I will be getting a MP 15 Sport later this fall . I have never had any AR before in fact have never held one. Just unlucky I guess! So my question to yall is what is Involved in changing the barrel on a new Sport to 20" .It's to much money to buy a new model with the 20" and most of the one's I've seen have the heavy contour which I do not want. I have looked for a pencil contorted thin barrel but have seen very few offered for sale.Since I am not familiar with the platform I want to make sure I can do it before I spend the cash.So can you drop a thin contor 20" barrel in a new Sport? What will I need to get? Do you have to change anything else? I have loss of use l.arm so weight is a factor. ..But if it's not to much (8oz?)..Does the factory barrel have much value? What would you expect to spend to have this done? If it's practical to do the swap I might not ever shoot it with the 16".Is the gas system part of the barrel assembly? I really thank yall for sharing your knowledge of the AR platform ! My goal is to have a new Sport with a 20"barrel 1 in 8 twist for less than $ 800.00 but I might be dreaming

May I ask what you're hoping to achieve by going to a 20" barrel? Are you simply looking to drop some weight, improve accuracy, increase the speed of the round?

You can switch out barrels, but I'm just curios as to what the end goal is that info may help you get better replies.

Welcome to the forum!

All you need is the barrel, gas tube, and appropriate front handguard retainer.

A barrel nut wrench, torque wrench, and the appropriate lube is needed. A clamp on FSB/gas block will be needed or a reputable shop to swap it out.

A government profile should be much easier to find and isn't much heavier.

Unless you shoot iron sights only and out to several hundred yards there's not much reason to go 20...but it'd sure look cool.

All my life I have liked to shoot guns and rounds that had just a little extra power and range. .My first handgun was a 357 mag 6" Ruger security six my second was a Dan Wesson 357 Max with a vent heavy shroud. But in this case I just feel the cartridge was designed to work best at the velocity only a 20" barrel can reliably deliver. Particularly over 100 yards.The main purpose of the Sport for me will be self and home defense for anything beyond 40 yards.My 930 Mossberg I feel is the very best tool for under that.Second my wife and granddaughters do not feel comfortable with the 12 gauge and I'm hoping the Sport could and would be used by them in case of a emergency home invasion type of incident. As far as the down side of the 20" barrel goes I don't think there is many for me. Professional soldiers/operator's in urban areas might need a shorter barrel but out in the boonies I would rather have the most punch/energy I can get. From what I've read a 20" with over 62 grain bullets gains the most from the extra length. Not really needed for targets or even for pests but for the bigger predators. ..

I have both 16" and 20" ARs. I have done a bunch of shooting with both. Out to 200 yards, they both exhibit the same accuracy, ~1MOA. So, I suggest not changing the barrel.

However, if you're dead set on a 20" barrel, here's what you'll need:

Tools-

• Punch 1/16" to drive out gas tube retaining pin
• Punch 1/8" to remove front sight taper pins
• Barrel nut wrench to remove/replace barrel nut and delta ring
• Upper receiver action clamp (without this you will ruin your upper receiver)
• Torque wrench that covers 30-70ft/lbs
• Bench vise

Parts-

• 20" barrel (.750" diameter if you want to re-use the front sight/gas block)
• Rifle length forearm, the one you have now is carbine length
• Rifle length gas tube

Replacing the barrel is a good time to make some other upgrades. If it were me, I'd go with a free float forearm and a low profile gas block.

Whatever you do, take your time. Shoot the Sport as is for a while. This will help you decide if you even want to change out the barrel. The factory barrel can be sold, but putting a couple hundred rounds through it first won't hurt the value much.

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Honestly...it seems to me it'd be a lot cheaper and easier to buy the gun you want, right from the start. You'll need specialty tools and parts to change out barrels, and barrels aren't always easy to come by.

I could see if you already had the gun and it didn't work for you...sure. But buying brand new and then turning right around and swapping barrels doesn't make a whole lot of sense.

Also using a clamp on (set screw) gas block, there is always the possibility it will work loose even if locktighted. A alternative I saw on You Tube was to cut the tower off a standard A2 sight and re use it as it is pinned in place. The person in video shows how to do a very neat job of it. Worth looking up if you have the time.

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Rastoff, are any changes needed to the buffer and buffer spring? Is a heavier buffer needed to counter the increased pressure from the longer barrel?

It's a good question. I don't honestly know. Both my guns have the same spring and buffer and both seem to work fine.

While the 20" barrel will push the bullet faster, I'm not sure the chamber pressure is that much higher.

