PDA

View Full Version : Long vs. short barrels: impact on ballistics


audihenry
03-11-2009, 4:31 PM
That's a pretty confusing title.

I'm interested in hearing your thoughts about, say, a Krinkov chambered in 7.62x39 and a full barrel length AK firing the same cartridge and the impact it will have on accuracy and final speed of the bullet.

Obviously, accuracy goes to the longer barrel, but does the speed advantage also go to the longer barrel? If so, by how much? (ballparks ok) How about bullet trajectory?

C.G.
03-11-2009, 4:49 PM
Accuracy has very little to do with barrel length; longer barrel will give you more velocity, thus longer range, but not necessarily more accuracy.

JeffM
03-11-2009, 4:54 PM
Quotes from another forum:

A rough "rule of thumb" is that you lose 50 fps for every inch of barrel taken off.
For a 16" barrel to a 10' barrel you'd lose roughly 300 fps.

I did a search and found some data on the 10" Yugo M92 stating that the muzzle velocity is about 645 m/s (2116 ft/s), while from what I can find, the standard size 16" AK-47 has a muzzle velocity of about 710 m/s (2329 ft/s). That's a difference of 213 ft/s, so "if" my math is correct (213/6), AK-47's loose about 35.5 ft/s per inch of barrel length, assuming a same twist rate of course.

Well, I took her out for a run this weekend along with my SAM-7 to get a rough idea of velocity loss.

Wolf black box from the 16" barrel gave a 10 round average velocity of 2350 fps. (+/-).

Same ammo from a 10" barrel gave a 10 round average velocity of 2125 fps (+/-).

Both shot through a "chrony" at 15' from the muzzle. Avg. Temperature 81 degrees.

So, it seems , for me at least I lost some velocity but I will add that both rifles grouped very well. My Arsenals' best 5 round group off the bench at 100yds was
1.8" ( naturally, it's an Arsenal). My yugo "krink" off the bench, best 5 round group was just a shade under 3.5".
Needless to say, I am quite pleased! So ,I'm sure most of you already new this, but short barreled rifles can shoot very accurately, they just do it slower.
At the "normal" combat range, the shorter M92 should be able to hit a man sized target out to 200 yds. Now mind you , when the rounds are flying back your way that accuracy is going to shrink significantly. :wink_smal

And a cool photo of what some M43 ammo does in tissue:
http://ammo.ar15.com/project/Misc_Images/DocGKR/RussianWP.jpg

bobfried
03-11-2009, 4:55 PM
Scientifically, the shorter the barrel the more accurate it will be. This is because a shorter barrel is more stiff than a longer one. It will also be more uniformed and have less defects over a longer one. Of course twist rate and bullet stabilization plays an important role as to how short a barrel can be. Optimally, a barrel that is only a micron long will be accurate down to the micron. But throw in real life and most will say that the bullet need to travel at least one revolution in the barrel to stabilize hence you won't see an AR barrel shorter than 7.5" (1x7 twist).

What the longer barrel gives you is velocity. Your question is sort of like asking how many MPG if you drove a car over 100 miles; you didn't say what car, what engine, what fuel, how will you drive it and how straight the roads are. The velocity gained or loss will heavily depend on many factors, but as a rule you will gain more velocity with a longer barrel. But how much you gain and how long you can go depends on the powder charge and a few other factors explicit to each combination.

C.G.
03-11-2009, 5:33 PM
Scientifically, the shorter the barrel the more accurate it will be. This is because a shorter barrel is more stiff than a longer one. It will also be more uniformed and have less defects over a longer one.

True, given the same profile, but longer barrels tend to be heavier in order to gain stiffness.

audihenry
03-11-2009, 5:36 PM
Thanks guys. So let's add a fake can to the equation (like on any CA-legal Krinks) and bring up the barrel to 16" (10" twisted, 6" untwisted). Is it safe to assume the extra 6 inches, despite lack of rifling, will produce greater velocity?

C.G.
03-11-2009, 5:40 PM
Thanks guys. So let's add a fake can to the equation (like on any CA-legal Krinks) and bring up the barrel to 16" (10" twisted, 6" untwisted). Is it safe to assume the extra 6 inches, despite lack of rifling, will produce greater velocity?

No, as you said, no rifling.

Fjold
03-11-2009, 7:57 PM
Thanks guys. So let's add a fake can to the equation (like on any CA-legal Krinks) and bring up the barrel to 16" (10" twisted, 6" untwisted). Is it safe to assume the extra 6 inches, despite lack of rifling, will produce greater velocity?


No, the bore of the can is larger and allows the the propellent gases to expand and leak around the sides of the bullet. Less pressure = less velocity.

audihenry
03-11-2009, 8:32 PM
No, the bore of the can is larger and allows the the propellent gases to expand and leak around the sides of the bullet. Less pressure = less velocity.

Ah, I see. Thanks! :)

tunder
03-11-2009, 10:18 PM
Here's a site that's not so much about accuracy, but velocity.

Three Guys, Two Chronographs, and 7000 Rounds of ammo.
http://www.ballisticsbytheinch.com/


Why "ballistics by the inch"? Well, just about forever people have wondered what kind of trade-off one made in choosing a gun with a short barrel - how much power were you giving up for convenience/concealability? There has been a lot of anecdotal information available - comparing this 2" .38 to that 6" .38, or a longslide .45 to an officer’s pistol with a shorter barrel - but there hasn’t been much in the way of consistent research made available to the average gun owner. In the 1980s American Rifleman did some tests using a .44 mag revolver, cutting the barrel down from 18" to 1", and back in the 1930s someone did something similar with a 30-30. But just try and find that data quickly. And further, how does that data compare to your 9mm or .32? Do they all lose power at the same rate? Are some ammos better for your purpose than others?

We were curious just exactly what the drop-off in velocity was for a given caliber over a range of barrel lengths, and using a variety of available ammunition. So, we decided to do some actual testing. And, we wanted to make this information freely available as a service to gun owners everywhere. This website is the result.

AlexBreya
03-11-2009, 10:24 PM
as others have said, a shorter barrel is more accurate, but there's a limit. after a certain point, it gets inaccurate when it is too short. but yes, longer barrel increases velocity, but even a change from 16" to 24" would only give it another 40-70 fps.