View Full Version : 7" barrel AR-15

March 12, 2010, 12:42 PM
I was trying to find out how effective an AR-15 with short of a barrel is in 5.56mm. How would it compare in effectiveness to a sub machine gun (pistol caliber firing machine guns)? What is the best kind of ammo to use out of something with such a short barrel? How much of an improvement in ballistic performance would the weapon be if it were chambered in 6.8mm?

In particular I was thinking of a weapon like the DPMS Panther PDW.

March 12, 2010, 02:11 PM
SWAT magazine did an article in November 2009 (http://www.swatmagazine.com/archive_2009/nov09.php) on the Troy Industries 7.5" SBR kit. It's an interesting read if you care to purchase the PDF.

The rifle was punching holes in a Level III Safariland vest at 50 yards. :eek:

When you're looking at putting rifle rounds in a pistol, you need to understand that you're loosing a bunch of velocity. The 55gr bullet out of an ar pistol will be getting some 2200 to 2500fps depending on the load. When you compare it to what's offered in the H&K MP7 or the FN p90, you'll find that the AR pistol has better ballistics than both of them. However, the fireball out the front end is impressive. There is a lot of un-burnt powder making it to the muzzle.

The short direct impingement system also soots up quite impressively. The short gas tube provides impressive pressure to the receiver, and a LOT of carbon in the receiver. The recoil is more violent with the shorter gas tube as well. A lot of guys run heavy buffers in them. (I run a Spikes ST-T2 buffer which is tungsten filled.)

I haven't had an opportunity to shoot a 6.8 SBR, yet. But I plan on getting an upper if I can find a deal on one.

This is the AR Pistol I play with. Built it for about $650. Of course the Eotech almost doubles that build price. ;)


PS. When you say "Sub Machine gun" you need to specify what gun you're talking about. Not all "sub guns" are the same. A lot of them run pistol calibers. I'll take a rifle caliber pistol over a pistol caliber rifle, personally.

March 12, 2010, 03:48 PM
PS. When you say "Sub Machine gun" you need to specify what gun you're talking about. Not all "sub guns" are the same. A lot of them run pistol calibers. I'll take a rifle caliber pistol over a pistol caliber rifle, personally.

Ok I fixed it. Yeah, I was wondering though how effective something like the .223 or even the 6.8mm would be vs these pistol calibers because they're being fired at a significantly lower velocity. My understanding is the reason a rifle cartridge is significantly more effective than a pistol cartridge is because the velocity it strikes the target, if the velocity is lowered enough because for example it's fired out of a very short barrel wouldn't it make more sense to use a heavier pistol cartridge?

March 12, 2010, 06:11 PM
There is a point of diminishing returns. The point at which the energy of a rifle cartridge is lost by the length of the barrel enough to match the energy of a 9mm or 40 cal. The pistol calibers, some argue, will gain effectiveness by putting them in a longer barrel. However, actual velocities show that the .223 out of a 7.5" barrel STILL retain 2300-2450 feet per second.

The question is what distances will you be using your SBR/AR Pistol? What do you want it to do? What kind of ammo are you going to be using?

The 7.5" AR barrel retains a lot of the usefulness of the AR platform out to around 100 yards. The velocity at 100 yards will STILL give enough energy to expand my bullet of choice, the 60gr Nosler Partition.

Here is a thread you would may find interesting over at ar15.com

LINK HERE (http://www.ar15.com/forums/topic.html?b=3&f=122&t=486625).

This is a discussion I had with a friend about my intentions with my AR pistol. He asked if I was planning on reloading to "regain" the velocities lost by going with a short barrel. I have never really been a fan of the short barreled ARs, before I built one. My brother talked me into doing it.

I am working up some loads specifically for this gun.

I don't think I need to try to "regain" any of the velocity lost by this round. To do so would be dangerous as you would end up with pressures too high for safe function.

This is the perspective my brother argued:

Look at the other "Personal Defense Weapons" on the market. Specifically the P90 from FN and the MP7 from H&K.

The MP7 uses the 4.6x30mm cartridge. It pushes a 31gr bullet at 2,250fps.
The P90 uses the 5.7x28mm cartridge. It pushes a 31gr bullet 2,350fps.

The velocities of a 55gr bullet in a 7.5" barrel is about 2,200-2,500fps depending on the tests you read.

So, which one is going to be better? Similar velocities, but heavier bullets. SWAT magazine did an article on the Tory Industries Short Barrel Rifle Kit in their November Edition. They found a 75gr round would penetrate a Class III vest at 50 yards. That gun uses the same 7.5" barrel as this pistol. For all practical purposes, the a 7.5" barrel should be able to keep 60gr Nosler partition at proper expansion velocities out to 90-110 yards.

The goal with developing a load is to retain the 2,200-2,500fps velocities, but decrease the muzzle flash. A lot of the powder in a .223 ends up as flame ball in a short barrel. So my goal is to find a load that will 1) Keep pressure acceptable 2) Cycle the gun 3) Maximize the velocity of the short barrel and 4) Reduce Muzzle Flash.

Once I find the right load, I plan on contracting a custom ammo manufacturer to load a couple thousand rounds that I can use for "Personal Defense." The thought being that I wouldn't want to manufacture self defense ammunition. It has been argued by Massad Ayoob founder of the Tactial Defnse Institute that if you ever had to use ballistic testing to prove a self defense shooting, your handloads would NOT be admissible in court. He served as an expert witness in a case in which the powder pattern didn't match the ballistics test because the handloads were MUCH lighter than FACTORY loads. The state did tests on the FACTORY loads, and concluded that the distance of the fatal shot would have been MUCH LONGER than what was reported. The court did not allow the handloads to be tested because the defendant "manufactured the evidence."

I'm going to be working with some different powders and recipes that will help give us what we're after.

March 12, 2010, 07:25 PM
Some interesting stuff, thanks.

March 12, 2010, 08:06 PM
The court did not allow the handloads to be tested because the defendant "manufactured the evidence."

Very interesting indeed, I never use reloads for SD, now I have another reason!