AR Gas System Characteristics

[lencac]

Hellooooooooooooo
Well, this is no doubt a question that has been posed previously. However, I would still like to kick it around a little to see what is currently the speed tip.
I know this could become a very intricate question with many, many aspects. Faster powder? Slower powder? How works on the inpingment system? Barrel harmonics? Stuff like that.
How will changing gas system length affect the overall operation and accuracy of the rifle?
In this particuler case I'm going from a carbine length (1/7) to mid-length (1/8), 16" melanite treated barrel. The gas system is now 2 inches longer than with the carbine barrel on it.
I have some preconceived notions on what I think should be happening but I'm open to discussion. My goal is to get this rifle to throw bullets as close to the muzzle velocity of a 20" barrel as possible.
Thanks guys

 

[Slamfire]

Probably nothing as the gas port hole will be larger on longer barrels.


As a general rule, for gas guns, I prefer powders that are faster than slower.

Regardless of burn rate, too much powder and too high of pressures is bad, bad, bad.

 

[lencac]

Really?

 

[TurtlePhish]

The gas tube shouldn't move much, if at all when firing. Effect on accuracy will probably be so small as to not matter. Length just changes how much gas and pressure goes back into the action and how much stress gets put into parts.

 

[chrome_austex]

The amount of barrel after the port on a 16"+carbine is about the same as the 18"+middy, which is what matters for gas system performance. In your case you probably won't see much if any difference from where you are now. Gas port sizes can factor in, so YYMV.

If you're having issues, you can change buffer weights, but like I said is should be a roughly equal swap. If you were going to a intermediate or rifle length system on an 18" barrel you would probably see less gas and might switch to a lighter buffer.

 

[justice06rr]

Going from Carbine to Midlength gas system should improve your cycling a bit.

 

[mtrmn]

If you're having pressure signs in your carbine-length, the mid length will most likely help with this because the bolt will remain locked longer on the midlength. This allows chamber pressures to drop a little more before the bolt unlocks and the extractor tries to pull the case from the chamber. I've had these problems with 5.56 ammo and carbine-length gas systems. The extractor tries to extract while the case is still "welded" to the chamber wall under pressure-this tends to tear the rim off the case or at least damage it.

 

[lencac]

Ok
so what's the deal with recoil buffers and weights

 

[mtrmn]

Heavier buffers will slow down the cycling as well, but won't DELAY the bolt unlocking. Usually a combination of heavier buffer/spring and mid length should be sufficient to correct your cycling problems.

Rifle-length gas systems (the way Stoner designed it) rarely have problems to begin with.

 

[Tirod]

Correct gas port timing doesn't have much to do with accuracy. It's about 5 -7 inches from the buzzle, and on an issue carbine with 14.5" or shorter barrel, still is. It's the aftermarket non-milspec guns with carbine gas on NFA legal 16" barrels that push things. And the invention of midlength is the industry's response - they did it to reduce bolt battering and CS claims.

Accuracy and getting the same velocity from a 16" as a 20" can be very counterproductive. Basically, any gun shooting it's optimumally accurate load is running under maximum velocity - there's a window of best performance that bullet speed will upset if you go too far.

Having the bullet go faster is just one variable in getting more power downrange. You can increase power levels other ways - increase bullet weight, or the amount of powder, or both. Exactly why alternate calibers exist, and they have higher power levels or more optimal accuracy, each depending on it's design intent. No one cartridge can do everything better, regardless of the hype of fanboys. One thing remains a constant tho, the overall restrictions on the AR15, which generally means you can have speed, or mass, but not both at the higher levels. It becomes an AR10 at that point.

Knowing what type target and distance is the first thing to determine, you then know how much power and how far downrange you need to deliver it for doing whatever it is - live target, paper, whatever. Basically, you pick what delivers enough power for the longest engagement range. THEN you know the cartridge, and THAT sets the barrel length, which THEN determines the optimum gas tube length.

Too many start backwards and end up with some nasty results even they are unhappy with, precisely because their favorite elements are simply incapable of delivering those results. But, shooters in denial riding a testosterone fueled emo trip over their latest idol aren't capable of logical thinking. They are literally "in love," and we all know love is blind.

