Barrel Length & Velocity???

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Saltydog235

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I posted this in the reloading forum as well but upon advice I added it here since it is a cross boundary question.

OK, last year I bought a 7mm08 SPS Varmint with a 26" barrel. I changed out almost every part on the gun to make it what I wanted and shot it. Originally I had intended to have the barrel taken down to 20" in order to make it more maneuverable in the deer stand. After I shot it, I decided not to, it was way too accurate and I didn't want to mess up a good thing.

I've kept my eyes open and bought another SPS Varmint in 7mm08 to experiment with. I will "build" this one the same way with the same components as the last one but I'm taking the barrel down on this one. However I have a dilemma, do I take it to 20" or can I go as far as 18" without too much velocity degradation. Most of my deer stand shots for this gun will be 200yds and under.

My load is a 150grn Nosler BT over 45.5grns of H4350 and a Win LR primer. Out of the 26" barrel I get 2700fps in the 700 and 2675 or so out of the same load in a Sako 75 with a 22 3/8" barrel that I own. I know all rifles are different but this is an exercise to find out if anyone has any similar experiences. It is flat out devastating on whitetails and accurate in the two that I already own. Hopefully it will be in the new one as well but who knows.

Why would I want to cut down a heavy barreled gun? I have a Savage 10 PC in .308 with a 20" barrel that shoots lights out, I just prefer the 7mm08 as a cartridge. I like to be different. I also like a heavy barrel over a light one if for nothing else but personal choice. Also I may have it threaded so that one day I can add a can were the option available.

What say the masses?
 
With the powder (4350) you are using there will be a greater loss in velocity with a short barrel. 4350 is a slow powder well suited for large cases and heavier bullets.

Can I safely asume the velocities you have quoted are over a chronograph.
 
Yeah, I've shot many a string over the chronograph in both rifles with very little deviation. Accuracy in both are good with the SPS 26" barrel being superb, .5" or less if I do my job. If I have to work up another load, it won't be the end of the world just be nice to be able to interchange one that already works well in two other rifles.

Who knows what will happen, that's part of the fun of working them out.
 
If you are not getting the velocities or performance you desire, consider working up a different load for the rifle. A faster burning powder or a slightly lighter bullet would probably get your velocity higher.

I don't think cutting it down will reduce velocity enough to be concerned with lacking the power it takes to take a deer down.
 
The Rem and Sako are different.
If I plug into Quickload that the Rem is getting 2700 fps 7mm-08 26" with 45.5 gr H4350 and 150 gr BT, then I have to fool with the start pressure and/or OAL to get QL to agree.
Under those conditions 22" would make 2598 fps, 18" would make 2368 fps.
All at 48,191 psi
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But to get QL to agree with the Sako ggetting 2675 fps with 23.375" barrel, then a 26" would get 2738 fps and an 18" would get 2511 fps.
all at 51,633 psi
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The 7mm-08 was SAAMI registered at 61,000 psi by Remington.
The same case head does 65,000 psi in the 22-250 Reminton.
Switching to H4895, another extreme powder, could drive that bullet at 2939 fps with a 7mm-08 26" barrel and 65,000 psi.
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I have pushed the 150 gr BT through a 26" barrel and a 7mmRM at 3375 fps, yet 3250 fps with the same load in a different rifle.
 
Here's SAAMI specs on muzzle velocity vs barrel length. It's based on each barrel length having the exact same chamber, bore and groove dimensions.

8187196265_87a436e9ef.jpg


If both barrel length's one is comparing velocity with don't have the same internal dimensions, the data will be incorrect. I've shot the same ammo (.308 Win. Fed. Gold Medal Match) through a 26 inch barrel with oversize bore and groove dimensions (Win. 70 .308 Win. factory match rifle, .301", .3085") that had a 50 fps slower muzzle velocity than a 22 inch one with bore and groove diameters a bit smaller than SAAMI minimum specs (M14NM match grade service rifle with tight bore and groove diameters, .299", .3075").
 
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