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Old November 23, 2013, 08:20 AM   #1
rugerdawg
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ar-15 pistol velocities

I am in the process of building an ar15 pistol and was wondering what sort of velocities to expect from the 7.5" - 10.5" barrel lengths.
Truthfully I see a lot of 7.5" pistols being built and wondered what kind of accuracy or velocity one might expect at say 100 yards.

Last edited by rugerdawg; November 25, 2013 at 07:23 AM.
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Old November 23, 2013, 10:47 AM   #2
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They're pretty low. I would never own a 5.56 with a barrel shorter that 10.5 inches, and even then I would probably go with something like 11.5 or 12.5. Check this website out, these guys will tell you all you need to know and more:

www.ballisticsbytheinch.com
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Old November 23, 2013, 08:35 PM   #3
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What is the overall length of these pistols?
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Old November 23, 2013, 09:54 PM   #4
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Click here to see a gel test (with chrono data) of 55gr Silver Bear 223 ammo and my 10.5'' SBR
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Old November 25, 2013, 07:21 AM   #5
rugerdawg
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I am well aware of the diminished velocities of shorter barrels. I do all the assembling myself and have a good time building different configurations even if the end result is not optimum. In the end, I just have a load of spare parts which allow me to build the flavor of the day and change it tommorrow.
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Old November 25, 2013, 08:42 AM   #6
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Quote:
They're pretty low.
Compared to what? 9mm? .357? .45acp?
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Old November 25, 2013, 09:44 AM   #7
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Originally Posted by Skans
Compared to what? 9mm? .357? .45acp?
Compared to longer barrels; .223 is more efficient out of longer barrels and isn't as good of an SBR cartridge as, say, a 300 Blackout. The velocity really starts to drop off when you go shorter than 10.5", which is why I would never go shorter than that myself.
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Old November 25, 2013, 09:50 AM   #8
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Quote:
Originally Posted by FALshootist
What is the overall length of these pistols?
I'm not 100% sure off the top of my head and I don't have one in front of me at the moment, but I think an AR pistol with a 10.5" barrel is around 25" long. Which means if you want to attach a foreword vertical grip you'll need to go with at least a 11.5" barrel to reach the legal overall length of 26".
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Old November 25, 2013, 09:54 AM   #9
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Compared to longer barrels; .223 is more efficient out of longer barrels
Everything is more efficient out of longer barrels, even .22LR. That's a given. The real question is whether there is a smaller, cheaper cartridge that can be used in place of .223 that will deliver identical power over a specified distance, given a similarly-sized "short" barrel length. For example, perhaps 9mm+p+ provides the same foot-pounds as .223 out of a 6" barrel between 3 and 10 feet. I don't know this for a fact, but IF THIS WERE TRUE, then you could say that 9mm+p+ is absolutely a better cartridge than .223 fired from a 6" barrel where the target is expected to be within 3 to 10 feet. The reasons it would be "better" would be due to lighter weight, less cost, higher capacity, and perhaps less recoil - with nearly identical ballistics for a given range.
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Old November 25, 2013, 10:08 AM   #10
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I just noticed that you added a new part to your original post:

Quote:
Originally Posted by rugerdawg
Truthfully I see a lot of 7.5" pistols being built and wondered what kind of accuracy or velocity one might expect at say 100 yards.
In my opinion, 7.5" ARs chambered in .223 are terrible. The gas system is so short that they're difficult to tune properly, and even if they're tuned just right they're still nowhere near as flexible with ammo as an AR with a longer gas system.

Also, the velocity starts to really drop off the face of the Earth when you go from 10.5" to 7.5". Check out the tables on that website I linked above.

As for accuracy, shorter barrels don't necessarily lower accuracy. What they do is shorten sight radius when you're using iron sights, making it more difficult for the shooter to be accurate. Also, because the barrel is shorter the velocity is lower and therefore there is more time for the bullet to be affected by wind. But it's a myth that a longer barrel is inherently more accurate.

Hey, if you want a fun range toy that spits out a huge fireball when you pull the trigger, go with a 7.5" AR. But if you want good performance from your AR and good performance from your cartridge, I suggest going with 10.5" or higher.
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Old November 25, 2013, 10:12 AM   #11
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Originally Posted by Skans
Everything is more efficient out of longer barrels, even .22LR. That's a given.
Yes, but some cartridges are much less efficient out of a shorter barrel than others and lose a lot of velocity when you start shortening the barrel. The .223 is one of these cartridges. That's why I like the 300 Blackout so much; it's much less affected by a shorter barrel length.
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Old November 26, 2013, 12:45 PM   #12
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I'm not 100% sure off the top of my head and I don't have one in front of me at the moment, but I think an AR pistol with a 10.5" barrel is around 25" long. Which means if you want to attach a foreword vertical grip you'll need to go with at least a 11.5" barrel to reach the legal overall length of 26".

Theohazard, did you confuse yourself on this one?

It seems if you start with a pistol why are you concerned with an overall length unless you are making a rifle out of it? Is that what you are doing here?
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Old November 26, 2013, 01:13 PM   #13
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With a 7'' barrel you might actually be better off with something like 9mm Luger. .223 out of a 7'' barrel generates less than 550ft/lbs of energy. We're talking about barely pushing a 55 grain bullet over 2000fps, plus lots of muzzle blast, which is just wasted energy.
9mm+P can fire a 90 grain bullet over 1600fps out of a similar length barrel, and generates almost as much energy as .223 with a 7'' barrel.
And I found a load from 'Magsafe' which lists a 9mm+P 64 grain load just under 2000fps, generating more energy than .223, through a short barrel, at that point.
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Old November 26, 2013, 01:58 PM   #14
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Quote:
Originally Posted by lcpiper
Theohazard, did you confuse yourself on this one?

It seems if you start with a pistol why are you concerned with an overall length unless you are making a rifle out of it? Is that what you are doing here?
Nope, not confused at all. If you attach a forward vertical grip to a pistol that has an overall length of less than 26", you've just turned it into an unregistered AOW and you're now eligible for 10 years in federal prison and a $250,000 fine.

If your AR pistol has an overall length of greater than 26" you can add any kind of forward grip you want.
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Old November 28, 2013, 07:58 PM   #15
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If your AR pistol has an overall length of greater than 26" you can add any kind of forward grip you want.
Just to make this clearer - the way federal law defines firearms a "pistol" is by definition concealable and the BATF has ruled that any firearm over 26 inches is not concealable. Thus, a firearm which does not have a shoulder stock but is over 26 inches in length is neither a rifle nor a pistol nor an AOW - it is simply a firearm. A semi-auto version of the M-1919A4 BMG also falls into this category.


Back to the original topic : When I built my short AR I went with an 11.5 inch barrel in 6.8X43mm (also known as 6.8 SPC). This round was designed for optimal performance from short barrels so it only loses about 10-15% of its potential in this shorter barrel. That is a lot more performance than either a handgun round or the 5.56 in the same length of barrel.
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