How hot to load for a semi-auto?

[fatelvis]

What is your personal pressure cut-off when loading for a semi-auto? Specifically for an AR style, DI action? I'm looking at my Quickload and trying to decide how hot to load it up for my 224 Valkyrie. (SAAMI says 55K psi max) I want to maximize the cartridge's efficiency, but I don't want to beat the rifle unnecessarily. Thanks Guys!

 

[243winxb]

Port pressure, powder burn rate have an effect on auto loaders. . Not just max pressure. An adjustable gas block is usefull.

Work up from starting load, stop when accuracy is ok.

 

[GLShooter]

I load to MAX frequently but powder choice can be a big factor and to deal with that I run an adjustable block. Some brass stands the pressure better than others in the semi.

 

[gunsandgunsmithing]

Another factor relating to max pressure for many gas guns is bolt thrust. A larger case head at the same pressure generates considerably more bolt thrust. That's why a 6.5 Grendel for example, has a lower published max pressure than a .223/5.56. Here's a link describing it and how to calculate bolt thrust. Scroll about halfway down the page.

A Look at Bolt Lug Strength - Lilja

All action designers are concerned primarily with producing a safe and functional product. All other considerations such as weight, finish, eye appeal and other cosmetics are secondary. From a safety standpoint, the strength of the bolt lugs is of prime concern. Bolt lug shear strength depends...

riflebarrels.com

 

[Rocketvapor]

The difference in diameter of the 5.56 case and the 224V increase the hoop stress on the case head and bolt thrust. Thus the recommended 55K.

 

[Twdmar]

With 5.56, an AR can be loaded pretty hot. Look at GI ammunition, it's hot. Unfortunately the Grendel size bolts don't take as much beating with less steel in the bolt.
It would be a good question for the highpower shooters that use the 6 turbo or 6 fat rat. They do break bolts more frequently than 223 users. One guy I know can tell how many times cases have been reloaded by the ejector marks.
Loading manual values are probably pretty accurate and if you need more juice, try a boltgun. As others have said, different powders and gas block and its location can make a difference too.

 

[mac86951]

Load pressure tests. I look for case head swipe in addition to standard pressure signs. As others have said, powder selection matters; in theory one can turn off gas and do pressure test, then slowly turn gas back on in separate tests.
I cheat in that i have a baseline knowledge of my rifle and various powders, but trying a new one, I'll look for pressure signs and case head swipe and using an adjustable gas block, I'll have a better idea if i need to change gas system or if the load is just too hot.


This should be done on a hot day and foulers to heat the barrel if that's how you'll be shooting.

-Mac

 

[Fuj]

You can tell how hot the stuff is by where the empties get
stacked. Anything Federal NATO loaded stacked at 4 o'clock
IMI Israeli stacks at 2 o'clock out of my RRA Delta mid.

 

[jepp2]

What is your personal pressure cut-off when loading for a semi-auto? Specifically for an AR style, DI action?

I've processed a ton of military once fired brass. I would get batches occasionally that I lost as much as 30% or more due to enlarged primer pockets. All of them had case head flow into the ejector. I've also shot a lot of XM193 loaded ammo to get the brass. For the past several years, if I didn't fire them in the winter I had the case heads showing pressure marks.

Summary is, on LC brass, the case heads will tell you pretty quickly when you are too high in pressure. So when I read about folks popping primers, they are TOO HIGH in pressure. The Sierra guide for gas guns is a pretty good starting point for QL..

I just re-read your post and see you aren't loading for 223.