Scratching My Head On This One

[MN50Shooter]

I shoot in FCSA 50 BMG Matches. I just put a 34" K&P 15 Twist Barrel on.

I use Israeli TZZ-90 As my match brass. I loaded my "Old cases" (fired and annealed 4 times) for my match ammo. And then I loaded 25 rounds in my new (Old Stock) TZZ-90 cases that had never been fired. But they have been fully Match Prepped with everything from uniforming Primary Pockets, Annealing, Resizing, turning necks etc..

I loaded the "New" Cases with the exact some load as I did with my Match and my Sighter Rounds (mainly fire forming them) during practice..

Now, here's the Scratching My head issue. Why am I getting a 100 FPS faster reading from the "New Brass Rounds". that ae loaded exactly the same as my 4 time fired brass. And the speed difference is almost exactly 100 fps faster, every time I shot a new cased round.

 

[Howland]

I suspect the new case dimensions are smaller than your fired, sized and prepped case dimensions. Less case volume results in higher pressure and velocity, everything else being equal.

 

[Bill Norris]

I suspect the new case dimensions are smaller than your fired, sized and prepped case dimensions. Less case volume results in higher pressure and velocity, everything else being equal.

+ 1 on that one.

 

[D-4297]

I'll go a +2 on it . Had exactly the same occurrence when using "New" Lapua cases in my .308 TR rifle . Speed went up a very consistent 65-80 FPS , so it bears that it's not a caliber specific issue . After several firings , and adjusting the load down , and back up , the speed on the new cases came back in line . You'll probably get the same results with a couple of firings .

 

[dkhunt14]

I shoot in FCSA 50 BMG Matches. I just put a 34" K&P 15 Twist Barrel on.

I use Israeli TZZ-90 As my match brass. I loaded my "Old cases" (fired and annealed 4 times) for my match ammo. And then I loaded 25 rounds in my new (Old Stock) TZZ-90 cases that had never been fired. But they have been fully Match Prepped with everything from uniforming Primary Pockets, Annealing, Resizing, turning necks etc..

I loaded the "New" Cases with the exact some load as I did with my Match and my Sighter Rounds (mainly fire forming them) during practice..

Now, here's the Scratching My head issue. Why am I getting a 100 FPS faster reading from the "New Brass Rounds". that ae loaded exactly the same as my 4 time fired brass. And the speed difference is almost exactly 100 fps faster, every time I shot a new cased round.

When I shot the 308 Baer and fireformed, the rounds were faster. After fireforming, to match the velocity, I had to drop half a grain of powder. That was becuase of capacity differences. Matt

 

[MN50Shooter]

Thanks guys. I'm segregating those cases and will watch them as I use them for sight-in days. Even my buddies have been shaking their heads about this one.

 

[JOSHUA EARLE]

You said that you just put a new barrel on your rifle. Have you tested these loads in this barrel in the past? I have had identical loads gain or lose significant velocity, depending on what barrel they are fired in. I have a Criterion barrel that replaced a Criterion barrel just like it on one of my rifles. With the same headspace measurement, brass, powder charge etc, the new barrel is 70 fps faster than the barrel before it ever was. Just something to consider