Dangerous Pressure Increase from Overheating in Rapid-Fire Near-Max 5.56?

Gnash.Hyena

Gnash.Hyena

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Howdy y'all! I've been working to develop a good 55gr 5.56 handload for bulk training and match ammo (FMJ), hunting (soft point) and varminting/self-defense (V-Max), and I'm more or less there. But a friend brought up a good point and it's got me concerned. I'm firing relatively slowly, shooting for groups, from a bench, in a comfortable position with little to no chance for the gun to overheat. That's nice and all for precision, chronographing and developing a good quality load. But my ARs are fighting rifles. I need to be able to fire them quickly, reliably and safely, with the knowledge that no matter how hot the gun gets, the ammunition will be safe to use without signs of overpressure. I don't want to be four hours into an eight hour carbine course and my gun blows up on me because I was using handloads that were too hot because I was ignorant.

My FMJ and SP loads are sitting right at 27.0gr of Ramshot TAC at a COAL of 2.200". My V-Max loads are sitting at 27.2gr of Ramshot TAC at a COAL of 2.260". COAL chosen is based on the COALs recommended in the Hornady reloading manual, since they're all Hornady bullets. At 27.0gr for the FMJ and SP loads, that registers (at least on my Caldwell Chronograph) an average velocity of 3140.8 and 3162.5 fps. At 27.2 gr for the V-Max loads, that registers an average velocity of 3206.8fps. This is out of a 16" barreled 5.56 AR (my ZEV Core Elite). The maximum charge for 55gr according to Western Powders' guide is 27.3gr of Ramshot TAC pushing a 55gr projectile. I know better than to push past recommended factory maximums. I don't try to do the "pissin hot!" thing, but I do like great groups with as fast a velocity as I can push the bullets *safely*. The only issue is that my chamber is never getting much more than ambient air temperature of anywhere from 50 to 95 degrees, based on the ambient air temps I've shot at my range developing these loads. Maybe a bit more "because gunpowder burns hot!" So my question is, how dangerous will it be firing these near-max loads in rapid fire, magazine after magazine, in a training, match or SHTF scenario? Is it fine and I'm just making a mountain out of a molehill? Or should I really back off and work towards a slower, lower-pressure load? I feel like the people here are more likely to have much more experience than I have, especially when it comes to real-world fighting and training applications with handloads. So before I go spending even more time making more handloads to test to get data that may or may not tell me what I need to know, I'll ask here.

So far, only the actually 27.3gr loads (and the one time I tried 27.4gr out of curiosity) have even shown the faintest signs of primer cratering (CCI #41) or headstamp swipe (LC 5.56 brass), and even then only with the soft point bullets and only a few instances of that. I've been plugging numbers into Gordon's Reloading Tool to check my pressures at their max temp of 140 degrees, and it's either fine or not, depending on whether or not the case volume is the user-data-listed 31-ish grain H2O case volume, or the Wikipedia-listed 28.5gr H2O case volume. So now it's looking like I need to find a good method of measuring average case volume. Does anyone have any experience frequently shooting lots of near-max handloads out of their 5.56 ARs that can provide some insight on the safety of this endeavor?

One factor in all of this that bothers me, is that because Ramshot TAC has such a high nitroglycerine content, it doesn't give you as steady of a warning sign that you're going overpressure as other powders might. It performs awesome, my rifles really seem to like it. But supposedly its pressure signs are ok... ok... ok... ok... BOOM! Not ok. So that's less than ideal.

Thanks!
 
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I tend to over think things like this and what you wrote above looks like something I'd worry about.
I took a training class once where the instructor addressed this from a different perspective. Guns that work fine in slow fire sometimes fail when you get the hot. So, to test this he had us mag dump two or three magazines. Get the gun too hot to touch, then load a 4th magazine and chamber a round keeping the gun pointed down range. Give it a minute or two, then fire a few shots. Then rest it a minute and fire it a little more. Repeat through the whole magazine and verify if you have poi shift.
That tests the whole system for reliability.
 
I would ask the folks that make the powder, I think it’s Hodgdon. Might help to do a couple quick mag dumps and then check the temp of the chamber since that’ll be the temp of the next chambered round if not fired for a min.
 
I use a good bit of TAC mostly in the big bore rifles and it is superb in many cases............ It is also a great powder in 223. I have a 22 inch WIn M70 test gun. I tested 55 Nosler Ballistic Tips with 27/TAC for 3290 fps at 55000 PSI. I consider 55000 PSI top end, max safe pressure in 223. This was Fed Small Rifle and Mil Spec brass.

26.5/TAC and Winchester Small Rifle, Mil Spec brass tested at 3246 fps at 52800 PSI and 26/TAC Fed Small Rifle at 3142 fps at 50700 PSI.

The Hornady 55 Soft Point is a very good common expanding bullet when impact velocities are kept at bay. I ran 26.5/TAC and Win Small RIfle with this at 3220 fps at 52200 PSI

Keep in mind several points....... My TAC is not Your TAC..... there can be differences, not just in Lot# but from keg to keg as well, depending on how it was stored. All Primers are not created equal as well. And, absolutely different bullets will test different as well, a 55 Nosler is not a 55 V-Max, I have typically found the various V-Max bullets sometimes produce a little more pressure than many other bullets.

The most devastating bullet available for Terminals is the 50 Raptor from CEB. Its what I have invested in. Precious Metals require an investment! I run RL10X with these which does not give the highest velocity, but it
does give the most accuracy in a variety of 223 rifles I have. I tested 27/TAC with the 50 Raptors, Fed Small Rifle, Mil Spec brass for 3362 fps at 53200 PSI. If I had one or two rifles that shot that well, then that would be my standard load. However, I found that all rifles did not burn TAC the same and in the end I went with RL 10X for standard loads with the 50 Raptors for slightly less velocity and close to equal pressures.

For your concerns, I would drop the load to 26.5/TAC. With the bullets you are using, the difference in velocity is a moot point, and in fact, many times, less is actually MORE.............This would probably give you around a 2500 or more PSI to play with before getting to Max Safe or well over Max pressures...........
 
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