Ballistics on a 10.5" barrel???

I was having a discussion with a friend regarding the use of short barreled ARs for entry teams. My point was the ability to easily maneuver a shorter barrel in tight spaces for CQB applications, and the 5.56mm is more versatile than the 9mm sub guns. My buddy feels that having a shorter barrel is a disadvantage due to it's inferior ballistics, and was told by a gunsite instructor that short barreled ARs don't reliably penetrate body armor. I don't know what the ballistics are out of a 10.5" barrel, 1:7 twist, with a 55gr 5.56mm, but I don't think it would make a huge difference in CQB engagements. What do you guys think? Is there any hard data on the subject?

Depends on the body armor. A 10.5 should penetrate any soft panals.

For entry work, the distances you'd be shooting people at are probably fine for a 10.5. I prefer a 11.5 for the additional velocity and dwell time.

At the distances that you describe 55gr, even a hollow point, will reliably penetrate soft armor panels and kevlar helmets. Unfortunately this has been best illustrated in blue on blue incidents operationaly and during training accidents.

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Originally Posted by tucsonan

At the distances that you describe 55gr, even a hollow point, will reliably penetrate soft armor panels and kevlar helmets. Unfortunately this has been best illustrated in blue on blue incidents operationaly and during training accidents.

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Any reports that I can get my hands on? Any public articles on the web regarding these negligent incidences? I'm trying to collect some hard data that will support my argument, not for my ego, but for educational reasons.

I would think that truly detailed instances would be difficult to document, you typically run into things like when Trooper Brinkerhoff was killed here in NY:

http://www.odmp.org/officer/18854-tr...id-brinkerhoff

http://www.nytimes.com/2007/04/28/ny...28trooper.html

http://www.officer.com/web/online/Top-News-Stories/Arizona-SWAT-Member-Hit-by-Friendly-Fire/1$47998


This is the first time I've tried to post a link. But if that doesn't work just google (arizona swat member hit by friendly fire.)

I have attached a bullet velocity chart. This gives a good reference for velocities out of various barrel lengths. The key to the 5.56mm round doing any damage is its ability to fragment. Generally speaking it will not fragment at velocities below 2,700 fps when it hits something. So looking at this chart, you will lose velocity the farther it gets away from the barrel. In a CQB situation, generally you should not ever see anything more than 25 yards. You will also be better off with a 11.5" barrel , because the 10.5" will be borderline. 55 grain ammo would be better with a shorter barrel because it is slightly faster than 62 grain and will fragment better.

As for penetration of body armor, any of these figures would penetrate level IIIA at these ranges.

http://img194.imageshack.us/img194/5...ocitychart.jpg

Here's some data from a range trip with my LMT 10.5" SBR

Win. 5.56 Q3131 55gr.
90 rounds fired:
Hi Vel: 2959
Low Vel: 2749
Average Vel: 2840

Black Hills 75 Gr. Match HP
25 rounds fired:
Hi Vel: 2405
Low Vel: 2322
Avg Vel: 2362

PMC 55 Gr. FMJBT
20 rounds fired (one box)
Hi Vel: 2554
Low Vel: 2426
Avg Vel: 2514

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Originally Posted by blhar15

http://img194.imageshack.us/img194/5...ocitychart.jpg

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What is the source of these velocities? I'd like to use this as a reference for a project, but I want to confirm the information imparted.

It is from a Bushmaster comparison test. Here is a link for more details on it.

http://counterstrikefox.freeservers.com/mv.htm