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View Full Version : Midlength vs. carbine in an ar-15...barrel life


Servo77
January 6, 2010, 12:26 PM
Will a midlength gas system actually have more barrel life than a carbine? I overheard a conversation to this effect in the local gun shop with the arguement going towards a midlength gas system having a longer barrel life. I am sure it would prolong the parts life of the bolt, but not sure on the rest.

What say you?

Technosavant
January 6, 2010, 12:49 PM
Not sure how it would affect the barrel; I'd honestly guess any difference would not be terribly noticeable.

The longer midlength gas system can mean easier extraction and cycling, but I've never heard anything about barrel life.

Bartholomew Roberts
January 6, 2010, 12:53 PM
News to me. The primary factor in barrel life is throat wear/heat at the throat. I do not see how a midlength gas system is going to reduce either of those.

Sounds to me like someone got their concepts confused.

Bird is the Word
January 6, 2010, 01:30 PM
I believe the argument that is made for Mid-length is that it reduces the stress on the bolt carrier group and the parts that come in contact with the BCG, potetially barrel as well. Additionally, it is proven to reduce felt recoil for better follow-up shots (perhaps the benefit most would agree on vs wear)

I went mid length because it was the same price as standard m4 and it was all upside.

DMK
January 6, 2010, 03:08 PM
Chrome lining should extend the barrel life. Gas system won't make a difference on barrel life either way.

A midlength or rifle gas system may extent the bolt's life.

musher
January 6, 2010, 04:02 PM
Perhaps they were thinking of higher wear on the barrel extension with the shorter gas system?

Yellowfin
January 6, 2010, 04:08 PM
So from what I'm reading here, mid length is especially good on 6.8 SPC where we're dealing a more powerful cartridge, correct?

DMK
January 6, 2010, 04:17 PM
So from what I'm reading here, mid length is especially good on 6.8 SPC where we're dealing a more powerful cartridge, correct? I don't think caliber matters. It's just as good for 6.8 as it is for .223/5.56

It's a matter of timing and pressure. As the bullet moves down the bore, the pressure behind it decreases because the gas is cooling and there is more volume to fill. The closer the gas port is to the breach, the higher the pressure at that point. Timing wise, the pressure in the cartridge will still be pressing it against the chamber walls. A gas port further down the barrel also allows the cartridge to "relax" more and be easier to extract. So you have less pressure pushing on the bolt/carrier and less resistance to extracting the cartridge.

To further complicate matters, the longer the amount of barrel after the gas port, the longer the pressure is acting against the bolt/carrier (dwell time). Once the bullet passes leaves the barrel, pressure in the gas tube quickly drops to zero.

Jimro
January 6, 2010, 05:33 PM
Barrel life is not affected by length.

Where the gas port is placed on the barrel has a lot to do with reliability, but issues such as gas cutting near the port doesn't come into play unless you are firing so much ammo so quickly that you are consistently running a very hot barrel, and doing it routinely. And by then, your barrel would have worn the chrome lining off in patches.

But you can still "shoot" with a "shot out" barrel as long as you find the accuracy acceptable. For uses at close range, the difference between a 2 MOA rifle and 6 MOA rifle are not so apparent.

Jimro

Bartholomew Roberts
January 6, 2010, 05:56 PM
Perhaps they were thinking of higher wear on the barrel extension with the shorter gas system?

Maybe; but are people wearing out their barrel extensions before the throat goes? I'm guessing not.

Willie Lowman
January 6, 2010, 06:09 PM
I overheard a conversation to this effect in the local gun shop

It has been my experience that gun shops are great places to get guns and terrible places to get information about guns.

But you can still "shoot" with a "shot out" barrel as long as you find the accuracy acceptable. For uses at close range, the difference between a 2 MOA rifle and 6 MOA rifle are not so apparent. This is true. I sold a Colt HBar upper that I had put over 10,000 rounds of mostly Wolf though to a friend to use on his 7.62 lightweight sporter. He is a casual shooter and 75 yards is a long shot for him. He loves that upper, he brags about it's accuracy! Like Jimro said, at that distance, it's hard to tell the difference.

Quentin2
January 6, 2010, 09:14 PM
I've heard that a midlength has less gas port erosion than carbine length. Probably not a huge issue as far as barrel life though.

Technosavant
January 6, 2010, 10:00 PM
I've never even heard of gas port erosion being an issue.

By the time you reach that point the throat would likely have already seen significant wear.

demigod
June 14, 2010, 03:12 PM
I'm looking at 3 Quickload charts right now. Port pressure on my 3 sample loads is reduced by around 17% by going from a carbine gas system to a middy.

That's a pretty good chunk of pressure (about 3000 psi) off of your action.

In my opinion, you would likely see less headspace deterioration (or barrel extension wear) over the life of the barrel, and thus, get more life out of the barrel.

