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marine6680
August 15, 2012, 05:32 AM
Looking into ARs, and wondering about which length gas system to get/is best.

Most complete rifles from Colt, BCM, and the like, are all carbine length, though some have the longer system. If I build my own, then I have more options.

I don't really care about barrel profile so much, other than I do not want a thin/lightweight profile. So M4 stepped profile, A2, or heavy, either work. Some say that heavy profile is best... if so, hey might as well go that route, but some research is in order. So, here I am. (meaning feel free to provide info on this subject as well... heck, any info you feel is good to know go ahead)

Thanks in advance

(Due to the advanced nature of the expressed gratitude, I am unable to determine if an individual post is deserving. If you're completely unhelpful, ignore any remarks of gratitude which you do not deserve :p )

plouffedaddy
August 15, 2012, 06:30 AM
The 'conventional' wisdom is that middys will have less wear on the gun and less perceived recoil. I have found this to be true but I've also never seen a rifle break because it was a carbine gas system vs mid length. Additionally, the brake has more impact on perceived recoil than the gas system IMO. Either way, the 556 is a very mild recoiling round so the difference is negligible; especially if you're going to use a HBAR.

RT
August 15, 2012, 07:05 AM
Mid will give you more rail space,slightly longer sight radius with irons and maybe a milder recoil. However, if you decide on a carbine, you can run a full-auto BCG and try a H or H2 buffer.

Crow Hunter
August 15, 2012, 07:18 AM
The only catch with mid-lengths is that there is no standard. Everyone does it slightly differently and who is to say they are doing it "right"?

Mark Westrom came up with the idea based on stuff he did when he worked at Crane and bought Armalite way back in the late 1990's and released it back then.

It didn't go anywhere then but now that the internet is more available than it was then, maybe it will be more successful. Just look at PMags vs Thermold and Orlite.:D

In theory it should work better but no one really knows and it doesn't have the history that the carbine and rifle gas systems have.

But the beauty of the AR system is that if you don't like it down the road, just push out two pins and swap the upper out.:cool:

madcratebuilder
August 15, 2012, 08:14 AM
The only catch with mid-lengths is that there is no standard. Everyone does it slightly differently and who is to say they are doing it "right"?

I thought all mid-length had the barrel gas port at 9.5 inches??? Carbine is 7.5" and rifle is 12".

The barrels gas port diameter has a big influence on the dwell time and recoil impulse. A carbine gas can be very similar to mid-length with the correct port diameter. Gas pressure at the carbine gas port is about 26K psi and if I recall, the mid-length is down to around 20K psi. The length of the barrel past the gas port is also very important in determining the gas port diameter. Most carbine and mid-length ports are .062.

A mid-length on a 16" barrel with a standard buffer and spring well run smoother than the same buffer/spring in a carbine most of the time.

Crow Hunter
August 15, 2012, 08:46 AM
I thought all mid-length had the barrel gas port at 9.5 inches??? Carbine is 7.5" and rifle is 12".

The barrels gas port diameter has a big influence on the dwell time and recoil impulse. A carbine gas can be very similar to mid-length with the correct port diameter. Gas pressure at the carbine gas port is about 26K psi and if I recall, the mid-length is down to around 20K psi. The length of the barrel past the gas port is also very important in determining the gas port diameter. Most carbine and mid-length ports are .062.

A mid-length on a 16" barrel with a standard buffer and spring well run smoother than the same buffer/spring in a carbine most of the time.

You are very much correct.

Location of the port is pretty much fixed because there are only so many barrel manufacturers. But port diameter isn't. Everyone uses a different specification and alot of companies use different barrel lengths beyond the gas port as well. Just because it says mid-length doesn't mean it will get all the advantages that Westrom originally discovered at Crane and brought into the civilian world.

That is why I say there is no "standard" and depending on who made it and what they used, some tweaking may be needed for certain ammo pressures. Tweaking that the average gun buyer might not be willing to acquire the knowledge to do.

ETA:

The carbine system has similar "issues" as they are actually designed for a 14.5" bbl and can often be overgassed. But for the average gun buyer, overgassed will probably be less of a hassle than undergassed. Most owners won't shoot their overgassed carbine length rifles to the point of failure anyway assuming adequate construction but a rifle that starts short stroking when shooting Wolf or UMC .223 will often cause a case of the vapors.:D

Te Anau
August 15, 2012, 09:07 AM
Mid length is clearly superior.

customaquatics
August 15, 2012, 10:20 AM
then that means rifle length is god?

plouffedaddy
August 15, 2012, 10:51 AM
Mid length is clearly superior.


