# Bullet speed vs. barrel length



## Gebirgsjäger (May 3, 2010)

I have a question merely out of curiosity. A lot of ammuniton manufacturers are printing physical performance data on their ammuniton boxes (incl. velocity, energy, and trajectory) and in most cases the data are based on 4 inch test barrels. How do this performance data change with shorter or longer barrels. Let's do an example:

Ammunition is a 9mm FMJ 115grains. Velocity (Muzzle) is 1135 Fps and Velocity (50 Yards) is 1027 Fps. Energy (Muzzle) Joule 446 and Energy (50 Yards) Joule 358 .

How would those data change with 
1. 3.58 inch barrel with polygonal rifling
2. 4.25 inch barrel with polygonal rifling
3. 4.49 inch barrel with polygonal rifling

And by the way: How much is the difference between a conventional rifled bore and a polygonal rifled bore of the same lengths when it comes to speed (roundabout)? Is it a slighty different or significant? I read that polygonal riflings have higher bullet velocity.

I asked a couple of people but never got good answers. Of course they said as longer the barrel as faster the bullet and as shorter the barrel as slower the bullet. But how much slower will the bullet out of the above mentioned 3.58 inch barrel travel compared to the 4 inch barrel ?

Maybe I should have not slept through all my physic courses in High School. LOL

Thanks for your answers.


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## TOF (Sep 7, 2006)

There is more to it than barrel length. Gun Powder comes in various forms and compositions. I have had loads using fast burn powder yield greater velocity from short barrels than long and the reverse.

My friends 9MM Glock (poly barrel) and my 9MM M&P (conventional barrel) yield within 10 FPS or so of each other at 1100 FPS range. The glock may be faster today and slower tomorrow.

If you try to fit things into simple rigid catagories you will be wrong 50% of the time.


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## Gebirgsjäger (May 3, 2010)

hmmm....like I said, I am just curious. For example I wonder how much speed difference would be there between a 9mm USP and a 9mm USP Compact with the same ammo. How much speed diffence would make the difference 0.67 inches. 

So you would say with the on kind of ammo the bullet from the shorter barrel will travel longer, in another case the bullet from the longer barrel will travel longer?


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## TOF (Sep 7, 2006)

As indicated above: (You have to think a bit) If the barrel is to long for the powder used the bullet will slow down when pressure from powder burn begins to drop before bullet exits barrel. In the extreme the bullet might not exit at all.

If the barrel is to short for the powder used the bullet will exit before complete burn and maximum pressure, therefore velocity is reached.

None of us know what powder the factories are using so can't answer your question.

Empirical testing is required to derive the correct answer to your question because it could go either way with any given load.


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## EliWolfe (Nov 20, 2010)

IMHO barrel length ballistics should be an ammunition research project. The manufacturers spend millions coming up with specific loads that work in short barrrels, long barrels, for target, hunting, SD. I believe it is a fast burn powder that is used in the shorter barrels. Using a round with a slower burn rate in the short barrel allows some of the powder to escape and ignite outside the muzzle with attendant flash and muzzle blast. I use Speer Gold Dot 38+P "Short barrel" in my 442 snubby.
If you take the time, there is a wealth of information about ballistics from the manufacturers online. Pick one that "fits" your needs and don't sweat the barrel length. Look at bullet performance (expansion, ft/lbs. etc.) if SD is the goal.
Eli :mrgreen:
PS...when I was Civil War reenacting I crewed a battery section that used a 3" Mountain Howitzer and a 3" Ordinance Rifle. Short tube/long tube. Well, the crowds seemed to love that little howitzer because of the boom and smoke rings (me too). Same charge in the big gun was neither as cool sounding or looking! Just an example, it was show-biz after all. :smt1099


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## fiasconva (Jun 14, 2009)

Generally speaking, you are going to get about 50 to 70 fps more speed per inch of barrel length.


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## gschnarr (Jan 26, 2009)

Here is a link that may give you an idea of how barrel length effects bullet velocity. It shows differences in barrels by inches with different ammo. As stated above, type of ammo, make and which way the wind is blowing in Fiji have effects on the results. :smt120 http://www.ballisticsbytheinch.com/results.html


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## Gebirgsjäger (May 3, 2010)

Thank you Gschnarr. That was exactly what I was looking for.


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