Throat erosion doesn't always mean the death of a barrel. I have a colt commando barrel that has visual throat erosion. It still headspaces ok, and shoots very accurately.

P-990
June 14, 2010, 08:15 PM
I'm looking at 3 Quickload charts right now. Port pressure on my 3 sample loads is reduced by around 17% by going from a carbine gas system to a middy.

That's a pretty good chunk of pressure (about 3000 psi) off of your action.

In my opinion, you would likely see less headspace deterioration (or barrel extension wear) over the life of the barrel, and thus, get more life out of the barrel.

Throat erosion doesn't always mean the death of a barrel. I have a colt commando barrel that has visual throat erosion. It still headspaces ok, and shoots very accurately.

Throat erosion is THE end of a barrel. Well, that and possible muzzle wear/damage.

Highpower shooters tomato stake lots of .223 barrels that will headspace perfectly, but have horrible throat wear. I have a Wilson SS barrel on my AR Service Rifle right now that I'm already starting to anticipate reaching the end of it's useful life. I have a few thousandths more worth of throat wear (measured by how long I have to seat an 80gr bullet to reach the rifling) before I won't be able to chase it any more. My last barrel started throwing random fliers at 600 yards first, then at 300 yards, so I knew it was time to chuck it.

Of course, at 100 yards and in, it will be less noticeable, and in an application where the required accuracy isn't as precise, the barrel will last longer because it will still be shooting well acceptably well.

demigod
June 15, 2010, 08:41 AM
I don't disagree with any of your reply. But in the context of mid length vs. carbine gas system.... most users are running that type of gun inside of 300 yards.

ISC
June 15, 2010, 09:14 AM
I like midlength ARs because they have a longer sight radius, making them more accurate with open sights.

Also, they are better proportioned than an M4 with an extra 1 1/2 inches tacked on to it. The bayonet lug becomes functional too.

trigger45
June 15, 2010, 09:32 PM
what about the dissipator? i like mine.

ISC
June 15, 2010, 11:28 PM
The dissapater gives you even more sight radius, so is even more accurate with open sights. That's a good thing.

They are also potentially less reliable because the gas port is so close to the muzzle. That's a bad thing. They also can't attach a bayonet. That's a bad thing.

trigger45
June 16, 2010, 05:23 AM
They are also potentially less reliable because the gas port is so close to the muzzle. That's a bad thing. They also can't attach a bayonet. That's a bad thing.

hows that?

ISC
June 16, 2010, 07:36 AM
I was thinking that the shorter distance to the gas port from the muzzle allows the gas pressure to decrease faster than it would if the gas port was further away from the muzzle, and that this is probably compensated for in the design of the disapater by increasing the size of the gas port. After a little research I learned that the dissapater actually has its gas block located in the same position as it would be on an M4, underneath the handguard, and that they have a front sight base located forward of the gas block. This solution eliminated the reliability problems that the initial dissapator prototypes were prone to.

As for the bayonet lug, it's location is too far from the muzzle brake to allow a standard bayonet to be used.

demigod
June 16, 2010, 09:06 AM
The dissipator is the worst of both worlds. You have a full 16" barrel and are stuck with the high port pressure of the carbine gas system.

Back when the dissy first came out, it had a niche, and there wasn't much thought of the midlength in the mainstream. But now the better option is the mid length dissipator... or the ADCO custom cruiser.

You get the exact same benefits of the old dissy without the downsides of the carbine gas system.

THORN74
June 16, 2010, 09:24 AM
lenght of barrel is most directly going to effect 2 things.

Accuracy
Velocity

lenght will have no effect of barrel life

demigod
June 16, 2010, 12:10 PM
lenght will have no effect of barrel life

It might have an impact on barrel extension and bolt lug wear... which could prolong the useful life of a barrel. I don't think anyone who knows the AR platform would deny that the carbine gas system increases wear and abuse on the bolt group compared to the longer gas systems.

It'd also depend on other factors including firing schedule. A barrel shot at high concentrated volume is likely to get bad throat erosion sooner than the headspace is out of spec.

In the grand scheme.... the extra life is probably neglibible.

trigger45
June 16, 2010, 04:29 PM
mine has the rifle gas system and 16" heavy barrel. no dummy gas block front sight.

its a DPMS tac 16. so if it aint a dissy, looks and acts like one.

no reliability problems what so ever, except low on ammo. with a 6x 1 chewed up hole.

Hound74
June 16, 2010, 11:03 PM
MID-LENGTH!!!

demigod
June 17, 2010, 08:49 AM
MID-LENGTH!!!

I just had a BCM 14.5" Middy show up on my door last night! :D

It's a T&E unit from BravoCompanyUSA loaded up with cool stuff I wouldn't usually get. I'll get some pics and reviews up here as soon as I can.

I plan to start out on a rifle stock and buffer, but I also want to run it with the carbine stock too. I plan to try it head to head with a regular 14.5 in both semi and full auto.