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Thanks for your impute yall! The low price of the Sport (under$600.00) is much better than the lowest price I found for a factory built 20"AR ...That was way over $1,000.00..The downside was also none under 7.5 lbs. ..Most listed the contour as a bull or heavy barrel and that was to much. I started looking at just barrel weight and prices and it seemed 16" vs 20" barrels cost about the same for the same quality . I didn't think about the forearm cost or the gas block.What would be the value of the factory barrel? Would that cover the cost of a gun smith's labor? Can anyone tell me what the lowest weight factory built 20" barreled AR is? I'm starting to think I will have to build the gun I want or rather have built. .Plain, simple stock, sights because of my budget. .But the 20" barrel is important to me. Can yall suggest the most economical path/company to end up with the basic AR I'm going for? Simple low cost stock GI thin profile barrel with a eye to low weight? I was really hoping to stay under $800.00.

The gas pressure from the longer barrel is actually LOWER than the pressure from a 16 inch barrel-especially given the gas port location on the M4 type platform. They compensate for this by using a larger gas port hole.

So, no spring/buffer changes.

i agree with the shoot it as is first group. The M16/16A1 did originally have a barrel with a thinner profile than the later M16A2 (commercially reffered to as HBAR) 0.625" vs 0750" at the gas block. My understanding of the term "pencil barrel" refers to a carbine length barrel with a contour much less than the M4 barrel under the handguard and 0.625 " at the gas block. Does not have M203 barrel notch.

A quality barrel is going to run at least $250, the gas tube about $10, handguards about $30. I'd expect the factory barrel would cover gunsmith fees with some left over. But....that assumes the new barrel will have correct headspace with the current bolt. No, they aren't completely interchangable.

Frankly, reading your intended use, there is no real advantage to the 20" barrel over 16". My 16" barrel clocks about 200 fps less than a comparable 20" barrel with the same ammo. That's not a measureable difference in the real world at any distance you'll be able to convice a prosecutor/jury you were in immediate and unavoidable danger of death or SBI. The 16 inch 1-9 twist barrel will handle up to 75 gr. bullets (Hornaday HPBT-not A-Max). The 64 gr Winchester Power Point was designed as a deer load.

I'd suggest John Farnams (DTI) book on the shotgun/carbine for self defense as required reading before purchase.

I also like the 20". I "came of age" watching reports from Vietnam on the news and to me an AR is the classic A1 configuration. However, I'd shoot the Sport as is for a while first. It's a remarkably handy and accurate carbine. All other things being equal a heavier profile barrel is usually a more accurate barrel. Plus, I'd probably install a target trigger before I'd change a barrel. Drop in triggers are a little over $100.

However if you do decide to change to a 20" pencil barrel, the M16A1 configuration from the 1960s/70s used a pencil barrel. Weight of an A1 is about 6.5-7.0 lbs so a Sport with its heavy profile 16" is about the same weight as an A1 with a 20" pencil barrel. Currently Green Mountain and McKay Enterprises make pencil barrels. The McKay is 1:7 twist and probably better with the bullet weights available today. The GM barrels are 1:12 like the original M16 barrels.

So you could switch out the stock 16 for a 20 and have about the same weight, which I understand is a concern for you. With the current surplus of ARs and AR parts on the market I'd suggest putting together another upper for a 20" pencil instead of changing barrels. Gunsmith labor to mount and headspace the barrel will be the same. You should be able to build a new 20" barreled upper using a basic flat top upper receiver for about $450-$500 including a bolt.

An Hbar is full profile under the handguards.

For more power and range a 6.8 is the way to go.

The difference between a 16" and 20" is not dramatic. The concept behind 5.56 rifles was effectiveness 300 yards and closer and maintaining enough accuracy to make hits to 500 yards.

This is some good reading regarding the technical merits:
Barrel Length Studies in 5.56mm NATO Weapons

I had a 20" and made a 16" out of it similar to Bushmaster's "Dissipator"

I think to just get into the platform the Sport is the best value in a complete rifle. .It would be hard to build one for less.LE price is less than $550.00 delivered, so for a basic AR it's really a nice package. What I might do is get the Sport shoot it for awhile then buy a complete upper built like I think would best serve me. That's a 20" lite profile, 1 in 7 twist,chrome plated barrel .

Natchez has a flier out with AR barrels for $99.00. M4 style, 16 inch, but I would think a little internet searching would find you a 20 inch light barrel. Check out a copy of Shotgun News, as they have a bunch of aftermarket AR sellers / manufacturers that have what you are looking for.

I would bet a gunsmith would make the swap in trade for your new barrel - Its not that difficult if you have the right tools.