That's a very broad generalization. For a barrel under 14'' for instance, they're clearly not.

Fishbed77
August 15, 2012, 01:19 PM
Mid length is clearly superior.

Huh?

I run a 16" carbine with an M16 bolt carrier and a Spikes ST-T2 buffer. It shoots as softly as any mid-length I've ever fired. I use an Aimpoint on it, so the shorter sight radius makes no difference.

marine6680
August 15, 2012, 01:32 PM
Yeah, I would be using cheap ammo, bit of steel and brass. It really depends on how much hassle I find it to be to reload. If I reload, then more brass than steel. I of course want it to run reliably with a wide selection of ammo regardless.

Been looking at Colt and BCM for prebuilt. Also looking at LWRC, the treated BDG and barrel treatment are the biggest draw there. Being piston would keep the receiver and BCG cleaner, which is nice. The piston will get dirty, but seems that would be easier to clean.

The I was looking at buying separate upper and lowers as i can build two rifles for the cost of the LWRC and just a few hundred more than the other two.

Looked at PSA as everyone seems to think they are high on the list as far as quality goes. They offer a wide range of uppers. Their barrels are made by FN, so I would think they are build well.

customaquatics
August 15, 2012, 01:36 PM
as i do love my PSA :) really accurate for $750 for the whole rifle. the new AR build im doing is going to have a PSA barrel for it too.

marine6680
August 15, 2012, 08:09 PM
If I build, or even if I use a separate upper and lower, I may go PSA. If I get an assembled rifle, looking at the mid length BCM as well, just a little more than the carbine length, but harder to find it seems.

I think the fiance has decided that 5.56 is good... she wanted 7.62. She also wants a DMR setup, so I am thinking a heavy profile 18" barrel upper will be a good bet. (maybe a 20" as PSA does not have 18" 5.56)

So I will be getting two rifles... I am thinking I will go with Magpul OD green furniture on mine... she can go with black or FDE. :cool:

madcratebuilder
August 16, 2012, 07:15 AM
That is why I say there is no "standard" and depending on who made it and what they used, some tweaking may be needed for certain ammo pressures. Tweaking that the average gun buyer might not be willing to acquire the knowledge to do.

I'm on the same page now:)

It's strange so many only consider the gas port location, ie carbine length, mid-length, etc. The amount of barrel length past the gas port can be even more important. I learned that the first time I cut a 20" barrel back to 16".

jmr40
August 16, 2012, 08:07 AM
To get a slightly longer sight radius without having a longer barrel is reason enough for me to prefer the mid length. But carbines are more readily available and often at better prices.

thesheepdog
August 16, 2012, 08:15 AM
Midlength offers better gas timing (slower and softer recoil impulse), which in turn-due to the physics-offers less wear on the operating parts of the rifle.

Carbine are great too, and if you're stuck with one, you can always buy a Sirac Ordnance AGB to tune the gas timing that way too.

I read an article recently about the proper gas length to barrel length setups, and it is highly recommended-for the purposes of battle ready reliability-that your barrel be 7" longer than the gas system.

I have seen 3 gunners with some strange gas-system setups on their guns. But those are competition weapons designed for speed, low recoil, and most guys are getting by with the minimum powder charges to cycle the weapon.

Middy's are a great middle mark (lighter recoil impulse, while still maintaining battle ready reliability).

Quentin2
August 16, 2012, 10:59 AM
Sounds like a great article, thesheepdog. Do you have a link to it?

thesheepdog
August 16, 2012, 11:41 AM
Trying to locate it.

plouffedaddy
August 16, 2012, 12:45 PM
In order to understand this, there are a few things about the functioning of the AR that have to be defined, I am away from most of my notes and stuff, so most of the figures given are from memory... but should be pretty close. For this description, the standard rifle gas system is with the gas port located at 13.0" and having a 20" barrel... the standard carbine system is with the port at 7.5" and a barrel of 14.5"

The pressures at the gas ports are: 13.5K for the rifle and 26K for the carbine -- or twice as much.

The dwell time (the time that the gas system is charged with high pressure) is determined by the amount of barrel after the gas port. These are nearly identical between the rifle and the carbine.

Pressure from the port is regulated only by the size of the gas port and the diameter of the barrel.

These two factors determine the internal bolt pressure, the maximum pressure that is obtained in the bolt carrier/piston combination -- for the rifle this pressure is about 1000psi and for the carbine it is over 1500psi, half again as much.

