Range Report Big drop in velocity with new 6mm CM barrel

[Big Moose]

Just had my 6mm CM gas gun re barreled. I went to the range today to break in the barrel. I was running the same loads from the same batch of reloads I was using with the older barrel, however these chronographed appx. 150 fps slower. Does anyone have any idea what might cause this? The load is 40 grains of H4350 behind a 108 ELD. Out of the old barrel these chronoed at 2960 fps, and out of the new barrel 2810 fps. 2810 seems excessively slow, looking at various data for this load combination.

 

[Pvt.Donut]

New barrels tend to speed up during break in. Get like 100-150 rounds down it and see if velocity improves. Some barrels will produce different velocities but I'd expect your barrel will speed up after a couple hundred rounds.

 

[Big Moose]

New barrels tend to speed up during break in. Get like 100-150 rounds down it and see if velocity improves. Some barrels will produce different velocities but I'd expect your barrel will speed up after a couple hundred rounds.

Thanks for that, just to expand, the old barrel only had just over 500 rounds down it before the swap, so it wasn't shot out. If you look at the load data, Hornady list 39 grains of H4350 as starting load behind the 108 ELD, with a velocity of 2900 fps. I'm a full grain over that at 40 grains, but only getting 2810 fps. I'm getting absolutely no pressure signs at all. I understand your point about breaking in the barrel and barrels being different, but I never saw the same velocity drop when breaking in the original barrel.

 

[lash]

It’s not uncommon for a new barrel to be slow at start and speed up after 100-200 rounds, as said above.

Barrels from different manufacturers can vary in speed for the same load. For example, it seems that typically Tikka barrels are quite a bit slower than others. Even barrels by the same manufacturer can vary in speed somewhat.

Chamber dimensions can vary enough from barrel to barrel, unless the exact same reamer is used.

Any of these factors can affect your initial barrel speed and sometimes during the whole life of the barrel. That’s why I often don’t worry about this too much with a new barrel until after there are 100+ rounds down the tube. Even then, you may find that you’ll have to redevelop/tweak your load to get the same performance from this new barrel.

 

[morganlamprecht]

what has been said ^^

and not all barrels are the same...the book velocities are just general estimates of what you can expect

my last 6 creed was 2945 fps on shot #1...it sped up all the way until shot #120 and then leveled off at 3020 fps with the same load

 

[Big Moose]

what has been said ^^

and not all barrels are the same...the book velocities are just general estimates of what you can expect

my last 6 creed was 2945 fps on shot #1...it sped up all the way until shot #120 and then leveled off at 3020 fps with the same load

Thanks, what load are using and in what barrel? I'm running a Bartlein 7.7 twist.

 

[morganlamprecht]

26” 8 twist, 40 gr of h4350 under a 108 eld, I think it was a bartlein but I don’t remember, I’ve got a mix of them from various manufacturers

 

[LawnMM]

It hasn't sped up yet, run 150 rounds through it and check again

 

[Big Moose]

It hasn't sped up yet, run 150 rounds through it and check again

Thanks will do.

 

[Big Moose]

Thanks will do.

Thanks