Sunday, July 1, 2012

M4-Based Short Barreled Rifle Barrel Selection Considerations

I am planning to assemble my first Short Barreled Rifle (SBR) based on the M4 Carbine platform of my own, and I have been trying to do a lot of reading about how to go about. In the course of my research, I have come across various interesting aspects that I would like to share with other newbies out there.

One of the more interesting facts I found out are the factors one has to consider when choosing the barrel length for an SBR. First, just a quick review of the term "Short Barreled Rifle": It is taken from the United States' Bureau of Alcohol, Tobacco and Firearms (BATF) definition of any shoulder-fired, rifled firearm with a length of less than 16".

Also, note that the standard M16/M4 barrel comes in four general classifications. Below are a listing of them, with the maximum length for each classification noted down next to it:
- Rifle Length, 20"
- Mid-Length, 16"
- Carbine Length, 14.5"
- Pistol Length, 7.5"

Over the years, there has been variations of barrel length in between those length classifications, like 10.5", 12.5", etc., but the classifications remain more or less the same.

'Fragmentation Range'
It turns out one of the major factors to consider when choosing the barrel length is the so-called "Fragmentation Range" of the 5.56 x 45 mm ammunition. This is basically the range where the standard US military issue ammunition is expected to "fragment", and therefore has enough "stopping power" to bring down a person consistently.

This "Fragmentation Range" varies from one barrel length to the other because the shorter the barrel length, the lower the muzzle velocity, and the lower the muzzle velocity, the sooner the time, or the shorter the distance it will require before the bullet velocity goes down below 2,700 feet per second, which is the minimum bullet velocity for the standard US military issue ammunition to fragment.

Based on these, these are the listed Fragmentation Range for each barrel length:

* 20" Barrel
M193: 190-200m
M855: 140-150m

* 16" Barrel
M193: 140-150m
M855: 90-95m

* 14.5" Barrel
M193: 95-100m
M855: 45-50m

* 11.5" Barrel
M193: 40-45m
M855: 12-15m

Notice that these figures are specifically for the M193 and M855 US military issue ammunition for the 5.56 x 45 mm round. Other ammunition might have better or worst fragmentation qualities, but these are the most common rounds available around the world, and the ones I will likely get to use.

The figures seem to indicate that for every 3-4" of barrel length that you lose, you also lose around 50 meters in terms of the Fragmentation Range. The Fragmentation Range for 7.5" barrels are not even listed, because I assume it ZERO. That means a 5.56 x 45 mm round coming out of a Pistol Length barrel is likely to be not so effective in terms of stopping power.

'Excessive Loud, Concussive Report'
One apparent problem with these short Carbine length barrels are the excessively loud and concussive noise they generate. It seems the 5.56 x 45 mm rounds are optimized for longer barrels, so if you use shorter barrels, more partially burnt gas exit the muzzle. This results in a louder, more concussive "bang" when these guns are shot, to the point that most people find it intolerable anymore, despite wearing ear protection.

The shorter the barrel, the louder they get. And vice versa, the longer the barrel, the less loud they get. Reports are that at least 16" barrels are the threshold of having "tolerable" loudness and concussion. This noise issue could be a big concern if you fire the weapon without hearing protection, which could occur in some emergency situations. Even worst is if you are forced to fire them on an enclosed space, and the possibility of temporary or even permanent hearing damage could occur.

This is probably why Sound Suppressors are pretty popular with these SBRs. But these Suppressors are expensive, and add weight and bulk. Plus there's the issue of their legality here in the Philippines. At any rate, it is one more item to worry about in terms of physical maintainance, and also getting permits and licenses.

An alternate solution are these so-called "Linear Compensators" that also has been popularly used with SBRs. What they do is that they basically redirects the expanding, partially burnt gases FORWARDS, away from the back and the sides of the weapon. This means less noise and concussion will be heard by the shooter or anybody at his sides.

'Linear Compensator Factor'
The need for at least a Linear Compensator (LC) to cancel out noise on M4 SBRs complicates the barrel selection as they do add to the length of the barrel, which is counter productive as one reason for going to an SBR in the first place is to have a shorter weapon. These LCs tend to be longer than the standard A2 Flash Hider in length, ranging from around 2.2" to 3.28". In comparison, a standard A2 Flash Hider is only around 1.75".

So for example, you have 14.5" barrel with a Noveske KX3 Linear Compensator which has an Over All Length (OAL) of 3.28", that makes the total barrel length to be around 17.78". This is actually longer than a 16" barrel with an A2 Flash Hider, which would only be 17.75" in length. Of course, you could get a shorter LC, say a Levang LC which is shorter at 2.2". But then that would mean only a length savings of around 1". So in effect you save only 1" in length, and yet lose around 50 meters in terms of Fragmentation Range effectiveness.

'Parting Shot'
I never thought that choosing an SBR barrel length could be this complicated. Much of the issue seems to revolve around the fact that the 5.56 x 45 mm round just wasn't designed for use with shorter barrels, hence you see a lot of these issues on Fragmentation Range, Noise and Concussion. As a result, an M4 SBR will always be a compromise between effectiveness and convenience.

It all boils down on which compromise you are willing to live with. At this point, the ideal barrel length for me would be 11.5" as I just find the 14.5" barrel a bit too long and ungainly to move around with. That barrel length would also give me some leeway in terms of its stopping power with its 50 meter Fragmentation Range.

However, I am still not ruling out the option of 7.5" barrels, because when I asked around for the availability of barrels at the local gunstores, these have the best availability, and the cheaper price at under P10k. Apparently, a lot of other people also like the 11.5" and up barrels as they are the first ones to go, despite them being expensive by at least USD 100.

7.5" SBRs are quite cute, very aesthetically pleasing, and very handy. They may not have a lot of stopping power, but these can still kill, I doubt if anybody would want to stand in front of it more just because of its zero Fragmentation Range. At any rate, I hope to make a decision in the coming weeks or months. There's a gunshow coming up this month, I hope to get a good deal out there somewhere.

SOURCES:

- AR15 Barrels, http://ar15buyersguide.com/AR15-Barrel/

- The AR-15 Ammo Oracle - Q. At what range will M193 fragment? How about M855?, http://ammo.ar15.com/ammo/project/term_fragrange.html

- Short-barreled rifle, http://en.wikipedia.org/wiki/Short_barreled_rifle

- KIES Blast Master Linear Compensator-- 1/2" x 28TPI, http://www.joeboboutfitters.com/ProductDetails.asp?

- Noveske Krink KX3 Flash Suppressor with Pig, http://www.bravocompanyusa.com/Noveske-Krink-KX3-Flash-Suppressor-with-Pig-p/kx3%20pig.htmProductCode=KIES-Blst%20Mstr22

- AR-15/M16/308AR CLAYMORE MUZZLE BRAKE, http://www.brownells.com/.aspx/pid=44924/Product/AR-15-M16-308AR-CLAYMORE-MUZZLE-BRAKE

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