AR disease strikes again

[mehavey]

Will do both -- pull some Federal/Winchester live rounds
(Actually, before/after might be no less instructive)

[stagpanther]

I'm going to do some more testing of hodgdon's published data--when I enter their data into QL it consistently returns pmax of around 10,000 psi or more compared to Hodgdon's pressure ratings. Using even Hodgdon's start loads, with an increased start pressure, I can easily exceed the max pressure rating of 55,000 psi.

[mehavey]

[stagpanther]

What do you mean by "base thickness?"

[mehavey]

Subtract the pocket depth if/as necessary, but the real issue is that .223 measures at 0.190"+
for dealing w/ both 55ksi pressure and chamber chamfer unsupported exposure.

Note also relative wall thicknesses as the case sides meet the web (or what serves as the web.

[stagpanther]

I suspected the issue had something to do with total headspace tolerance and the depth of the head to top of the web--the only thing unknown from your measurements is if it is possible that there was some compression due to firing--a possibility at higher pressures.

[mehavey]

New (Starline) case gives me the exact same depth reading.
(little if any compression)

It was Starline that failed on you ?

[stagpanther]

Yes--you can see it on this thread a few pages back

[stagpanther]

I've never seen any brass anywhere that comes even close to 1.71--Have you? I'm going crazy trying to figure out why there is such a wide discrepancy between published loads--my loads--and what QL forecasts. I keep coming back over and over to the case failing to seal to the chamber and backing to the bolt breech face before peak pressure is attained.

[mehavey]

... case failing to seal to the chamber and backing to
the bolt breech face before peak pressure is attained

Whether the neck expands/locks in forward section, or the case moves back against the bolt face early, or the case seals but stretches to meet the bolt face -- the shorter Starline web appears to be moving back/beyond the protective chamber wall and into the unsupported chamfer space.

Thin case wall so exposed cannot contain the high pressure.

Combining max-but-within tolerances ....
- Long chamfer zone,
- Long bolt face setback (headspace) distance
- Short web

.... voila.

[stagpanther]

the shorter Starline web appears to be moving back/beyond the protective chamber wall and into the unsupported chamfer space.

Thin case wall so exposed cannot contain the high pressure.

Combining max-but-within tolerances ....
- Long chamfer zone,
- Long bolt face setback (headspace) distance
- Short web

.... voila.

That's pretty much the conclusion I came to already, the real question is how to time the cartridge just right so that it has enough pressure quick enough to seal to the chamber and not move back, there's a heck of a lot of tolerance slack involved between the headspace and case lengths. Even if the case support problem goes away, inconsistency in timing will yield inconsistency in performance/accuracy.

[mehavey]

Stag,

One way or another the case head is going to meet the bolt face at those pressures -- stretch or slip.

[stagpanther]

Stag,

One way or another the case head is going to meet the bolt face at those pressures -- stretch or slip.

If you fire some American whitetail 170 interlocs you'll see hornady manages to get the pressure up high enough to keep a good seal to the chamber without going over-board on pressure.

[9x19]

Weren't you taking a break with a new Glock project?

[stagpanther]

Weren't you taking a break with a new Glock project?

Yeah--but it seems to be taking three weeks for each part to come in.

[stagpanther]

Hornady once-fired brass. I don't have a mic like mehavy's that has precision of an electron microscope, unfortunately.

[P Flados]

The short web on the starline does sound like a good explanation for the blow out.

However, the extrusion into the ejector hole says there was also an over pressure event. The brass would probably have been "belted" had the entire case wall been fully contained.

All of us 357AR and 350L reloaders are "lab rats" to some extent at this point. I need to go through my brass, find a representative "shortest web" and do something to determine if the case wall is fully contained. Until we better understand the cause of the over pressure events, using cases with walls that are not fully supported is really just plain not smart.

[mehavey]

Which is why I just ordered 400 Winchester cases....

[stagpanther]

Which is why I just ordered 400 Winchester cases....

The hornady stuff is pretty pricey, but I have a hundred of those coming in along with 2 hundred winchester cases, should be here tomorrow or Friday. I've noticed a pronounced drop in bolt thrust pressure since I switched to the Faxon BCG--I've removed the heavy spring and H3 buffer and restored conventional carbine weights.

[stagpanther]

I just went out and did some test firing--the only cartriddge I could duplicate from Hodgdon's data that was also in my QL database is the 147 gr xtp--so I whipped up their lowest load and fired it (using IMR 4227) and for the very first time my velocity results almost matched theirs (even though my barrel is shorter). However, their cartridge specifications essentially will not feed properly in my AR (I doubt anyone else's, either) being far too short with a COL of 2.00".

I'm convinced Hodgdon must have used a bolt gun for their tests. CMMG must have a pretty darn well tuned set-up in their 350 legend AR if they are using the same specs as everyone else--I personally believe it's just not a cartridge design that will work consistently and reliably with as many cartridges in an AR as would work in a bolt gun. To top it off, I finished my shooting session with some handloads in 9mm through my 9mm PCC frankengun cheapo special, and using an unmagnified red dot turned up to around 5MOA I was able to shoot a tighter group than anything I did with my 350 legend and leupold 3x 9 scope. I left pretty disgusted.

[jetinteriorguy]

If that's your 9mm at 50yds that's a pretty darn nice group, if that's at 100yds that's an outstanding group. You shoot like me, there's always just that one round that doesn't quite make it.

[stagpanther]

Thanks, It is only 50 yds with the 9mm carbine--but with the same scope as is on my legend I'm sure I could beat it at @100 with this particular load--at least at the moment.

This is one of my "Especiale" loads; 65 gr extreme defense @ 2300 fps and over 750 ftlbs energy.

[stagpanther]

Mehavy--the problem I see with the 350L in an AR is that when you fire it--two projectiles leave going in opposite directions.

[mehavey]

Because it's anything but a straight shot, does anybody got any really nifty ideas on how to get CerroSafe into an AR chamber without ....

(a) Going everywhere but the chamber
(b) Solidifying in the (metal) funnel tube before it gets to the chamber ?

[stagpanther]

Brownells has good instructions on how to do it at https://www.brownells.com/guntech/ce....htm?lid=10667

I would add for AR's run a patch or two down the bore from the muzzle and stop it just after the lands. Resist the temptation to make the patch too tight fitting--you'll regret it if you do. Make a cone out of paper or other suitable material that fits down into the chamber opening and is high enough and narrow enough to clear the extension lugs but wide enough to fit a metal funnel into. Some people can manage to pour into the chamber and know exactly when to stop before flowing out into the extension without needing a "prevention cone"--I'm not one of them. Heat the cerro over a low flame gradually to get a nice even silvery pool of material and then carefully pour into the chamber with the barrel held vertically (easier if it's in a vice). As soon as you fill the chamber secure your heat sourced tap the plug out with a dowel from the muzzle end within a minute or two--the longer it cools, the more it will expand. It should pop right out easily.