M&P 15 Barrel Shortening

I decided I'd like to reduce my barrel length to around 12" from 16". Other than having to submit a NFA Form 1 and pay the $200 tax, are there any downsides to shortening my AR barrel?
How much with accuracy suffer? Will I need to adjust my gas system as a result? I could even make a little shorten as 10.5 or 11". Any issues with doing this?

Why not just buy a dedicated SBR upper?


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Because I can have my barrel cut down for less than $100. A complete upper would cost much more. Plus most of the SBR uppers I have seen are all 10.5" or 14.5". Is there something I'm missing here?

10.5 inches was the original barrel length of the Colt Commando/XM-177. There was a humongous flash suppressor that took it to longer overall length. 14.5 inch is/was the length (w/o flash suppressor) established for the M4.

Call those lengths traditional. The gas port diameters have been worked out for reliable functioning. Dropping barrels shorter may result in poor functional reliability. I expect that a gas port for 14-16 inches would be close for 12 inches. You'd probably be better advised to find someone who makes a barrel that length and check with them.

Any effect on accuracy would largely be due to quality of work, depending upon weight of bullet. Velocity loss may be an issue. Some folks who do quite a bit of entry work have noted that unless you're using a sound suppressor (and want/need to keep a similar overall length), going below 14-16 inches costs you velocity you need for bullet performance. The sound suppressor doesn't get you that terminal performance back, but the advantages of the reduced sound signature/flash may outweigh the velocity loss.

Downsides? NOISE and flash. Plus the previously noted loss of some degree of terminal performance.

Jamers99 said:

Because I can have my barrel cut down for less than $100. A complete upper would cost much more. Plus most of the SBR uppers I have seen are all 10.5" or 14.5". Is there something I'm missing here?

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Less than $100 to cut, crown, ReThread and open the gas port? What about a new handguard, and gas block?

A complete 10.5" upper with BCG can be had for $275, less if you find a sale. I'd personally rather go that route. You could sell your rifle upper to cover a big chunk of the cost after or keep it and have options. JMHO.


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Is there a particular reason you want to do this? I mean, you're gonna spend a bunch of money to make a modification that can't be reversed (without buying a new barrel). Just to make it 4" shorter?

I just don't see the value in that. So, help me understand.

Well that's why I'm asking you guys. I'm not sure how much my S&W M&P15 Sport upper is worth? It's brand new. I just thought it would be cheaper to cut it down to 12". The gas block area would remain intact. The complete upper I'd like to have is $799. BCM 11.5" Carbine (Enhanced Lightweight **FLUTED**) Upper Receiver Group w/ BCM KMR 10" Handguard (NFA)
It just seems much cheaper to chop the current barrel. I believe my current barrel is 1:9 twist. I wonder if if the barrel twist will still work fine at 12"? I noticed the 11.5" Bravo upper has a 1:7 twist.

10/6/14 EDIT: The OP discloses later in the thread he has an early production 15-Sport with a 1:8 5R progressive gain twist barrel. That changes my opinion from below. I've left this post alone for those who have a barrel with a constant twist rate.


Dude, if you want to cut down your barrel and have it re-crowned and threaded go for it. Your S&W upper is worth what you think it's worth. If chopping it makes it worth more to you, go for it.

Here's my opinion...

Keep your M&P 15 intact. If it were me, I would go one of two ways. The caveat is before building the upper or ordering the parts is to pay for the tax stamp and apply to SBR your current serialized lower.

1. Buy a pistol length barrel, low pro gas block, gas tube, and inexpensive but decent free float handguard. Swap out the parts on the current upper receiver, transfer over your BCG & charging handle.

2. Slowly acquire all the parts you'll need to build a 11.5" upper. Get the parts when they're on sale or promo. Look for cosmetically blemished uppers (PSA has some). Shop several websites for the barrel. Look for sales on complete BCG's. Look for deals on the small parts you'll need.

Then you can get a gunsmith to assemble it for you. If you have any friends who have access to the tools you'll need, you can build it yourself. It's not hard.

I'm just realizing my M&P Sport has 1:8 twist with 5R which is a progressive twist. If I reduce the barrel length to 12" then I will probably screw up design of the 5R. I guess I should look at a different option.

Jamers99 said:

I'm just realizing my M&P Sport has 1:8 twist with 5R which is a progressive twist. If I reduce the barrel length to 12" then I will probably screw up design of the 5R. I guess I should look at a different option.

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All things considered, save that barrel and upper, and get the dedicated upper with a shorter barrel.

Jamers99 said:

I'm just realizing my M&P Sport has 1:8 twist with 5R which is a progressive twist. If I reduce the barrel length to 12" then I will probably screw up design of the 5R. I guess I should look at a different option.

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Now that I know you have an earlier production Sport equipped with a Thompson Center 1:8 5R progressive gain twist Melonite treated barrel, leave it alone. It's not the 5R rifling that is the issue. It's the progressive gain twist. It starts slow at the breech and speeds up toward the muzzle. Cutting down a progressive gain twist rifled barrel will change the twist rate of the barrel.

Leave that barrel intact. It's a gem of a barrel. Rebarrel your upper or buy/build a new upper.

Wow , just wow.

It seems the OP has decided to not cut is bbl, but here goes on the subject. Cutting , crowning , and rethreading is indeed simple. What gets complicated is the gas port, the sizing gets critical for proper , or at least quasi reliable functioning. Even with expected to be correct sizing , it is common to use some combination of different buffer weights , extrapower springs , or orthopedic muzzle devices for proper cycling.