When the rifle is fired, primer shot sets the bullet forward until it contacts the rifling, at this point the powder charge detonates and sets the shell case fully back, binds the action and start to propel the bullet. The bullet jumps slightly again and is etched by the rifling... it stops again very briefly as the pressures build to a point for the bullet to overcome the mechanical advantage of the rifling twist and the bullet starts to spin, at this point the chamber pressure is at max, 50K plus (there are some that believe there is another, third stop the bullet makes and some testing suggest this may be true).

As the chamber pressures start to climb, the brass case expands and becomes plastic, this is essential to seal the case in the chamber -- the correct term for this is Obturation, when the case is obturated and sealed, it is stuck in the chamber, practically welded in really.

The Lock Time, or the time that the action remains locked with no attempt to start unlocking is very important... on the rifle, the lock time is about 550 microseconds, the lock time for the carbine is about 375 microseconds -- this may not seem like much, but it is much shorter of a time, also keep mind that the chamber pressures are twice as high in the carbine when the unlocking starts.

What does all of this mean? When the carbine is fired, the system attempts to unlock earlier than intended and while the case is still fully obturated... this results in the action bind delaying the unlocking and stressing the system. As the 5.56N is not drastically tapered, "squirting" is not a big problem in most guns. When the internal bolt pressures finally unlock the bolt, the velocity of the reward movement in the carbine is much higher than what the rifle was designed for, it also must start extraction of the obturated case... as you know, the AR does not have any sort of initial extraction, perhaps the single biggest shortcoming of the design. This has been known to cause ripped case heads...

At this point, as the bolt starts to unlock, it is rotated to unlock...here there is a phenomenon called extractor lift where the extractor lifts off the rim of the cartridge case -- some argue that the pressure of the extracted cartridge case keeps the case head against the bolt face, but the fact is that the extractor does float and the contact with the case rim becomes "soft". For this reason, it is much more likely that the extractor will simply pop off, rather than actually rip the case.

Balanced extractors and different designs have been developed (LMT), but the best solution to date has been stronger extractor springs and spring buffers. That about covers the FTE issues...

Back to bolt velocity. The high speed of the bolt has a couple of other detrimental effects, one of the most common is that the bolt is cycled so fast that as it returns to battery, it actually has enough force to "bounce" off of the barrel extension when closing and locking... this bounce back is very small, but can be enough to cause the weapon not to fire... this "bolt bounce" is pretty well known.

One other problem is that the bolt can cycle so fast the magazine spring can not keep up with it and the round stack is not properly aligned and forced back into place before the bolt returns to batter -- therefore there is no new cartridge picked up and the bolt closes on an empty chamber, this is what some call "ghost loading", or bolt-over-base jams... this is far worse in full auto fire as the bolt does actually move faster in full auto than semi auto; this is due to the fact that the top cartridge in the magazine does not apply force to the bottom to the bolt causing drag.

The common solution to this issue is to use a stronger recoil spring and a heavier buffer... this works, but is treating the symptom, not the problem.

-stolen from ARFCOM

marine6680
August 16, 2012, 02:15 PM
So from all that... it seems the mid length is a compromise between the rifle and carbine actions.

I know my fiance's rifle will be a 20 inch barrel over a lower with a carbine extension and adjustable stock. She doesn't like the fixed stock.

I will be going with a mid, at 16 inches.

I would think that I need a lighter buffer than the standard carbine buffer... especially for the 20" barrel.

Te Anau
August 16, 2012, 04:00 PM
I will be going with a mid, at 16 inches
Good job!

Crow Hunter
August 16, 2012, 08:44 PM
This is one of the reasons that good part quality is so important. (Particularly in the M4gery guns).

That H buffer and the Full Auto Bolt Carrier Group add weight to the system and help hold it closed a little bit longer.

The forged (instead of cast) extractor helps with extractor life as does the proper extractor spring and buffer.

This is also the reason a lot of people will say that they have Brand X and it has gone 20,000 round without a problem.

However, they are often talking about full size A2 style rifles (often DCM service rifles).

These can easily get by with parts that the M4gery type guns can't because of the significantly reduced operating stresses.

Ideally, I would like to see someone make a full sized rifle without the heavy barrel and a collapsible stock.

(Actually, I would like the military to get on board with the SCAR 16 or other new generation rifle that has material/manufacturing/design improvements to greatly improve the life of the weapon and have it trickle down to the civilian world)

marine6680
August 16, 2012, 10:08 PM
Yeah... the Scar was one of the smoothest feeling rifles when I was hand cycling the action, of all the ones I have handled. Its just too expensive right now.