AR-15 barrel change inquiry

[bi2thelly]

I currently have a an S&W M&P 15 carbine with a 16" barrel (may have barrel damage because I'm an idiot). I'm looking into changing the barrel to a 20" heavy barrel (possibly stainless but haven't decided yet). But I had some questions that I was hoping someone would be able to help me with.

• If I go to the longer 20" barrel do I also have to change the buffer spring assembly as well?
• Do they make carbine length gas systems for 20" barrel (just curious)?
• Is it better to get a new bolt carrier assembly when you get a new barrel or can I stick with my current one?

 

[lx1x]

I currently have a an S&W M&P 15 carbine with a 16" barrel (may have barrel damage because I'm an idiot). I'm looking into changing the barrel to a 20" heavy barrel (possibly stainless but haven't decided yet). But I had some questions that I was hoping someone would be able to help me with.

• If I go to the longer 20" barrel do I also have to change the buffer spring assembly as well?
• Do they make carbine length gas systems for 20" barrel (just curious)?
• Is it better to get a new bolt carrier assembly when you get a new barrel or can I stick with my current one?

1-doubt you need a new buffer spring.. they are all the same
3-bolt carrier are fine.. should work no matter what barrel you have.

2-may have to measure the gas hole on the barrel.. i think they should be the same length.. (somebody may chime in.. i have not changed barrel myself).

 

[smores]

1-doubt you need a new buffer spring.. they are all the same
3-bolt carrier are fine.. should work no matter what barrel you have.

2-may have to measure the gas hole on the barrel.. i think they should be the same length.. (somebody may chime in.. i have not changed barrel myself).

1 - Rifle and carbine action springs are different, so as long as you don't change the stock you're good to go.

2 - The 20" barrel has the gas port in a different location than the 16" barrel. There are also Pistol length and Mid-Length applications. A lot of people like the mid-length system on a 16" barrel.

3 - Bolt and bolt carrier should not matter as long as the headspace is correct.

 

[midcountyg]

If changing to a 20" barrel, I would recomend using the rifle length gas system. You should change your buffer assembly if going to the rifle length gas sysytem. Honestly, it would be quite an expense to do it all, and probably cheaper to just start over. Just rebarreling with another 16" barrel would be much more cost effective.

 

[Mdman]

why do you think you have barrel damage?

 

[E.Shell]

Barrels are easily changed out, and each barrel (20" or 16") will have the proper size gas port in the correct location.

To swap out a straight 16" carbine to a straight 20" rifle ONLY requires the "front end", which is the barrel, front sight base/gas block, handguard cap, handguards and gas tube.

The bolt carrier, delta ring assembly and upper forging are universal and do not need to be changed.

The buffer and buffer spring are butt stock specific and there are two basic types of buffer/spring system, "rifle" and "carbine". The free length of a serviceable "rifle" spring is different from the free length of a "carbine" spring

An A1, A2 or any other stock that uses a standard (A1) receiver extension will use a "rifle" buffer and buffer spring. A telestock-style receiver extension will use the carbine spring, no matter what length barrels are installed on the upper.

 

[bi2thelly]

why do you think you have barrel damage?

I believe I may have barrel damage because like I said I was being an idiot. I am relatively new to the AR-15 and was using a laser bore sighter (that kind that goes into the end of the barrel) to help sight in my rifle. I'm sure you can figure out the rest of that story.

Inside the barrel after I clean it there always seems to be a really dark mark at the muzzle end of the barrel that I can't get rid of.

 

[bi2thelly]

Thank you all for the advice. I really appreciate it. It looks like I may be able to knock a couple of hundred off of my modification.

 

[alucard0822]

The buffer may be fine, maybe not, generally the rifle length gas systems on 20" barrels are not as sensitive as the carbine setups, they usually work, but if you are getting jams, especially with lighter factory loads you might need to go with a different setup, and it might be a good excuse to get a decent stock for it like the PRS and a rifle length tube/buffer. Ed Shell explained the difference between the fixed stock, and telescoping stock pretty well, while the fixed "rifle" stock is normally used on 20" rifles, and the telescoping "carbine" stock is generally used on carbines, they can be swapped as assemblies from fixed to a carbine, or telescoping to a rifle.

The gas system is timed so the gas heads back to the carrier at a certain pressure, dependant on the gas port size, then have a duration suitable for the length of barrel ahead of the port. The rifle length system is more forgiving than the shorter setups, no reason not to go with it, and being the barrel will probably be drilled for it anyway you probably won't have a choice.

The bolt does wear over time, and that can affect headspace and fitment to a degree, especially if it a high mileage part, you are better off getting a new bolt head, and keeping the old one with the old barrel, the carrier itself can be reused. You also can get a matched bolt head/barrel where they are fitted and perhaps trued to one another, or at least headspaced together. In most cases you can swap bolt and carriers amongst rifles, but you always want to check headspace when building, and again after a few hundred rounds to make sure it seated up OK.

You will also need different hanguards, and some of the hardware. Before too long, you will look around, see enough parts and think "hey, if I get a lower, LPK, XX and XX, then I can have 2 rifles for $XXX, it's almost like saving money", then after a couple years you end up with a half dozen rifles you don't remember buying.

 

[Mdman]

I would imagine you could just recrown the barrel and be fine, even then I would shoot it and see how it works.

 

[slayr15]

How close is the dark spot to the end of your barrel? You could be looking at your gas port hole? I would have it looked at before you spent a bunch of money.

 

[longshot1911]

Or you may trim the barrel back and have an extended flash suppressor permanently installed.