 

[lencac]

Bravo! Nice write up tirod and mtrmn.
I would have to agree with everything you said tirod. Very linear and logical. I used to build and tune hotrods so I know that linear and logical thing goes a long way to a successful outcome.
I'm replacing the old YH 1/7 twist 16" heavy carbine-length barrel with a melanite treated 16", 1/8 twist, mid-length, M4 contour without the M203 cutout. Both 6 groove. The rifle functioned perfectly and provided good accuracy. It was pushing 75 gr Hornady Match (22 gr H335) at 2525 fps. Nothing special. If I could get another 100 fps out of it, and I think I can, I'd be happy. Not terribly worried about ultimate accuracy as long as it functions perfectly.
This summer my club is going to do a one gun shoot with everything from 15 yrds. to 300 yrds. so I need something that is point blank out to 300 yrds. And no magnifying optics allowed. Sounds like fun uh?
That, and I want to play with a melinite treated barrel and see what kind of data I can glean from the change. I'll run this same load through it and see if we see a gain in velocity. The melanite treatment process is supposed to be a plus for a number of reasons. Back in the day forged steel crankshafts were nitrided (Tuft-Rided was the Chevy name) and that was the same process. It put a tough as nails casehardened .008 in. deep slippery surface and it relieved stress in the steel.
Anyway, I'm waiting on a couple of parts this week. So it'll get interesting.
So here's another thought. The powder burn rate has a "spike". Now from what I've being reading is that optimum is to have the "spike" occur just before the projectile goes past the gas port. What say you?

 

[mtrmn]

I'm not qualified to to speak of optimum pressure spikes. I've seen a lot of arguments and sticky threads over on ar15.com and m4carbine.net, but be warned you may get your feelings hurt over there.

 

[Bartholomew Roberts]

The only way I can see to squeeze another 100fps out of 75gr in a 16" tube is to go to something like the Hornady Superperformance ammo that uses their new powder; but I don't believe it is available for reloading.

 

[cameron.personal]

Bartholomew Roberts is right. You will not gain any velocity by changing from a carbine length gas system to a mid length in a 16" barrel. As the barrel length is the same the burn time for the powder is the same. You will need hotter ammunition to increase the velocity in barrels of the same length. I have noted however that some barrel materials will give higher velocities than another. My 16" Lothar Walther LW50 SS barrel will provide more consistent and very slightly higher velocities than my 16" Colt mil spec chrome lined barrel. I'm not sure if that is from stainless steel versus chrome or if it is that the LW50 barrel has a tighter chamber or better gas seal in the bore. The difference averaged less than 10fps.

Cameron

 

[crossrhodes]

Menilite / Chrome

Off topic. But what's the barrel life of a menilite barrel compared to a chrome lined barrel?

 

[lencac]

My understanding is that a melanite treated bore of a 5.56/223 barrel is almost indestructable with barrel life being greatly increased due to much greater resistance to throat errosion. I also understand that because the solution the barrel is soaked in is at a temp of 1075F so the barrel gets stress releaf.
Well, I got my other parts today so I will mostly likely get a chance to run this new set-up over the chrono. Same batch of ammo as went down the other barrel @2525fps.

 

[MachIVshooter]

My goal is to get this rifle to throw bullets as close to the muzzle velocity of a 20" barrel as possible.

Not possible. The same load will have higher MV in a 20" than a 16", period.

 

[blackwalnut]

RE: more info on AR gas systems...

Patrick Sweeney has authored a 3rd book on AR 15s and covers the topic of gas systems and barrel lengths very well if you are looking for reading material on this other than this forum. I am interested in ARs but only just to read about them at this point. I find the AK rifles to be much simpler and dont have to deal with these issues.

The AK was designed with a short barrel and even if they are longer the gas port is still in the same place. This applies to Saigas and a few other imports.

Besides I fall into the category of simple minded, uneducated, peasant so AKs makes perfect sense to me and I can forget about cleaning it and I can fix it with a hammer, pliers, a screwdriver of anykind or a wooded stick and chewing gum. It makes a fair club too. A Mosin Nagant is better.

Good luck. Hope you enjoyed the humor.?

 

[northark147]

Blackwalnut... Ive only messed with a few saiga conversions, so I certainly no AK expert, but if you ask me they are a lot harder to work on than an AR.

 

[TonyAngel]

The only thing a longer gas system will get you is smoother shooting, that is, assuming that all other things are equal.

As far as velocity goes, I seriously doubt that you will be able to get anywhere near the velocity that you would get from a 20" barrel with a like load.

 

[lencac]

Range Report

Well, I took it to the range today after work. Here's the results as shown in the photos. The rilfe all assembled perfectly with the barrel-nut coming to an index point at 45 ft/lb. Headspace checked excellent. I thoroughly cleaned and brushed the barrel before installation. The 2 in. longer open sights is defineatly a marked improvment over the carbine length iron sights.
I noticed that the second shot on paper. Because I have a YH flip up front sight and is an integral part of the gas block. So there was about a half dozen rounds shot for me to loosenthe front gas block and "bump" it to get it zero'ed with the rear peep sight at zero left or right.
Then a couple of clicks up on the front sight post and I wasin business.
I shot groups with the open sights. With a Bushnell ACOG type sight, no magnification. And with a Leupold MK4 4x12x40. I didn't keep the target from the iron sights but I'd say if I got down to brass tacs I could get close to an MOA. The 4 shot group (.9 MOA) is with the Bushnell ACOG and the 5 shot group (.5 MOA) is with the scope.
Ok, here's how I came up with comparable velocities. I have 2/15 shot strings from the old barrel with this particular load. I threw out the high and low extreme and averaged the other 28. The average is 2523.6 fps. With a 15 shot string for the new barrel I have an average, minus the high and the low at 2551.1 fps. So whatever happened it looks like it was good for about 25 fps. increase. Not much but something. However, I will call it good it as I'm very pleased with the accuracy. I could tell right away as soon as I got it on paper off the bench at 100 yrds. that it was going to be a shooter. Also a small observation is from my initial inspection it would appear that the longer gas system does not flatten the primers quit as much.

