Muzzle velocity vs barrel length

Thanks guys this is really good info.
Does the pressure changes as well?

If I was betting, i would guess your velocity will be about 100 fps higher than what the book advertises with a 24 inch .

Give or take 125.

Mc Fraser said:

Hi Everyone,
I have a Win M70 in 300WM with a 28" Krieger barrel.
My question is: most of reloading books are based on a 24"in barrel, the 4in difference on my barrel will increase or decrease the muzzle velocity compared to one advertised in the book.
TIA.

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You should get anywhere from 100-200fps greater velocity running equal pressures.

Your main advantage is that you can take full advantage of powders like RL26 my favorite, RL33, retumbo, and H1000 getting higher velocities with even less pressure vs the faster powders needed to get equal velocities from shorter barrels.

Mc Fraser said:

Thank you.
Is there a rule of thumb to determine by how much? In the absence of a chronograph.

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In this case size between 35-50fps per inch.

Weatherby Fan said:

As Bravo 4 pointed out every rifle is different, my brother and I have identical Mark V SS 300 Wbys 26" barrels and with factory ammo out of the same box which is advertised at 3240 FPS his rifle shoots it at 3280 FPS and mine won't shoot it over 3200 FPS,
So there's 80 FPS difference in the same 26" length barrel,
In theory you 28" barrel with a magnum cartridge should benefit but I notice in my 28" 6.5 CM it's the same speed as my 22" CM ?

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Smaller case. Both the 6.5cm and .260 were specificaly developed with short barreled tactical rifles in mind optimizing at around 20-22". There's going to be very little gain in equal barrels going longer.

Does anyone know how much twist rate affects velocity? For example, going from a 1-10 to a 1-9 on a .308 barrel?

LambornLR said:

Does anyone know how much twist rate affects velocity? For example, going from a 1-10 to a 1-9 on a .308 barrel?

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The faster the twist all else being equal the slower the round is going to pass down the tube.

I'm sure there's a formula somewhere that has been worked out by someone but I couldn't point you to it.

What I can say for a certainty is that the fastest barrels ever produced are all smoothbores.

We have had smoothbore cannon for over sixty years capable of over 6,000fps. None of our rifled barrels comes close.

As far as the velocity change from 1:9 to 1:10 I seriously doubt it could be more than 10-20fps.

Bryan Litz of Applied Ballistics did a study with .308 Winchester and started with a 1:11 and ended with a 1:9. All same loaded round and reamer along with length. The velocity difference between the 1:9 and the 1:11 was like 9 FPS. So I would say no the barrel twist doesn't effect speed enough to worry about.

Rhovee said:

Bryan Litz of Applied Ballistics did a study with .308 Winchester and started with a 1:11 and ended with a 1:9. All same loaded round and reamer along with length. The velocity difference between the 1:9 and the 1:11 was like 9 FPS. So I would say no the barrel twist doesn't effect speed enough to worry about.

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It will become much more pronounced with faster twists but in the ranges we're working within more than 20fps doesn't seem reasonable.

The tighter the twist the more friction which is why gain twist barrels are so hard on bullets. With them you're literally cutting new grooves the full length of the barrel but you can get insane velocities that can't be achieved with a standard twist due to the bullet getting such a run up before it tightens.

The Berger loading manual has the velocity loss/gain for each round in their manual. Typically, the loss or gain ranges from low 20's fps to around 30's fps per inch difference in barrel length.

good info

In general as people have said yes length will increase but there are too many variables to get very accurate without a chronograph (twist, bullet bearing area, rifling, temperature etc). Trying to hit too far out will be difficult without that piece of equipment. You could probably guess within 100-150 fps but that would be huge at distance. The magneto speed sporter or decent trap style are fairly affordable.

As mentioned earlier, the biggest gains can be had with the newer powders.

troupe said:

As mentioned earlier, the biggest gains can be had with the newer powders.

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And when you combine both the newer, slower powders with longer barrels you get a much higher increase in velocity without running into pressure problems.