Huge drop in FPS while fireforming

[Mram10us]

I was using the chrono on a wildcat I just built (longer 6.5 creedmoor using 260 brass) I took some virgin 260 brass with 43g 4831sc with a 140bthp out of a 24" barrel. The book said 2610ish was expected. The first 5 shots I did the shoot once, clean. The next two shots I did were 2418 and 2462. I'll keep track of the next 7 shots. Very surprised to see the speed fireforming steals. Charges were thrown from a rcbs powder thrower.

 

[L.Sherm]

I'm guessing it's because your changing the internal volume in the case.

 

[Mram10us]

I'm guessing it's because your changing the internal volume in the case.

You are right, I just never expected that much of a drop.

 

[Mikecr]

Recent discussion about it: https://www.longrangehunting.com/th...nce-between-new-brass-and-fired-brass.235729/

 

[Mram10us]

Recent discussion about it: https://www.longrangehunting.com/th...nce-between-new-brass-and-fired-brass.235729/

Thanks Mike. I never expect almost 200fps with a wildcat.

 

[Mikecr]

I haven't dug into details, but you should see the velocities go back up now that your brass is way better chamber fitting.