Rock River Armory, I believe, makes a model like you are interested in, as I am sure others do. RRA makes a good AR. I have one, and is a good gun.

Larry

I think this is a good plan. I also think that once you have a bunch of rounds downrange, you'll rethink the 20" barrel.

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People I respect in my research who were in real battles and firefights some who told me personally said the 20" barrel hit harder and stopped better...I was at the VA in Gainsville Florida in 1994 for 7 weeks with a anuransum in my shoulder and hung out with a tunnel rat from Nam and about 5 to 10 other combat vets in the 4th floor smoking lounge playing poker and bingo for chits good at the hospital store. I'm Navy and it was the first time I have been accepted by real ground combat vets. I kept my mouth shut and listened..The 20" AR was called the 'meat chopper'.by them. .But the favorite was the 12ga pump with the deer/bear brass 00 buckshot. The tunnel rat used a 22cal. Pistol .Listening to the real experts on killing/fighting talk shop..If they liked the 20" barrel thats enough reason for me. I felt honored to just sit with them..

Depending on the ammo, there is little difference between 16 and 20 inch. With ball ammo, The 20 inch gives a bit more range with fragmentation of the bullet. You are not limited to ball so a good softpoint/hollowpoint will work just fine.

You can probably find a custom builder that will make an upper to your specs. Then sell your stock upper.

If you want more power, I would upgrade to a different caliber.

Bushmaster was making an AR-15 with a 20" skinny barrel rifle. I don't know if they still do or not. It had an A1 upper & an A2 lower, with a full size fixed stock. FWIW. I have a SPORTER & love it. It just fits right & shoots great! I believe you should get what you want to begin with. Cheaper in the long run. Good Luck!

Thank you everyone for all the good advice. Since I started my research I've found that many AR owners really favor the shorter barrel lengths. It does seem the 20" is in the minority both with manufacturer's and for home builders. As far as ballistics go there is a advantages with a longer barrel if the load was made to take advantage of it . I can see where a heavier bullet using the right powder could benefit from the extra length.

An aside, and don't know squat about ARs; I recently read an article about short barrel ARs being less reliable due to added wear and tear from the modifications needed to run the action of a SBR. Gas ports needing to be larger to make full use of the shorter barrel's less faster release of gas pressure, and resulting "harder"/sharper working of the action produced excessive wear and occasional broken bolt lugs. 16" stock barrels vs. 12"-14" barrels.

Would the extra 4" of barrel and the resulting addition of "bullet barrel time" ease the pressures on the action? Would the 20" barrel action be more reliable over time?

Another point to consider..The AR was designed for a 20" and quite a few sources I found have also said the rifle length gas system is the best most reliable length.The first and many after government loads were optimized for that length . I've read too that most of controversy over the ineffectiveness in battle of the 5.56 round has been in AR configurations of less than 20".I would guess most military missions are now completely different than the Vietnam era and a shorter barreled AR is considered better. I think there is still a place for a full length rifle in civilian use and to me there is more up side than down. .The more I look into it the more I think the 20" will be what I get. I just wish Smith and Wesson would offer the low cost Sport with a light weight profile 20" barrel. Sorta like Mossberg did with there ' hunter' version AR.

The full length gas system is the original design and the gas pressures in the BCG are lower than in shorter gas systems since the pressures at the gas port will be higher. I'd forget whatever you read about relative gas port sizes. I will point out that you don't need a 20 inch barrel to have a full length gas system. You can find 16 and 18 inch barrels with full length gas systems if you shop around. Mid-length gas systems are markedly lower in BCG pressures than carbine length (which, per one authority, are 50% higher than in the rifle length system).

Cracked lugs and cracks at the cam pin hole are common to bolts in heavily used ARs of all gas system lengths. It's the design, which was never intended to have the service life of a Garand, M-14, G3 or FN FAL. Bolts are wear items in the AR design.

SBRs are strange creatures. They're finicky to get reliable and tend to have a short life. They're simply too far from the original design. Kinda like 1200 hp drag racing engines that must be torn down after each run.

Manufacturers make what people will buy. Right now the low price/entry level AR market is owned by ARs configured like the Sport. The advantage to the Sport is that S&W took great care with the design and engineering so that the Sport is a high quality firearm.

It doesn't appear that many people looking for lower priced ARs are interested in a 20". That may change, but until then it looks like your option for a 20" requires saving some additional cash to fund a 20". But, by waiting for exactly what you want it will be all that much sweeter when you get it.

FWIW, personally I like the Canadian C7 version of the 20". A1 sights with a shell deflector.