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Discussion Starter · #1 ·
Let's say I wanted to do the paperwork and build an AR based Short Barrel Rifle. Has anyone seen anything that states the minimum barrel length required for a complete powder burn, and thus a minimized flash potential?
In other words, the powder burns completely before the projectile leaves the barrel muzzle.
 
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Discussion Starter · #2 ·
This is an interesting subject and one I'm surprised that no one else seems to want to touch with a 10 foot pole, but to better be able to help, there are some questions below that would need to be answered first before anyone could recommend one. I'll get to that in a minute.

When building and choosing a short barrel for a rifle, powder burn is not even on the list. That's what flash suppressors (or even sound suppressors) are for. Of course, I'm taking for granted you're allowed to own either where you live.

For full powder burn on a standard 5.56x45 round, won't occur until you get to about a 22 inch barrel and totally eliminates short barrels altogether. And hence why even the 20 inch models will utilize a flash suppressor.

With that said, bullet stabilization according to bullet weight and bullet length and barrel twist and even somewhat of the barrel length is generally what is looked at when choosing.

Basically, what needs to be asked is how well will the barrel stabilize the bullet going through it.

Some questions that would need to be answered are:

What weight ammo do you want to use? (this will answer which twist rate would suit your purposes best and also may help answer which chambering to recommend).

Do you plan on ever using a suppressor or other muzzle attachment? (this will answer if you would need a threaded or non-threaded barrel).

Will you be using the extra heavy subsonic ammo? (again would help determine which chambering and barrel twist rate to recommend).

There are 3 basic chamberings for an AR: .223 (match), Wylde (hybrid) and 5.56 (Nato--this is the most versatile, but often not the most accurate)

What kind of accuracy do you want to achieve at XX distance? Most SBR's are limited to 150 yards or less for a lot of reasons. Depending on your answers, I may get into some of that or may not.

But to make it easy, if you get a 5.56 chamber with a 1:7 twist rate, and threaded barrel (if you're allowed) this is a great starting point and will open up the most potential in the future, but the heavier ammo is usually more expensive to shoot, so there is a trade off there.

The barrels will also be harder to find because the most common twist rate nowadays is a 1 to 9 twist, and that will allow you to shoot the more common (and inexpensive) 55 gr stuff well.

As for length to choose, there has been a lot of debate about what's best for the most reliability. Some say 10.5, some say 11.5, some even say 16 inches.

There are several things you can do to make any SBR as reliable as any other AR15 if you do run into problems with a particular load that should otherwise work just fine for that barrel. Some that come to mind are using H buffers, Chrome Silicone/flat coil buffer springs, adjustable gas tubes, etc.

Though many self proclaimed guru's will recommend enlarging the gas port hole, I however do not for a couple of reasons. 1. all this does is make the back pressure higher and in my opinion is not necessary to cycle the action. 2. will make the gun run dirtier and more susceptable to fouling 3. will increase the risk of bolt bounce or extractor failure.

OK, that's about as far as I want to get into this at the moment. If you can answer the questions above, that would help find a starting point for what would serve you the best.
 
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Discussion Starter · #5 ·
My 7.5" upper is on order. I figure if you're paying $200 to go short, you might as well go all the way. You won't find a lot of people who recommend going that short because there are a lot of figures and facts out there about fragmentation of the 55 gr FMJ round and velocity thresholds. I don't pay much attention because I'm not limited to 55 gr FMJ. I'm a big fan of 60-gr poly-tipped bullets myself, but we'll have to wait and see how they shoot in my gun.

So- yeah, as others have said, for full powder burn you'd be looking at something you could buy out of any gun store, no NFA stamp required.
 
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Discussion Starter · #6 ·
My 7.5" upper is on order. I figure if you're paying $200 to go short, you might as well go all the way. You won't find a lot of people who recommend going that short because there are a lot of figures and facts out there about fragmentation of the 55 gr FMJ round and velocity thresholds. I don't pay much attention because I'm not limited to 55 gr FMJ. I'm a big fan of 60-gr poly-tipped bullets myself, but we'll have to wait and see how they shoot in my gun.

