AR 15 uppers and gas systems

[Soybomb]

So I'm slowly picking parts out for my first ar15 and had a couple questions. As I understand it the purpose of free floating handguards is primarily that your accuracy remains more consistent as the rifle heats up and resting the handguads on a surface when shooting doesn't cause shots to deviate. You usually get them covered in rail area because people seem to love their rails.

I've also seen some 1 piece handguard/upper assemblies coming out like the larue monolith and vltor vis. I guess the primary advantage of these is that you can put your optics anywhere across the top and don't have to worry about straddling the handguard rail and upper receiver rail if you're inclined. I assume the barrel also floats? I read a post by the larue people saying that with monolithic uppers you lose a lot of heat transfer and will wind up with a much hotter chamber.

Do I have that all pretty much right or have I missed something important? All I can really tell is that I think the monolithic uppers look pretty spiffy.

Also what barrels lengths do you put the different gas systems on? I think I'll be using a 16" barrel, I assume that means a carbine or mid length gas system? What would be the advantages and disadvantages of either choice?

 

[Victor Romen]

Correct on the free-floating hand guards and accuracy.

The barrel does float on the monolith uppers.

There is a lot of debate on the monolithic uppers and heat transfer. If you aren’t doing a lot of mag dumps it might be a moot point. Some of it has to do with the difference in metals as well (one design may be better then another).

Not really up on my carbine vs. mid-length fu, but I think it has to do with pressure curves during cycling process.

 

[Soybomb]

Heh yeah I figured on the whole it wouldn't matter either way for any of my uses really.

Anyone else on the gas system differences on a 16" barrel?

 

[Bartholomew Roberts]

The THR Rifle Library thread stickied to the top of this Forum has some lengthy and detailed discussions of the gas system issue if you want detail.

I like the midlength gas system for the 16". All of my 16" are now midlengths.

 

[Soybomb]

Thanks for the pointer, really great read. It certaily makes it sound as though the midlength or rifle gas system is better (what barrel length do you need to be able to run rifle length anyway? 18" or 20"?). So my next question would be, why do people run carbine gas systems in 16" barrels at all then?

 

[4v50 Gary]

Mios dos centavos (sorry all Spanish speakers for my gringo Espanol)

OK, I'm ignorant on the "gas" system of the AR. I know that gas is impinged and it travels down the gas tube before acting on the bolt carrier. But the real issue is the difference between the gas system on a 16" bbl and a barrel of a different length.

So, here's what I learned at one school. For a gun to function properly with a wide range of ammunition, the gas system must be able to deliver enough energy such against the bolt carrier such that the bolt is unlocked, the buffer pushed back sufficiently for the bolt's extractor to fully extract the spent cartridge and the ejector to pop it out of the port. Furthermore, it must travel back far enough for the magazine to push a fresh cartridge up before the bolt comes slamming home. If the bolt didn't travel back far enough, the bolt may not pick up the fresh cartridge and the shooter is doing a malfunction drill.

What's the difference? Absolutely none in the gas system per se. It still works alike whether you have a 16" or 20" or 24" barrel. What needs to be different is the gas port. You see, the shorter the barrel, the faster the bullet leaves the bore, right? Well, the faster the bullet leaves the bore, the faster the gas pressure drops. If the gas pressure drops too fast, there's not enough energy to do all the things described above. The solution is to increase the size of the hole such that sufficient gas is introduced into the tube before the bullet exits the bore. The shorter the barrel, the larger the hole. Now, as to what size hole, that's something the engineers have to play with and graph. The graph will show the pressure curve and the timing. That's something I'm not smart enough to figure out and besides, with what fancy scy-en-tiff-fec equipment?

So, there's my two cents worth.

Your humble scribe,

Gary

 

[mc223]

Very well said 4v50 Gary.

 

[Bartholomew Roberts]

(what barrel length do you need to be able to run rifle length anyway? 18" or 20"?).

17" is pretty much the minimum barrel length you want to run with a rifle gas system.

So my next question would be, why do people run carbine gas systems in 16" barrels at all then?

The original short M16s were the Colt Commando and CAR-15. Both of these used 10.5" barrels initially. If you place the gas port as close to the muzzle as is feasible for a 10.5" barrel, it happens to be right where the carbine gas port is located. The problem is that the gas pressure problem Gary mentioned pops up on a 10.5" barrel. The gas pressure drops so fast that there isn't time to get enough gas because the muzzle is right after the gas port. The only way to fix that is to open the gas port up even more and start trying to extract the brass before it is ready to come out (increased cyclic rates, wear, etc.). Basically the window for proper function is pretty small. So first they added a sound moderator (because they are loud as well) - this had the twin benefits of reducing sound to rifle levels and adding enough backpressure to give a slightly longer dwell time; but it still wasn't enough so they lengthed the barrel to 11.5" for better reliability.

The M4 carbine didn't come about until the late 1980s and it used all the existing carbine parts already in the system - including the handguards and gas system. Since the moderator was now classified by ATF as an NFA item, they also went with a 14.5" barrel (reliable functioning and could add the grenade launcher stepdown).

In the civilian world, 16" was the shortest barrel you could have outside of the NFA route so they went with it instead. I think the midlength is a definite advantage over the carbine system (just look at all the mods the military has introduced to the M4 as a result of the carbine gas system); but at this point in time the improvement probably doesn't justify the cost for the military. They already have all the parts (heavy buffer, bolt upgrade, M4 feed ramps) to improve the M4 in the system so not much point in adding the logistical nightmare of a whole new gas system for moderate improvements that you have already obtained through modifying 3-4 other parts.

 

[Soybomb]

Interesting, so really the people running a carbine gas system on a 16" barrel now really are just following the military's example and don't know any better it seems.

 

[Bartholomew Roberts]

Armalite's Mark Westrom has been preaching the midlength gas system for years as the solution to the numerous issues identified with the carbine; but it has been slow to catch on.

I imagine too that the carbine gas system is still so reliable that few people see those issues to begin with. If you aren't having problems, then buying something that fixes the problems you aren't having is a tough sell.

Others do know about the issue and have had problems in the past; but like the military that are now running heavy buffers, feed ramps, upgraded extractors and ejectors, heavy-duty mag springs and O-rings so no point in making the switch.