AR Question.

[Cheech]

I'm stupid about AR's. Can some one explain the relationship between barrel lengh and gas tube length/diameter and gas block location. Can a long (20-24") barrel be used with a short carbine length tube so the gas block can be located closer to the action leaving more barrel exposed? Is the gas tube to vent/relieve pressure for the action to cycle or is it fed from a hole in the barrel causing pressure to cycle the action? How does it work? Adjustable gas blocks?

Don't be too hard on me fellas. It's all new to me. I've been a "bolt" guy for a long time.

Cheers

 

[mateofeo85]

Re: AR Question.

AR barrels are primarily available in carbine, midlength, and rifle (shortest to longest), as far as gas system length is concerned. the barrel length itself can vary on all of these systems from 7"-16" on a carbine length gas system up to a 20"-24" rifle length gas system. look here: http://www.ar15barrels.com/profiles.shtml for different barrel profiles and how they measure up.

as far as how a direct impingement gas system works, use google, wiki, any search for direct impingement. way better explanation than what i care to provide here.

 

[BasraBoy]

Re: AR Question.

Cheech, there are others here who know infinitely more about these rifles/mechanisms than me but....

1) The barrel has a hole drilled in the top (the gas port). This is aligned with a gas block to which is pinned the gas tube. This enters the upper receiver and is covered (with the bolt closed) by the gas key on the bolt. When the rifle is fired the gas venting through the gas block/tube forces the bolt back into the recoli buffer/spring resetting the trigger and then in turn pushing the bolt forward thereby cycling the action.

There are some good online animations available that show how the system works - here's a couple:

http://www.youtube.com/watch?v=WSqYvWib1og
http://www.youtube.com/watch?v=eemyE2JQ5W4&NR=1
http://www.youtube.com/watch?v=RKGlthExwRQ&feature=related
http://www.youtube.com/watch?v=O0HjEkZ6Vr4&feature=related
http://www.youtube.com/watch?v=cpkbORiOXbo&feature=related

2) In theory I suppose you could run any length gas system (carbine, mid-length, extended mid-length, rifle) on a suitably sized barrel. But why?

I am sure there are others who can maybe give you figures, but as I understand the shorter the gas system, the worse the recoil.

Accepted wisdom seems to point towards a mid or rifle length system being kinder and better suited to precision rigs.

3) You can fit any sized free-float front end you choose and there are gas blocks that fit under handguards if, for example, you want to run a carbine length system on a rifle length foreend.

My question (again) would be, why would you want to have a short foreend on a long barrel? Is it for cosmetic preferences or are you worried about cooling, weight or accuracy?

If you're running a precision rifle, I personaly prefer to have the bipod as far out front as is practically possible and would run a mid or rifle length free-float on an 18" or longer barrel. There are some very light foreends available (like the Daniel Defense products)that (IMHO) means their weight is pretty negligible in the overall equation).

On foreends, I'm now favouring monolithic uppers (Like Mega Arms MTS or Vltor VIS) over separate upper/foreend combos.

Maybe a bit of a more detailed explanation of what you are trying to achieve with the rifle and the type of shooting you are planning may help get a bit more information you may find helpful?

 

[Cheech]

Re: AR Question.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BasraBoy</div><div class="ubbcode-body">Cheech, there are others here who know infinitely more about these rifles/mechanisms than me but....

1) The barrel has a hole drilled in the top (the gas port). This is aligned with a gas block to which is pinned the gas tube. This enters the upper receiver and is covered (with the bolt closed) by the gas key on the bolt. When the rifle is fired the gas venting through the gas block/tube forces the bolt back into the recoil buffer/spring resetting the trigger and then in turn pushing the bolt forward thereby cycling the action.

There are some good online animations available that show how the system works - here's a couple:

http://www.youtube.com/watch?v=WSqYvWib1og
http://www.youtube.com/watch?v=eemyE2JQ5W4&NR=1
http://www.youtube.com/watch?v=RKGlthExwRQ&feature=related
http://www.youtube.com/watch?v=O0HjEkZ6Vr4&feature=related
http://www.youtube.com/watch?v=cpkbORiOXbo&feature=related

2) In theory I suppose you could run any length gas system (carbine, mid-length, extended mid-length, rifle) on a suitably sized barrel. But why?

I am sure there are others who can maybe give you figures, but as I understand the shorter the gas system, the worse the recoil.

Accepted wisdom seems to point towards a mid or rifle length system being kinder and better suited to precision rigs.

3) You can fit any sized free-float front end you choose and there are gas blocks that fit under handguards if, for example, you want to run a carbine length system on a rifle length foreend.

My question (again) would be, why would you want to have a short foreend on a long barrel?

If you're running a precision rifle, I'd personnaly prefer to have the bipod as far out front as is practically possible.

<span style="font-weight: bold">Maybe a bit of a more detailed explanation of what you are trying to do adn the type of shooting you are planning may help get a bit more information you may find helpful? </span></div></div>


first, Thanks guys for the replies.

I've been looking at, and trying to learn about AR's and really like the Rock River Arms selections. As for my reason in asking, first is to learn second is I was thinking about a longer,(20-24") SS bull barrel but don't care for the over sized hand guard.

