OK this will be a long post to fill in the background. I purchased a 264 win mag for a steal ($75) It's a mouser 98 large ring reciever. The rifle had a broken bolt (large crack by the front lug) I replaced the bolt and had a reputable shop check the headspace. I was told "tight but in tolerance" So I bought a few boxes of ammo and started plinking away just to get a feel for it and build up some brass for reloading. Now here comes the trouble, over half the factory ammo was so over pressured that I couldn't even reprime the brass. The primer pocket was .005 oversize! Visions of a broken bolt flashed through my head! So now I tear down the rifle and start measuring. The first thing I find is there was NO THROAT at all So every round was forced into the rifling with no "jump" at all. No problem right, rented a throater and fixed it right quick! Reloaded some brass with minimum loads from the book and ta daa seems to be working. Well I didn't want to re-use the brass too much so I picked up a bag of unprimed winchester premium brass. loaded a couple different powders in groups of 4's all minimum loads and back to the shooting bench! Every new brass round overpressured so bad I gave up after a few shots out of safety. Actually blew the primers right out of the case! Primer pockets were .020 oversize. Now I'm not a gunsmith by trade but I've been a machinist for 14 years and work as a inspector at an aerospace shop. Very mechanically inclined so tearing apart a bolt action was not a problem. I pulled the barrel from the reciever measured the chamber, picked up a reamer and cleaned it up (in case it was fubared down at the neck) remeasured everything including the reciever down to the bolt face, found out that the head space was off .007 over the nogo spec. Faced the barrel off and reassembled to the reciever. Everything is now perfect! First round with the new brass at 50 gns powder blows the primer right out! So I load one waaaay down to 20 gns. fire it and it's ok. Take the same case load it up to 60 gns (min spec by book) boom works perfect! So after all that rambling The main question is why will my rifle which is in perfect spec only fire correctly with "fire formed" brass? I gave all the info to help rule out any easy questions. I also measured the brass and compared it to my old stuff. It was consistant in every way. (weight, wall thickness, volume, dimensionally etc..) So Im hoping someone will recognize something I've missed.