primer backing out

[fatercat]

bought a older marlin 336 RC. bought two boxes of remington 170 gn. corelocks and two boxes of the new 160 gn. rev. the 170 gn. shot great 1" at 100 yds. the rev. primer backed out as if too much pressure. i sighted in 1" high at 100 yds with the rem. and will use them for hunting. is there such a thing as head space problems with the marlin 336?

[fornra]

 If you had a pressure problem I'd expect the primers to be flattened not backed out.
 Every rifle has some degree of tolerance built into the chamber, as to allow for different brands of ammo to fit. I think this is a rough quote from the Lee Modern Reloading manual.
 When a rifle is fired the force of the firing pin drives the cartridge  forward until the case shoulder, belt, or rim stopes it's forward motion, then the igniting powder gases force the primer rearward until it contacts the bolt face, as pressure builds the case will stretch until it completely fills the chamber and reseats the primer into the pocket. This stretching of the case is also what seals the chamber and prevents the powder gases from blowing back into your face.
 I'm not talking about a lot of stretching only a few .000s, but this is why the cases must be resized before loading them again.
 I guess it's possible that your rifle's chamber has been stretched enough to cause a head space problem, but I don't understand why the 170 gr loads didn't have backed out primers also.
 My suggestion is to get a box of 150 gr bullets and take all three weights along with a chronograph and do a little testing. What I would look for is a reasonable step from one bullet weight to the others, and if not then I'd not use the 160 gr load because you said it was an older gun, why stress it.
 You didn't mention if the cases were smoked up on the 160 gr load, but I suspect you may have a low pressure problem ,not high , but if theirs any doubt about the safety issue don't use them., Glenn

[jjamna]

Fornra pretty much said it all. Other thing I can think of is with the War on Terror going there is some issues on ammo producers. I would try the same load but a different Brand. If it stills does it it is the Rifle if it don't you may have some bad ammo.