With a shorter tube and the gas block located closer to the action a shorter hand guard, say from the 16-18" version could be used?? Recoil shouldn't be much of a concern, I'm used to shooting a 7mm mag and 300mag. My friends AR has a real light recoil with an 18" barrel. I just want a basic AR but with a long bull barrel and short or mid length hand guard. The loooong hand guards just look odd to me hence the reason for my question.

Thanks for the help and the links. They explain a lot and help an AR moron understand a little more. I love learning new things.

Cheers, George Daly

 

[BasraBoy]

Re: AR Question.

George, I know what you mean about long railed hand guards - check out the Mega Arms tubular MTS:

http://www.megamachineshop.com/armscatalog/MTS-MODULAR/loader.html

Why have miles of rails you'll never need/use?

Alternatives could be the PRI or JP Enterprises free float tubes?

The build I have with Roedale right now will have an 18" barrel and a Mega Arms mid-length MTS. But lately I am wondering if I should have gone for a longer (rifle length) fore-end so as to get a better sight radius on the iron sights and a better balance on the bipod.

I spotted this pic in the gallery - looks the top rifle looks like a mid-length foreend with a 24" (or so) barrel to me....

http://www.snipershide.com/forum/ubbthreads.php?ubb=showgallery&Number=2412079

Cheers, Andy

 

[USACS]

Re: AR Question.

The problem with a carbine-length gas system is with recoil and reliability. With the recoil portion, it's not the amount of recoil, but the way the rifle handles the recoil and transmits it to you, the shooter. Carbine gas systems create a recoil pulse that is much more abrupt and faster than the longer gas systems, which has a tendency to push you off target easier during sustained fire. Mid and Rifle length gas systems have a slower and less abrupt recoil pulse, which allows you to stay on target easier. I am an obsessive gym rat whose second home is the weight room. Controlling recoil isn't an issue for me, but I can very easily feel the difference between my pistol length and carbine length gas systems, and that of my SPR which uses a rifle length system.

The second issue involved is reliability. The shorter the gas system, the higher the gas pressure will be going back into the BCG. This increased amount of bolt speed throws off the timing on the rifle, and can cause malfunctions. There are ways to compensate for this, like heavier springs or buffers and going to a different BCG like LMT's Enhanced BCG if it becomes necessary. Mid-length gas systems typically mitigate this issue enough so that it's not a real prevalent issue, and rifle systems work exceptionally well. If you don't want a the full length, look into a mid-length. There are several options out there with mid-length gas systems.

 

[mateofeo85]

Re: AR Question.

i would recommend a free float tube. model 1, dpms, RRA etc... have tubes for under $100 that will provide comfort and savings over a rail system you may never use. they are available in different lengths. in the future you could always attach a 2-6" rail section should you need one.

 

[mjh30]

Re: AR Question.

USACS, I think you sum it up really well, Cheech if you are going to be using a 20- 24 inch barrel, I really think you need to looking at a rifle lenght gas system on the rifle for the purpose of reliablity, you might use a pigtail gas tube as an alternative or an ajustable gas block, but I think the rifle lenght gas system on a longer barrel is the way to go.Theres a reason why the 18 inch SPR barrel has a rifle lenght gas system. Basraboy, I used a rifle lenght rail on my mid-lenght 18 inch 264lbc upper for the very reasons you mentioned, extended sight picture and bipod stability and I am very pleased with the results.

 

[SilentVectorX]

Re: AR Question.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Cheech</div><div class="ubbcode-body">
first, Thanks guys for the replies.

I've been looking at, and trying to learn about AR's and really like the Rock River Arms selections. As for my reason in asking, first is to learn second is I was thinking about a longer,(20-24") SS bull barrel but don't care for the over sized hand guard.

With a shorter tube and the gas block located closer to the action a shorter hand guard, say from the 16-18" version could be used?? Recoil shouldn't be much of a concern, I'm used to shooting a 7mm mag and 300mag. My friends AR has a real light recoil with an 18" barrel. I just want a basic AR but with a long bull barrel and short or mid length hand guard. The loooong hand guards just look odd to me hence the reason for my question.

Thanks for the help and the links. They explain a lot and help an AR moron understand a little more. I love learning new things.

Cheers, George Daly</div></div>

I think that a carbine gas system on a rifle barrel would also require the port size to be tweaked. I mean, not only do you have the higher presure initially because the port is closer to the chamber, but you also have a much longer gas port duration since the barrel is so long.

I doubt larger manufacturers will go through the trouble of making a custom gas port for you (specific loads would probably make a significant difference in pressure curves also).

Can anyone offer more input on this?

 

[Loopm5]

Re: AR Question.

I have found that Brownells sells a Bolt carrier weight system. If you want a longer rifle but a shorter gas system and be reliable.Then I would recommend a M-16 bolt carrier and a weight system. It will bring the pulse down to a reliable level. There are some suppressors being made that cover about 7 or 8 inches of the barrel behind the flash hider so a carbine length gas system is necessary to reduce overall length. There are gas port dimension differences,but I have modified those on a drill press not a lot of accuracy needed to increase the size. Now if you want to go from a rifle length to a carbine length or even a pistol length {300 whisper}then that is a hole other proses.

Just another note the ATF has said that you can have a M-16 bolt but NOT have ANY other M16 parts in the gun. All NFA rules apply. Colt started using M-16 bolts in there civilian guns and so have most other manufactures due to the operation of the gun is more reliable and it is less costly to manufacture one bolt style than 2 due to the increased military actions in the last few years.

I hope this helps a bit in your purchase.