So- yeah, as others have said, for full powder burn you'd be looking at something you could buy out of any gun store, no NFA stamp required.
Heavier bullets might not even fragment :p
 
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Discussion Starter · #8 ·
Fragmenting isn't everything, but where rifle rounds are concerned, is a nice benefit.

But depending on the barrel length and bullet choice, it would be incorrect to think a short barrel would be incapable of fragmenting a bullet under no terms. That's just incorrect.

Though, that's why a lot of folks prefer the heavy .223/5.56 rounds so they'll begin to tumble within a couple of inches below impact and cause pretty impressive damage and wound channel, even though the bullet may not fragment as much due to the lower velocity of the round. In a short barrel, as far as rounds are concerned, heavier is better in my opinion.

Myself, I like to use 75 gr out of my 8-1/2 inch AR upper.

I have one other thought about Short barrels that may meet your criteria that hadn't crossed my mind when I first read your post and replied and I apologize for not thinking about it sooner.

Pistol calibers
let's see, there is 9x19, .45 acp, .22LR (all great for using a suppressor). You don't typically have to worry about fragmentation with them either as they're pretty much designed to mushroom instead of self destructing, but if you're using it for defense, I don't think the 22LR would be up to the task.


Many alternate pistol calibers other than 5.56 will provide enough space for complete (or near complete) powder burn and do a fantastic job using the shorter barrels the rounds are designed for.

Pistol caliber SBR's are very effective and worth considering in my humble opinion. Though many folks will bash Olympic Arms products, they've got the pistol caliber uppers down to an art with no lower reciever modifications, though a magblock or modified magazines would be in order when using the pistol upper. When not using a pistol caliber upper, you can use a standard 5.56/223 upper with no modifications. :cool:

And considering for a moment a super heavy 77 gr round .223 is about half the weight of the 147 gr 9mm, the 9 will provide better terminal ballistics in comparison out of a short (10 inch barrel) all things considered.
 
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Discussion Starter · #9 ·
As for muzzle velocity, this is the best article I've seen on the matter. In the fastest load on that page you'll lose 557fps going from a 22" barrel to a 10" barrel (3336fps - 2779fps), but 2779fps is still plenty to fragment a decent 5.56 bullet.

I went with an 11.5" barrel on my SBR (I was originally thinking 10.5"). Just because I liked the look of it, and I got a good deal. ;)
 
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Discussion Starter · #10 ·
Heavier bullets might not even fragment :p
I'm not expecting fragmentation, I'm thinking more the loadings with the 60 gr poly-tipped Hornady V-max. Think Hornady TAP. Black hills has a similar loading as do a few others. It's not going to defeat body armor but does an amazing job in soft targets. The poly-tipped stuff is designed to hold together at high velocity but mushrooms well even at lower velocities. As long as groups are tight with my gun, that's probably what I'll stockpile.
 
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Discussion Starter · #11 ·
Let me clarify that by saying I'm not expecting the standard turn and snap at cannelure-type fragmentation. More like nose-pushed-in, jacket comes apart type fragmentation. :D
 
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Discussion Starter · #12 ·
As for muzzle velocity, this is the best article I've seen on the matter. In the fastest load on that page you'll lose 557fps going from a 22" barrel to a 10" barrel (3336fps - 2779fps), but 2779fps is still plenty to fragment a decent 5.56 bullet.
+1, and thanks for the link. That's the first time I'd seen that. I wonder what distance the chrono was set up?
 
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Discussion Starter · #13 ·
if you are even considering going suppressed in the future, dont go any shorter than 10.5. Suppressor companies dont recommend going shorter, and they may not honor any warranty if the can is used on a shorter bbl.
 
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