I’ve written a lot on these pages and in my books about ills that can befall AR-15 platform gas systems, and will write more. In a nutshell: excessive pressure getting into the gas system too quickly causes all manner of ill manners.
- Excessively quick bolt unlocking
- Excessively high bolt carrier velocity (back and forth)
- Excessive hammering of action parts
The “shorter” the guns get, the more these issues arise, and also the greater are their effects.
- Rifle-length barrel and gas port location: not much really to worry about.
- Carbine-length: more to worry about.
- Pistol-length: much to worry about.
The reason is that there’s a whopping lot more gas port pressure nearer the chamber than there is farther down the barrel.
For a reference, “rifle-length” means 20-inch barrel and gas port located approximately 12-inches ahead of the receiver face. Carbine-length is a 16-inch at seven inches ahead. Pistol length may be as short as a 7.5-inch barrel (or even less) with a gas port a scant four inches ahead.
There’s also “mid-length” and “intermediate-length” adjustable gas systems. But more on that in a short bit.
Starting some 20 years ago, we put these on longer-barreled NRA High Power Rifle competition guns, along with relocated (forward) gas ports. Back in that day, these were provided as custom parts by some of the better builders. The idea was to offset the effect of the longer barrel (higher gas pressure in the system) and, largely, to improve spent case condition.
“Adjustable” Gas Blocks
There’s an idea out there running rampant that the best way to fix an excessively gassed AR-15 is to “just slap an adjustable gas block on there” and live happily forever.
Well… about that.
These are indeed worthwhile and effective devices, but only when used for the right reasons for the right circumstances.
An adjustable gas block is best described as a “valved” gas block. The function they provide comes in one of two essential ways: some restrict, some vent.
Those that restrict provide a means to effectively alter the gas port opening size, kind of like opening or closing a water faucet. Those that vent provide an outlet, a bypass, for excessive gas to exit without entering the system—usually out the front of the block itself.
Both of these are user-adjustable. I don’t know that there’s a clearly superior approach, but those that vent tend to hold up a little better.
So, all that sounds like a great idea. If there’s too much gas, make it so there’s not too much gas. And they indeed “work.”
When To Use One
In my experience, which is where I get my opinions, I think an adjustable gas block is a wonderful thing on a rifle-length system. If that rifle is to be used for competitive or otherwise range-only use, I run one.
On my 18-inch barreled, rifle gas port, practical competition rifle, for instance, I use it because that rifle has overall “architecture” that fits into the parameters I have set for an adjustable block—and because it takes every last bit of movement out of the sight.
It’s a worthwhile tradeoff (to me at least) for that. So why not enjoy all that goodness on all of them?
This block from Odin is simple and, so far, has maintained itself well. Technically it’s “tuneable,” not adjustable, and the two large screws tend to stay moveable.
A point that gets missed is that the adjustable gas block becomes a part of the gas system, and it’s the same gas system that was overactive. And it was overactive because it contained just too doggone much high-pressure, really. Hot gas.
And this adjustable gas block is in the same location. The main cause of the excessive gas effect is the gas port location being closer to the chamber.
Adjustable gas blocks won’t last forever, and neither does the adjustment that was once carefully put on it. They get dirty and the heat erodes the innards.
Look in the barrel of a well-worn AR-15 and you’ll see an eroded area just beyond (toward the muzzle) the gas port.
That’s from flame-cutting, the same culprit as what causes chamber throat erosion. The trick is that, after time, the adjustable gas block may not remain adjustable.
Both the erosion and the carbon deposits that get into the valving effectively alter the pressure, getting into and then through the system. This is one area of some difference in the restricting or venting designs, but the result is that function will change.
And there’s a chance nothing can be done about that. I’ve seen them lock, slap-up and no longer adjust.
Heavy buffers and stouter springs are a longer-term solution for short guns, and they allow pressure to stay “up” enough to reliably cycle.
A Couple Words of Caution
Do not run adjustable gas systems on a pistol-length gun! It will not work well and it will not work long. I also won’t run one on a carbine-length setup for the same reasons.
There’s such a huge dose of hot gas hitting the block interior that it’s not going to hang out over the long haul. And. There has to be enough pressure in the system to cycle reliably.
With the already-abbreviated volume available within the system, there’s less room for error in a short system respecting “enough.” Longer systems are more flexible.
Again, I can recommend adjustable blocks only for use on rifle-length systems. The block is then far enough away that the already-milder dose of excessive gas won’t hurt it… as much.
Two tips on getting the best performance over the longest life with an adjustable block:
- Don’t get too greedy on restricting gas flow into the system. Get function 100 percent and then open it up at least a little bit more. This is very important for a gun that may be used in different temperatures and with different brands of ammo.
- Shop for a block that can be cleaned and rebuilt, and then clean and rebuild it! I can’t say for certain how many rounds should pass between refurbishings, but I’d sure check it after 1,500.
On guns this short, adjustable gas systems are not the right answer. They will get eaten up in short order.
Alternatives for Adjustable Gas Systems
I quickly and freely admit that, in some instances, retrofitting an adjustable gas block onto a problematic gun might seem like the most direct and easy solution, and that could be, in fact, right. That is also one of the reasons for their popularity.
However! If you decide that you have to have one to make the gun work, look for solutions elsewhere. There are easier solutions.
Ramp up buffer weight and also buffer spring power, for example. Both delay bolt unlocking to give a little more time for pressures to drop.
Next time around, though, the far and away best solution to reduce the effects of excessive gas port pressure is to locate the gas port farther forward.
I very strongly recommend installing a “mid-length” gas system on any 5.56/.223 carbine (port location +2 inches ahead compared to carbine), and I also recommend moving the port out to carbine location on an AR-platform pistol in the same chambering.
If, and only if, you can live with a long enough barrel to make that possible. An 11.5-inch can work with that location.
Even though they treat the symptoms, not the cause, heavy buffers and stouter springs are a better solution for a shorter gun.
How to Tune an Adjustable Block
Finally, before I end this post, I thought I’d include a section on tuning these adjustable gas systems.
After umpteen years of messing with these things, I suggest closing it down all the way to start. Load only one round at a time (an empty magazine is sitting in the gun, round chambered). Shoot and test. Open the valve apparatus until the bolt locks back.
Then test it with a few magazine-fed rounds. And then open the doggone thing another fourth of a turn! Play it safe. The reason for the empty magazine is because it takes a little more to trip the bolt latch than to feed the next round.
I also, when possible, put a drop of light-duty threadlocker on the adjusting screw. That keeps it in place and reduces the likelihood of corrosion.
Note: The preceding is a specially-adapted excerpt from Glen’s book, The Practical AR15.
Do you think adjustable gas systems are a good idea? Why or why not? Let us know in the comments below
Source: Cheaper Than Dirt