Pressure in a Bolt Action vs. Gas Gun

[Bully]

i have a couple of guns that are new or new to me. One is a Tikka T3/Tac 223 in 1:8 the other an AR 223 in 1:7.

In shooting some moderate reloads (69TMK over CFE223) I noticed that in the Tikka there was nothing amiss but in the AR the primers were flattened. I reached out to a friend that mentioned that gas guns generally produce pressure signs at lower powder charges than bolt guns.

I'm wondering why that is.

Any help is appreciated.

 

[people]

That is what I am thinking. If it was your brass getting tore up during ejection then a high port pressure would be the thing.

 

[243winxb]

The auto action opens while pressures are higher.?? http://m.hornady.com/ammunition/superformance-in-gas-operated-firearms

 

[BoydAllen]

The sequence goes like this. The case is knocked forward in the chamber until the shoulder of the case is stopped by the shoulder of the chamber. This happens from the striker force and the primer force against the bottom of its pocket. As the powder starts to burn and pressure rises, the case body expands and grabs the body part of the chamber maintaining the gap between the case head and bolt face that was the result of the case initially being knocked forward, and the clearance created from sizing (shoulder bump). As the pressure rises, the primer, which is unsupported for an instant between head and bolt face, expands slightly, and then the case stretches, at a point slightly above the extractor groove which lets the head come back to the bolt face. This re-seats the primer to flush, and sharpens its corners, depending on how much it expanded during the instant that it was unsupported. More pressure equals more expansion, and sharper corners (flattening) Because semiautomatics require more clearance between their ammunition and their chambers, the effect will be greater than with bolt actions that can safely digest ammo that has a tighter fit.

 

[Bully]

Thanks all. Good info as always.