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Barrel vs velocity?

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I have .308 with a 26” barrel. I get a verified 2900 fps MV with 178 Hornady Amax bullets and 44 grains of R15. It has killed a pile critters at 300-500 yards. 
 

How much MV can I expect to loss if I have it cut down to 22”?  

I expect I will need to use a faster burning powder With the shorter barrel. Maybe drop down to the 168 grain. 
 

I have several other rifles for deer/elk but they are kind of heavy and am looking to have a lighter weight option for rougher/steeper country. 

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Usually  plus or minus 30-40fps per barrel inch. 

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Several yrs ago I played with barrel lengths and what effect a shorter barrel had on velocity’s. Long story short. I started with a 26” barrel on a 7 stw and cut one inch off at a time. From 27 inches down to 23 inches I lost 60 FPS....Then a funny thing happened....From 22 inches all the way down to 18 inches Velocities returned back to what I was getting with a 26 inch barrel. No powder or bullet changes and all testing was done under the same conditions. I have confirmed this several times over the yrs with different calibers....

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My guess would be closer to 50 fps per inch. How much weight do you want to lose? There are other options. Fluting the barrel, hogging out the stock, lighter bases and rings, aluminum bolt shroud

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4 minutes ago, azelkhunter2 said:

Several yrs ago I played with barrel lengths and what effect a shorter barrel had on velocity’s. Long story short. I started with a 26” barrel on a 7 stw and cut one inch off at a time. From 27 inches down to 23 inches I lost 60 FPS....Then a funny thing happened....From 22 inches all the way down to 18 inches Velocities returned back to what I was getting with a 26 inch barrel. No powder or bullet changes and all testing was done under the same conditions. I have confirmed this several times over the yrs with different calibers....

why does it do that 

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I wish I knew why....But I haven’t the slightest clue.....I’m sure lance could provide some insight....But I know several benchrest shooters that have had the same experience...

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Wow that's pretty cool, I thought it would loose a lot more, what about accuracy? 

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For whatever it's worth, some long forgotten gun writer claimed that with a .308, velocitys didn't change given different barrel lengths. At that time I had a Remington 700V with a 24 inch barrel and a Remington 788 with I think it was a 20 inch barrel. With the same load they did chronograph the same velocity.

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I think it is very hard to make correlations of different barrel length velocities in different rifles, or even when switching barrels on the same rifle.  Different barrels will shoot different velocities, even at the same length. 

A tight bore will get you more velocity than a loose bore.  

Groove numbers, design, twist rate, shape, cut will net different velocities.

Factory ammo can vary a lot by each shot, each box, each lot.

Powder selection can make a big difference in velocities vs. barrel length.  Faster burning powders are more efficient in shorter barrels.  So they see smaller gains in longer barrels or losses in shorter.  

Cartridge case capacities can make a difference in barrel length vs velocities.  If you are trying to burn 100 grains of Retumbo in a 20" barrel, expect a huge fireball at the muzzle, because I would bet a lot of that powder won't be burned by the time the bullet leaves the barrel.  But a .221 Fireball in a 30" barrel with 20gr of Benchmark powder has probably all burned up by 16" of barrel length.  Nothing left to keep pushing velocities for the next 14".

Bullet weights can have an effect on velocities in different barrel lengths.

So many variables go into the equation, it is hard to have the same results every time.  Only your rifle, barrel, cartridge, and load will tell you what YOUR rifle will do.

To me, there are a few reasons to want to shorten a barrel from 26" to 20".  Lighter carry weight, stiffer barrel, more handy in the thick woods.

You can probably change your loads, even with the same bullet, and lessen your drop in velocity when cutting off 6" of barrel length.  Powder, primer, seating depth.  

As for physics, I have no clue how cutting off 6" of barrel length, with all other factors staying unchanged, will not effect velocity.  But it does happen.  I would place my money on powder burn rate.

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11 hours ago, azelkhunter2 said:

Several yrs ago I played with barrel lengths and what effect a shorter barrel had on velocity’s. Long story short. I started with a 26” barrel on a 7 stw and cut one inch off at a time. From 27 inches down to 23 inches I lost 60 FPS....Then a funny thing happened....From 22 inches all the way down to 18 inches Velocities returned back to what I was getting with a 26 inch barrel. No powder or bullet changes and all testing was done under the same conditions. I have confirmed this several times over the yrs with different calibers....

thats interesting......

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13 hours ago, trphyhntr said:

why does it do that 

Chrono probably started reading the muzzle blast.

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13 hours ago, trphyhntr said:

why does it do that 

The bullet is under load longer in the longer barrel.  Also the bullet starts slowing immediately upon exit of the barrel, conversely it is speeding up and under pressure while it is in the barrel.

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29 minutes ago, rcdinaz said:

The bullet is under load longer in the longer barrel.  Also the bullet starts slowing immediately upon exit of the barrel, conversely it is speeding up and under pressure while it is in the barrel.

but why did it get faster with shorter barrel

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