this is happening to anyone else? The manufacturers I've talked to say they haven't heard of anything like this, though I strongly suspect liability concerns might prevent them from saying anything even if they had.

Also P Flados made a good suggestion--did you measure the case depth to head in a before and after case comparison? I've noticed some slight compression in the depth of the head section--though it's hard for my calipers to discern between flattening of the headstamp and actual flattening of the head.

[stagpanther]

me havy--I've just noticed in your photos that there appears to be a slight ring around your case just behind the mouth--is that from a crimp, or could it possibly be forced into the step-down prior to the throat (or am I imagining things)?

[mehavey]

You're not imagining things -- and I've noticed from the beginning that fired/unsized cases would not accept bullets until re-expanded .... 0.3720" mouth as fired.
(ominous roll of distant thunder permitted here)

At the same time, I never get even the slightest sticky-case when seating loaded 0.3800" mouth diameters using gas bullets. Yet..... the strange mouth "crimp-looking" witness mark, and tight as-fired case mouth. Go figure.

I'm about to take a very brute force approach to resolving if/where there's chamber anomaly in all this.

Film at Eleven ....some time a week-10 days from now....

[stagpanther]

I never get those rings in my fired brass--plus I did a chamber cast that doesn't show any deviation from SAAMI specs, more or less impossible for the case mouth to get forced past the step-down without a bullet-deforming crimp, but yet I get the same expansion in the head in my failed case as you have in your "belted" case. You might consider doing a cast and see if it's like the one Flados shows to see if your chamber has the same issue.

[stagpanther]

Truth in lending: The Lil'Gun anomaly occurred using the LEE 358-200 cast at 42ksi indicated -- and measured velocity was waaaay outta whack* to where I couldn't even jigger Lil'Gun's burn parameters to match it at all -- but the incipient failure mode was the same as yours.

One thing about your QL settings--are they set for start pressures for rifle FMJ core bullets? It probably comes up with a default start pressure for pistol bullets (2000 psi or under vs. 6500 psi or more). Adjusting for the start pressure weighting factor, I can get my velocity results within what I'm actually recording on labradar, even though they are well over "published" results. When you adjust that start pressure--you can also see that peak chamber pressure is correspondingly attained much faster. I would love to get my hands on a conventional bolt gun and test the same ammo--but I'm not willing to waste that kind of money on this at this point.

Using a start pressure of 6500 psi for Hodgdon's published load 26.7 max for H110 for a 170 interlock, QL returns a velocity of around 2300 fps--which in fact was what I got when I shot it (interestingly, top velocities were achieved with charges well under that, so I'm assuming a lot of the added pressure is simply being dumped when the case releases and leaks in the chamber). QL also returned a pressure rating of almost 64,000 psi for that load. I believe it.

[mehavey]

Start pressure was 1160 (lead/cast bullets)
Note velocity predicated (1,915fps) vs actual (LABRADAR 2,153 & 2155) .. then quit.
Absolutely nothing I could adjust/alter in QL could come close to accounting for it.
First time ever in 10 years of experience.





postscript: Federal Commercial cases/loads don't show that unexplained mouth/rim artifact (same length cases).
Again, go figure.

[stagpanther]

That's pretty impressive for 200 gr! I'd be concerned about the ring aft of the mouth. Not a criticism, but my hunch is your start pressure has gone up for some reason--even if it was just that one cartridge. Try adjusting the start pressure in QL and I bet you can get in range of that velocity.

[P Flados]

As I recall, Mehavey's chamber neck is on the big side (it can chamber loads with 0.358" bullets), but the throat is tight and/or short (correct me if I am wrong).

Is there any chance that a previous round had left a ring of lead (shaved off) at the step just in front of the case. I have seen this quite a bit on a 9mm Glock that I load for. If there was lead ring and then a full length round is jammed into the chamber, I could see the lead ring being pushed inward and up against the bullet (again this does happen on the Glock) . This would tend to hold the bullet in place more than normal. This could have elevated your "start pressure" for the one round that had the pressure spike consistent with what you seem to have figured out from QL.

For Stagpanther, he was using new brass. At times, I have had new brass act very "sticky" and require much more than normal force to to expand the brass and/or have high/inconsistent seating force. If his one bad round just happened to "stick" to his jacketed bullet, again it would cause an elevated start pressure.

For both events, there seems to have been a substantial pressure excursion when compared to similar rounds that did OK. When I try to come up with some explanation for a big pressure increase with no increase in powder and no change in components, there are not a lot of choices. The initial breakaway force needed to move the bullet seems to be the least unlikely at this point.

[mehavey]

Increasing start pressure first raises -- then lowers -- velocity as available energy is eaten up.
Going all the way through/to 60,000psi-start never saw velocity rise out of the 1,900's.

Remember also, whatever this condition was it caused two successive shots at near exact same velocity.

[stagpanther]

My new Faxon BCG just came in. The bolt face does appear to be .001 shallower than my other bolts--but that may or may not be relevant depending on the lug's length and how they lock up with the extension. We're about do have a pretty messy storm here so I'm not going to run a full test (my labradar would get blown over among other things) but I will fire off a few cartridges to see if I notice anything different.

[stagpanther]

As is often the case nowadays--I ran out the door forgetting my handloads next to the press--but fortunately I had some American Whitetail cartridges still in my range bag so I was still able to test the new BCG.

The American whitetail had also formally show signs of over-pressure in my gun--but after installing the Faxon BCG they all disappeared: flattening of the primer, flattening of the head stamp lettering, incipient failure line at top of web, ejector scrape extensive gas leakage--all these signs were gone. Also, the BCG failed to lock back at the previous setting and I had to turn it back clockwise a half turn to get it to start locking back again. Without a doubt, gas pressure/bolt thrust have been significantly reduced. Is it conclusively because of less headspace due to one or two thousandths of an inch closer bolt breech face? I can't say for sure--but I strongly suspect that whatever reason the case is getting a better seal and case support, at least with the hornady factory ammo. I'll try to get out and test my own loads once our brewing storm passes.

[mehavey]

Intel-lesting.......



`got a Hornady sectioned ??

[stagpanther]

I guess I could--what are you showing here?

[mehavey]

The sectioned Starline has slightely thicker walls when measured, but the Federal base
has a head much deeper than that of Starline -- most likely extending well past/into
the chamber/beyond the chamfer juncture tolerance.

[stagpanther]

That is interesting. This is unfired brass? Can you measure the distance from rim face to top of web on each?

[mehavey]

Fired brass in both cases (pun intended)
I'll trim-to-common-length and measure depth.

[stagpanther]

It would perhaps be better to measure unfired brass since it likely would not be distorted, no?