Antelope Sniper asked for my opinions on gas systems a few days ago and I thought it might be better to just start a new thread to talk about it.
Before I start I've gotta explain where I'm coming from. Most of my shooting is in competition, so what I need from an AR might be different than somebody else. But because how my rifle runs can make the difference between winning and losing or between going home with a prize that will pay for more ammo and going home empty handed, I pay more attention to different details of my AR than somebody who shoots with different goals than me would. It's not that one type of shooting is better or worse than another, it's just that different kinds of shooting demand different things from a gun.
So some of this may be applicable to you and some may not. Take it for what it's worth.
In general, with a 14.5" barrel a carbine system will run whatever ammo you want to feed it. A 14.5" mid-length barrel may or may not function on weaker .223 ammo (depending on the manufacturer and what gas port size they used and what buffer you have). BCM is known for making very smooth shooting 14.5" middies. I've shot one and with an H2 and VLTOR A5H2 buffer and it was incredibly smooth shooting. But the BCM barrels may not function with the weaker .223 ammo.
In a 16" barrel I would definitely go with a mid-length gas system. I just can't see any reason not to do it. It'll reduce the pressure sent to your bolt carrier group and keep unnecessary wear off of your bolt.
In an 18" barrel go rifle gas. Again, I can't see a reason not to. I shoot an 18" rifle gas barrel, along with a lot of friends, and we've never had a problem with them not functioning because of a lack of gas. There's a reason that competitive shooters are all using 18" rifle gas guns....they've got enough gas to work, but not so much that they beat you up.
Which brings up something worth mentioning. When I talk about the recoil on an AR I'm not talking about what you felt the first time you fired your dad's deer rifle. I'm talking more about recoil impulse than actual felt recoil. Most people probably wouldn't notice the difference in recoil between a 16" carbine gun and an 18" rifle gun. But if you look through your scope and shoot offhand at 50 yards you'll see a difference between the two.....You won't FEEL recoil, but you'll SEE it. That's why it's so important to competitive shooters. You've gotta be able to see where you're hitting right now or where you're going to be shooting next, but you've gotta be able to watch what's happening through your scope. And if your gun is jostling and coming off target with each shot you're going to be slower than the next guy. I posted some videos of a match I shot with DocRocket last weekend. If you watch closely, you'll see the muzzle of his AR bouncing with each shot and you'll see that mine barely moves. It's not that mine is better than his, mine is just built with a different purpose. But just imagine looking through a scope on each gun...one will be bouncing off target with each shot and one will stay on target. Which one will be faster?
So none of this might matter to you. If you're shooting super fast at 7 yards, you'll probably never notice the difference. If you're shooting coyotes one or maybe two at a time you might not notice. But if you're firing multiple rounds on a single target passed about 50 yards or making a lot of transitions between targets you'll definitely see the benefit of a smoother gas system. I can watch my hits through my scope on long range shots and make windage-elevation adjustments as needed that I could have never done with my old 16" carbine barrel.
But everything has to be taken with a grain of salt. Some manufacturers will make sure their guns run with the weakest ammo on the market and will accomplish that by simply putting a larger gas port in the barrel. Not all "mid-length" or "carbine" barrels are created equally.
A lot of folks overlook the role that buffers play in the whole system. If your gun is getting too much gas you can slow the BCG group down and smooth out the recoil impulse with a heavier buffer. If your buffer is too heavy though your BCG won't move back far enough to strip off fresh rounds from the mag or lock back on an empty magazine. Once again, it's a balancing act. I'm using a VLTOR A5 buffer system that allows you to use a rifle length spring on a collapsible stock. I've switched that A5 lower and a lower with a standard carbine buffer back and forth on the same upper before and there's a noticeable difference in how smooth the gun runs. For anybody who cares, the "mil-spec" Colts have an H buffer in them. Most guns will run fine with an H2 buffer since they've usually got more gas than they need. I've switched to an H3 on that VLTOR system and haven't had any issues with regular .223 or 5.56 ammo on 16" mid and 18" rifle barrels. If you want to check, your buffer will be marked on its end and you can see what weight it is by just opening up the upper-lower. If it's blank, it's a carbine buffer (the lightest). Otherwise it'll be marked "H", "H2", "H3". If you're stuck with a carbine gas barrel and want to make some changes to your gun, the buffer is a $30 and 5 minute change to make. Move from a carbine buffer to an H2 during a trip to the range, firing one right after the other, and I think you'll like what you find. The gun just seems "smoother".
These are some general rules of thumb that you can use when buying a new rifle, they're definitely what I look for. The BCG does all the work in the gun, don't beat it up with more gas than you have to.