The minimum aproach would be to purchase a bbl assembly to fit to your upper reciever. But as mentioned , having a dedicated upper has a lot going for it, if only to let you be able to switch your gun back into Title 1 configuration for those occasions you wouldn't care to deal with Form 20 , or any state regulations. While you can spend as much as you care to spend on big name designer complete uppers , I was able to get a complete 10.5 upper on sale from PSA for about half of what I spent on my Genuine vintage Colt Commando upper.

But before you even start any of the paperwork ( which needs to be done before you start work , or even accumulate the parts) you need to decide what bbl length you desire. I'll presume you already thought out the pro/ cons of short bbl AR's , and decided in favor, so I'll skip THAT debate.

But the next eternal debate is what is the *best* balance of handiness vs function. [ Unless you have a particular military configuration in mind you already wish to emulate.]

The simplest is 14.5/14.7in . A "normal" 16in bbl carbine gas system could be chopped , and would probably improve the cycling. But you are spending a lot of time and hassle to get minimum shortening. You might as well go with one of the M4 bbls in origional length with a perminently attached flash hider and avoid the NFA requirements.

Shorter than 10.5 , and the reliability issues go up exponentially, not to mention the muzzle blast and velocity loss. There is a strong argument in favor of 12.5 giving a substantial increase of Dwell Time over 11.5 in exchange for 1in longer. * Biggfoot's Recomendation * is for you to study on AR gas system theory , and flip your personal coin between 11.5 and 12.5 . On a dedicated upper.

Addendum

Of course 3 min after posting , a lightning bolt of inspiration hit me , so here's. :

*Biggfoot's OTHER Recomendation*

This way let's you test drive before you commit. Purchase a complete upper with 11.5in bbl with "perminent" 5.5in flash hider. Shoot the heck out of it , and see how you like it , and how it works for you. Then you have three options :

1. File your Form 4 , and have your friendly local 007/SOT remove said FH , clean up threads , etc

2. Decide you're having fun with it as is , you impress your friends and neighbors with the cool looks and dramatic muzzle blast , and Chicks (sufficiently) Dig It.

3. Discover it was an intresting experement , but the SBR experience didn't float your boat in quite the manner you expected , and you have scratched your itch all you need to. You can now sell your used complete upper for only moderately less than you paid , and move on to planning your upcoming SPR build.

ADCO will cut, rethread, reprofile, and open the gas port for $80.

There is significant velocity loss, but no accuracy loss. Proper ammo selection can offset some loss of terminal performance. There are a few expanding projectiles that work just as well going 1000+ fps slower.


Top is an 8" barrel. Bottom 20."

Yes, they can be louder and will have more muzzle blast. A linear break or "flaming pig" style flash hider helps immensely.

Selling a complete upper is never profitable, especially in a common style or barrel length. It'll have more worth as a pistol/sbr upper.

Anything 10.5 to 12" is fine. If going 14.5/14.7" you may as well perm install a FH or break without the tax stamp hassle.

Everybody that has one knows, the shorter you go, the funner they are.

What part of my post about the progressive gain twist don't people understand.

The OP has an early production 15-Sport. At the time, they were equipped with the 1:8 5R progressive gain twist barrel. The twist starts slow then increases towards the muzzle. Shorten the barrel, you'll end up with an unknown twist rate.

CUTTING DOWN A PROGRESSIVE GAIN TWIST BARREL IS NOT AN OPTION.

IMO, while the current production 15-Sports are a good value the early production 15-Sports were a better value because of the Thompson Center 1:8 5R progressive gain twist Melonite treated barrel.

JaPes said:

Now that I know you have an earlier production Sport equipped with a Thompson Center 1:8 5R progressive gain twist Melonite treated barrel, leave it alone. It's not the 5R rifling that is the issue. It's the progressive gain twist. It starts slow at the breech and speeds up toward the muzzle. Cutting down a progressive gain twist rifled barrel will change the twist rate of the barrel.

Leave that barrel intact. It's a gem of a barrel. Rebarrel your upper or buy/build a new upper.

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It might change the twist rate, it might not. Who knows? I would not be surprised if it's 1-8 at 12in. Maybe closer to 1-9? Probably shoot better.

Op- some guys think there is magic in those 5R barrels. You could probably sell it, buy a 10.5in and not be out much money.

Reportedly about 11" down the twist increases.

Sheesh, OP.

Just sell the old barrel and get one in your desired length. The way people covet those 1/8 5R barrels you should come out fine.

Rick_A said:

Reportedly about 11" down the twist increases.

Sheesh, OP.

Just sell the old barrel and get one in your desired length. The way people covet those 1/8 5R barrels you should come out fine.

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Ya know.... if the barrel is unfired, it might be worth it to keep it that way. In 40 years (No joke) it would have some collectability value beyond being just another rifle barrel.

Thanks all for the advice. I decided to put the complete upper for sale in the Accessories/Misc - For Sale forum.

Jamers99 said:

I decided I'd like to reduce my barrel length to around 12" from 16". Other than having to submit a NFA Form 1 and pay the $200 tax, are there any downsides to shortening my AR barrel?
How much with accuracy suffer? Will I need to adjust my gas system as a result? I could even make a little shorten as 10.5 or 11". Any issues with doing this?

Click to expand...

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Just my opinion, the less interaction with the BATFE, the better.