Shopping for a AR stock/buffer

[WestKentucky]

I am only a stock/buffer kit away from another running rifle. The upper has a 20" 5.56 barrel with rifle length gas system. I ran across a deal on a collapsible stock kit but it's got a carbine buffer. Would it work fine or would it beat the snot out of the gun by being over gassed? I'm used to undergassing and not cycling, never ran a rifle upper on a carbine lower. It's a deal but it's not a steal so...should I wait for a rifle specific kit? I want collapsible so my wife and my sisters kids can shoot it.

 

[Varminterror]

I'm never quite sure why so many guys worry about hurting a carbine lower with a rifle upper - it's been the exact opposite in my experience.

You'll be fine with a rifle length gas system and a carbine spring, buffer, and receiver extension. If you want ultimate control over your gas flow, an adjustable gas block is the best route. Some "tuners" might claim a heavier buffer can help mitigate some overgassing issue as well - but in general, it hasn't seemed to really fix the problem of overgassing. That's more like how advil will help your headaches, but if they're caused by a brain tumor, Advil's probably not the only thing you need. I haven't seen many rifle length gas systems be nearly as high on gas flow as their carbine equivalents in factory models. A lot of guys argue you should match the buffer weight to a rifle buffer, but in reality, as long as it's not unlocking prematurely, that's not necessary (and in hundreds of AR builds, with a huge number of rifle systems on carbine lowers, I have not seen any regularity of unlocking prematurely. I DO, however, see advantages in running heavier buffers in almost ANY civilian AR, outside of action competition - so it's not a bad idea.

Almost every single AR I've ever serviced in the last 20yrs (over 500 of them) has been grossly overgassed by design, but especially so the carbines, so you'd be just as likely to have overgassing issues if you put it on a rifle lower or a carbine lower either one. Durability and longevity seem to be less important than reliability to manufacturers (fair position to take) - again, this is one reason I've always been such a proponent of adjustable gas blocks. At a minimum, assuming a non-A2 type front sight, simply sliding the non-adjustable gas block slightly out of position to throttle the gas flow works very well (boring and bushing the block is better, adjustable gas block is the best - I'll NEVER crimp another gas tube).

Also, keep in mind, "overgassing" is a multi-faceted issue. Unlocking too early in the pressure curve is one problem, but too great of bolt speed is another, and solving one or the other with weight or springs doesn't always solve both problems. Limiting gas generally fixes both issues at the same time, but you CAN find combinations where only one of these problems exists (i.e. light buffer and BC, heavy spring, oversized port, vs. heavy buffer and BC, weak spring, oversized port). Largely, unlocking early is a sign of not enough weight. Excessive bolt speed without premature unlock is typically a spring issue - if you have BOTH, then you assuredly have too much gas. Limiting gas will fix either problem, and both problems. Most newbies don't have a good gauge for excessive speed, however.

Some math, for those who enjoy it: A 16" barrel with a 7.8" carbine gas system has 8.2" of dwell time exposure to high pressure - in the range of about 48kpsi. A 20" barrel with a 13.2" rifle gas system has 6.8" of dwell time exposure to a much lower pressure - about 20kpsi. As such, 16" barrels with Carbine length gas systems typically have much smaller gas ports - typically 62-70thou diameter, whereas their rifle gas counterparts have much larger ports - typically 92-98 thou. That's more than TWICE as much flow area for the rifle system port than the carbine port. Carbine gas has approx 46% of the area with 240% the pressure, plus ~20% greater dwell time, that ends up over 35% more gas flow than a rifle system (not even taking into account the difference in bullet velocity through the dwell distances - less gas in the rifle!!) - so if they're actually designed right, you'd be more prone to UNDERGAS the carbine lower with a rifle upper than the reverse. Comparatively, a rifle buffer typically (by spec at least) should be 5.2oz, whereas a carbine buffer is 2.9oz - a carbine spring has ~38 coils compressed into 6 3/4" for a mil-spec tube or 7 5/8" for a commercial tube, while a rifle spring is 42coils compressed into 9 5/8" --> 38/7.625 vs. 42/9.625 gives about 15% more spring resistance in a carbine stock than that in a rifle. So... In other words, you have a stock designed with 15% more spring resistance, designed to have a carbine buffer at 2.9oz, and meant for 35% more gas than your ... Does it REALLY need to match in weight?

