Question for the true "gearheads" - Bullet Velocity loss with shorter barrel

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dean1818

New member
My son is beginning a science project for his freshman year in high school

The premise is checking results that are available on Youtube and other places that use wet newspaper in plastic jugs to measure bullet penetration
and expansion. We will also add Denim as well.

My son will recreate these tests, but with a 3 1/2 inch barrel vs a 4 inch barrel that is used.

I am wondering if all things being equal, what will happen to the speed of
the bullet out of a shorter barrel (I dont have a way to measure actual speed)

(I would assume that the bullet may lose a bit of speed, which would impact penetration and also expansion)

I am using my new Kahr K-9 for these tests

Has anyone checked how much the speed drops?

I am also looking for a 3 inch barrelled gun to check as well (well....my son is)
 
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Auto426

New member
All else being equal, a bullet fired from a shorter barrel will leave the barrel at a slower velocity than one fired from a longer barrel. That is because the longer barrel gives the expanding gases more time to accelerate the projectile. If your bullets have the same mass, then the faster one is going to have more energy due to it's higher velocity.

However, there are a few different variables that go into this. The biggest one is type of powder used in the load, since there are slow burning powders and fast burning powders, and each will give you different results from different length barrels. You want to make sure that you are using the same exact load for all of the tests, to make sure you control as many variables as possible.
 
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HawkeyeNRAlifer

New member
Another variable is the barrel itself. I constantly see differences is velocity in different barrels of the same length. Some barrels are just "faster" than others, maybe due to slightly different internal dimensions.
 
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HighValleyRanch

New member
velocity by the inch

Thanks for the link, IceMonkey.

I've been wondering about:
The difference between my glock 29 and the velocity gained by a glock 20,
and the difference is only 90 fps. according to the chart.

So what is the formula for calculating muzzle energy?
 
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lawnboy

New member
With only half an inch difference in barrels I doubt the difference will be noticeable without scientific gear.

You'd be better off finding platforms with more difference in barrel length. Say 4in and 6in at least.

Just my .02
 
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KyJim

New member
Guns with the same barrel length will differ on velocity for a variety of reasons, including the barrel itself, leakages of gas, and probably some others. Even the same model of gun will show differences.

You not only need to use the same load and ammo manufacturer, you really need to use the same lot number of ammo if you can. Basically, that means the same mix of gunpowder is used and this makes for more consistent velocities (helping take out one variable). However, there are still variations which is why those who check velocity with a chronograph usually use an average of several shots. True match ammo is loaded to more precise powder mixes and amounts than standard ammo and is therefore more expensive.
 
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highvel

New member
Energy is calculated from bullet weight and the velocity it's traveling.
#1 Velocity squared divided by 2.
#2 Bullets weight in grains divided by 7000 to change to pounds, divide again by 32.17 to convert weight to mass.
Multiply #2 times the half of the square of velocity found by #1 and the answer is energy. Whew!:)
 
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James K

Member In Memoriam
Too many variables for a definite answer, but a good rule of thumb is 50-75 fps per inch. So 1/2 inch barrel length would make little difference.

Penetration in something like paper or wood is also subject to too many variables, which is why standard ballistic gel is used in most testing.

Jim
 
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Catfishman

New member
Use a .22 cal pistol and a .22 cal rifle.

It has already been stated but, yes it will be very hard to tell with only a .5" difference in length.
 
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SW-14

New member
According to the ballisticsbytheinch.com data, you can expect about a 50 fps drop going from the 4 to 3.5 inch barrel, and a 75 fps drop going from 4 to 3.

I would say that the variance in the cartridges will probably be too high to measure any real difference in penetration between the barrels.
 
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