Charge weight adjustment in fire formed cases

So I have been working on load development for a new barrel on my 300 win mag, and I have a good load worked up in virgin unfired cases. I measured water capacity on a fired case and case capacity grew by about 1.3% should I adjust my charge buy 1.3% or start over and work up another load with the fire cases

No, keep the load as is. The brass offers little to no resistance while being fire formed to the chamber so that post-fired volume is what you’re working with whether it’s fire formed or full length sized.


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you can try the same load in your fired brass and see if it shoots the same as it did in your virgin brass . if it does that's great , if not I'd try to adjust the powder charge to get the velocity the same as it was in the virgin brass . if that doesn't work you'll need to rework the load . virgin brass can give a slower velocity than the same load in fired brass , due to the brass expansion / forming . my one 7mm-08 has a broughton bbl , using norma brass , is 50 FPS different between virgin and properly sized brass . I try not to use virgin brass for anything meaningful .

I wouldn’t change it.

Unlike the above, I often times get my best groups fire forming from several of my rifles. I read that the Army surplussed the once fired brass the match shooters used, even the hand loaded stuff from new. Somewhere I also read about an old Sierra engineer who wanted the cartridge to fit the chamber “like a rat turd in a violin case.” [emoji23]The longer I go at it, the more I agree.

I will use virgin brass to start finding a node or just to get a gun zeroed but won’t mess around with brand new brass seriously until it’s been fired in my rifle. Chased my tail a few times. To some folks the difference may not matter and with a well optimized load you may not notice the difference but I’ll do load with with FF’ed brass and try it in new cases. Not vice versa.

SJB358":1v84g2sa said:

I will use virgin brass to start finding a node or just to get a gun zeroed but won’t mess around with brand new brass seriously until it’s been fired in my rifle. Chased my tail a few times. To some folks the difference may not matter and with a well optimized load you may not notice the difference but I’ll do load with with FF’ed brass and try it in new cases. Not vice versa.

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For me I do the same since loading and shooting virgin brass would be the same as shooting factory ammo which we all know may shoot good in some rifles better then others and or reloads always surpass the factory stuff if it didn't we wouldn't do it.