 

[lencac]

Here's a photo of it the way I like it.

 

[blackwalnut]

Re: NorthArk and AR15 and AKs.....

I would agree with you that certain aspects of the AK do not lend themselves well to the idea of "modularity" that ARs do as well as the marketing hype that focuses on this. And yes it is overall easier to change a barrel on an AR than the Kalash. AK barrels are pressed into the trunnions and this alone becomes more of an exacting chore than doing an AR.

Overall though I think they are still more rugged and simpler. It was a "throw-away rifle" designed to be cheap to build, easy to repair to a degree and disposed of when its service life was over. However there are millions of them that have gone well past what they were made to do.

Conversions were something Mr. Kalashnikov never dreamed of along with our crazy import laws. I will be doing a conversion soon and my only hesitation is that you could be making a good serviceable rifle into a "unserviceable" one.

Some persons seem to have trouble with functioning after doing a convert and I have never been able to ascertain why other than skill must have something to do with it.

Like the pictures of your AR. Thats could be my next one...but still thinking. Maybe all Americans should have one to be patriotic.

I find firearms and their design and history fascinating regardless of what they are. Thanks for your reply.

 

[Tirod]

AK barrels are pressed into the trunnions and this alone becomes more of an exacting chore than doing an AR.

It's not just a bit more exacting - it requires a gunsmith level of training, and seriously expensive equipment.

Whereas, the AR can be assembled on a kitchen table, with the use of a block of wood fastened down to insert in the mag well, and the nut tightened to hold the barrel on.

I can forget about cleaning it and I can fix it with a hammer, pliers, a screwdriver of anykind or a wooded stick and chewing gum.

Which describes how I got most of my AR together. No, you can't do that with an AK precisely because the barrel has to be literally pressed into the receiver - as in hydraulic press, and the headspace is part of the adjustment. It's actually much more complex and difficult than the AR, which has a simple barrel extension that screws on, the headspace then set, and then fastened with a simple pin to hold it. That pin is also the index pin to orient the barrel. Much simpler process.

That's the 30 year difference in technology - not so much the operating system as superior technical innovation in assembly. It actually takes less skilled labor to make an AR.

It's really not what popular myth makes of it at all.

Machining the receivers? AK was more labor intensive originally, as it was milled by an operator manipulating the controls by hand. Later, the stamped versions came out to simplify it, then CNC was eventually adopted. The AR? Drop forged to eliminate most of the machining, bored and broached largely with the tools final cut preset to dimension to eliminate operator error. That was quickly converted to CNC early on, and machine operators simply changed out platters for finish cut receivers.

Everyone thinks the AK is a dirt simple gun, but in reality it's one mired in old school high expense fabrication and assembly. It's largely the labor rate and subsidized manufacture that underwrite the giveaway price. However, in America, we actually make them less expensively. We just happen to be capitalists and make money doing it.

There's another ironic twist - totalitarian anti gun government making high cost guns and giving them away for less than cost, capitalist free enterprise gun rights country charging what the market will bear. At least here you CAN buy whatever you want.

 

[lencac]

Well, as long as my thread has been hijacked I believe I should be given opportunity to make comment. I've had all types of the AK 47 things even the 74's. Even had a Bulgarian AK 47 with a milled receiver. Do they go bang? Absolutely! Much beyond that it's a crap shoot though. By far the most inaccurate mil-spec rifle I've ever seen. Worse ergonomics ever. What most people consider “repairing" is merely only replacing normal maintanence items. AR's are far easier to do that than AK's. AK's are meant to be thrown away, not repaired. I have no doubt that if our military planners thought like the commies we would be making the world's best crappy combat rifles too. The AK is like a '72 Yugo, the AR is like a new ZR-1 Corvette. An AR can be fully disassembled down to the tiniest part with tools that can be carried in your pack virtually without effect. I believe the AK has something called a trunion which I also believe requires a jig and a press to disassemble.
Put two opposing forces, one with AK's and one with AR's at 600 yrds apart and see who dies.
In short, AK's are just stamped commie junk. Except when I was wringing out AK’s they cost about $150, tops! I payed $250 for the Bulgarian with the milled steel receiver. It shot like complete crap too. They really were disposable rifles. But at $450 to $700 for one now, not so disposable:banghead:
You can polish a turd, but it will always just be that, a turd. Granted a shiny one though.
Do you guys see the groups this rifle shoots? AK's don't do that, period!