As long as it doesn't unlock early, you don't need to increase the weight of the buffer. If it does, you can always throw in an H3 buffer - or again - throw on an adjustable gas block and throttle back your gas impulse.

But 99% of that is all just conjecture, as experience by millions of AR owners who have done the same exact thing you're about to attempt tells us you'll really be ok. If it ends up over gassed, get an AGB.

 

[WestKentucky]

Incredibly well spoken. It all made sense. Thank you.

 

[MistWolf]

Varminterror, you've got the numbers, but your conclusions are overly complicated and a bit off base. The differences in so called "dwell time" (the time the bullet is in the barrel between gas port and muzzle) are minute and make no practical difference. The difference of distance between gas port and muzzle is how it affects blow down time after the bullet has departed the barrel. The gas does not act on the action until after the bullet has left the barrel.

In simplest terms, the rifle buffer is not a heavy buffer. It is the standard. Buffers that weigh less than a rifle buffer are actually light buffers as they are lighter than the standard.

I do agree that gas flow is an important part of tuning an AR. But so is using enough mass and the correct spring. The optimal setup is the rifle gas system, 20 inch barrel, rifle buffer and rifle spring as it is what works best. It is the standard. Anything else is a deviation. Starting with an H or H2 buffer and tuning gas flow to match gets us as closer to the standard than tuning gas flow with the lighter carbine weight buffer. It ensures the AR will function under the widest set of conditions with the widest types of ammo without over gassing, eliminating bolt bounce and at the proper carrier speed.

A good tuner needs to know what the baseline is, before making any changes. Tuning the AR to the right buffer weight sets that baseline

 

[Varminterror]

The rifle buffer weight is only standard for a rifle buffer tube and rifle spring and rifle gas port size and position with a designed M16 rate of fire.

A carbine buffer weight is STANDARD for a carbine extension, carbine spring, and carbine port size and position with an M4 rate of fire.

A rifle buffer weight is NOT standard for a carbine spring or buffer tube.

 

[MistWolf]

Carbine WEIGHT buffers are too light (too little mass). The MASS of the rifle buffer is the standard. Colt civilian carbines are sold with H buffers and M4s are delivered to the military with H2 buffers, both have greater MASS than carbine buffers. The H2 buffer has greater MASS than the H buffer. The H buffer has less MASS than the rifle buffer. I don't recall if the H2 MASS is greater or lesser than the mass of the rifle buffer.

The mass of the rifle rifle buffer is the standard. It is the baseline. Any different mass is a deviation from the baseline

 

[Varminterror]

Closed minded, I can see...

H2's are lighter than rifle buffers.

Recall, the M4's delivered to the military by Colt are also typically 14.5" barrels, with a carbine length, meaning less dwell time than our civilian versions. And Colt isn't the only game in town who knows how to make an AR tick in time.

Mass keeps it locked through the pressure spike, spring limits velocity - lots of combinations to hit the right specs for bolt lag time and carrier velocity. As I said before, it does NOT take an H2 or H3 buffer to make a rifle upper operate properly on a carbine lower.

 

[MistWolf]

Not closed minded at all. Understanding how things work, what the baseline is and how deviation affect function is what I do for a profession. Not difficult to apply those skills in figuring out how an AR works.

The mass does not keep the bolt locked during peak pressure. The bolt is locked mechanically in the barrel extension until the carrier moves and the bolt cam turns the bolt. The mass and spring rate ensure the carrier doesn't move until the expansion chamber reaches the right pressure.

But, have it your way, boss

 

[Elkins45]

Mass only keeps it locked if it's a blowback design like a 9mm. A normal rifle is locked because the bolt lugs are locked into a recess until the carrier rotates it.

I had to hollow out and pour a rifle buffer full of lead to help slow down the unlock speed of my 9mm blowback gun. It was making these:




I think it could probably benefit from a heavier spring as well. Of course, none of this has anything to do with gas operated guns....

 

[Varminterror]

That case is amazing - pic saved! Incredible to get that much expansion without rupture.

I've lost interest. I've done the free body work on AR's myself, the inertia (driven by mass) of the bcg and buffer is the first resistance to movement, as the spring is at its least forceful state - the bcg and buffer not wanting to move (inertia) keeps the bolt rotated into the lugs.

Guys can get stuck in a one track mind, but there are more roads which lead to the same destination of a reliable AR with enhanced longevity. Adding buffer weight to a carbine is one road, not the only road.