Question on Brass expanding to fit your chamber

[rdinak]

I have read that may take brass two firings to fully expand enough to completely fit your chamber. Example- I am using virgin Lapua brass in a .243 that is brand new and back from having a new barrel installed. On the first trip out to the range, I sighted in the scope and did the standard ocw load development test. for round two of the ocw test to check for seating depth, I just neck sized the brass and went with the best load the target was giving feedback on. After the second firing I plan to use my gauges to measure the brass expansion to set my resizing die for bumping the shoulder back 1/1000th.

I just dont understand when firing a round that brass takes a second firing before it elastic enough to have the optimal chamber "foot print" for lack of a better word. for sake of discussion I am shooting loads that less than max and searching for accuracy instead of top velocity.

Then take it a step further for the guy shooting an AI cartridge or other wildcat.

Please enlighten me

 

[flashhole]

You should get very good chamber fill with any standard load. Did you do any measurements on fired cartridges that say otherwise?

 

[BuzzBoss915]

Yeah, pretty much a non problem. I fireform for six Ackleys and the second firing is the same as the tenth. I don't know what your motivation is for a footprint, other than resizing, but this is usually a function of pressure. If you use light for caliber bullets and a minimum load, it could take you a couple firings to blow out your brass, but I can tell you one thing. Not always, of course, but full house loads are accurate loads, in my rifles. So, there is little advantage in shooting a mild load, unless your objective is extended barrel life. Just my opinion, maybe I'm missing something? BB

 

[turbo54]

It does in fact take several firings to **fully** fireform a case to a chamber, though the difference between the 2nd and 3rd (or 3rd/4th) is way, way less than the difference between the 1st and 2nd.

Evidence of this is how many shots it takes, necksizing ONLY between shots, before the bolt closes tightly on the cartridge. This will definitely vary depending on how hot the charge is.

For all practical purposes though, you can consider a case fireformed after its first shot.

A slight contradiction to this are some of the ackley/improved cartridges. These require a pretty stiff charge to get a sweet fireform from a single shot. For example, in forming 6xc from 22-250, a lame charge of less than 37gr H4350 won't create nice sharp angle at the shoulder.