16" Midlength AR Problems

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Aug 21, 2011
A few weeks ago I traded for an Anderson/DS Arms build, a 16" Midlength gas system chambered in 5.56. I have put about 200 rounds through it and am having some issues.

It will cycle and fire full spec 5.56 ammo (mostly Wolf Gold and a bit of Lake City M193) 100% fine, however, it will short stroke -every- single shot with any lower powered ammo, including American Eagle .223, Independence 5.56 "XM193I", and Silver Bear 62 gr hollowpoints. I am not ok with a gun that won't cycle every round I feed it with.

The gun is clean,I have carefully checked the gas block (it is a set screw, railed type, but it is in the right place and stays put, and there are no signs of a gas leakage). It has a standard Carbine buffer (3 steel weights) and I even tried replacing one of the weights with rubber with no improvement. I also removed and carefully re installed the gas block with no improvement.

With the lower spec ammo (mostly American Eagle .223 and Independence 5.56), 75% of the time it will eject, and simply skip over the next round. The other 24% of the time it will jam with bolt halfway across the ejection port, with a round at a sharp angle caught between the opening of the chamber and the bottom of the bolt face, often with a nasty dent/gouge in the cartridge. I have had a few failure to extract/ejects with the American Eagle and Independence, and one stuck case with the Silver Bear.

I have the same problem when using this upper on my brother's carbine lower, and when using his bolt in my gun. I have the exact same issue whether I am using PMags or USGI, and the mags sit in the exact same place as my dad and brother's rifle, so it is not an issue with the Anderson lower or the mags, or mag seating.

The day before I got this rifle, I put over 200 rounds of the Silver Bear through my Dad's Stag Model 4 20" rifle at an Appleseed event (3 Rifleman scores! :) ) with no issues. All of these ammo types function flawlessly in my dad's 20" rifle and my brother's 16" carbine gas Diamondback build. The Stag has about 400 rounds through it, and my brother's 16" carbine Diamondback is brand new, about 120 rounds through it. A few years ago I had an M&P-15 Sport which ran all of these specific types of ammo 100%, over 900 rounds.

The bolt catch on my midlength rifle is also defective, the tab which catches the follower to engage the catch is too short by less than a hair, and probably 3/4 of the time, it will not lock back on an empty even when cycled manually. I have a new bolt catch on its way.

Help? The gun has -never- successfully cycled anything but Wolf Gold or Lake City XM193.
Sounds like it is under gassed. Could be restricted gas port or not enough barrel beyond the gas port. I had a Dissipator upper that was like that. Gas port too close to end of barrel. Sounds like lighter buffer did not help. I would check the gas port first.
It sounds like it's under gassed, but you could also have too heavy a buffer. If your lower has an H or heavier buffer swap it for a lighter buffer. If you're already running a standard CAR buffer then the upper is almost certainly under gassed. If that's the case contact the manufacturer to have the upper repaired or replaced under warranty. The repair may be as simple as the manufacturer opening up the gas port to get more gas volume in the rifle.
It has a standard carbine buffer and I even tried taking weight out of that. Unfortunately I think the previous owner voided the warranty by swapping the gas block.
I also think it is a gas issue, I'm trying to weigh options before working on the port as a last resort. I don't really have the tools to measure it precisely, and all I have to work on it would be a standard hand drill. I have done a lot of work on AKs, including converting Saigas, but I don't want to screw up the AR worse. It shoots good groups and has been entirely reliable... with a select variety of ammo. :rolleyes:
You don't want to go lighter than a CAR buffer. Either the gas port is too small, or something with the gas block is restricting gas flow. I'd bet it's the latter. You may need to have a smith look at it.
Later today I will pull of the block and look closer at it and the port.
Swapped the gas block? Might be a "smoking gun" right there. Maybe not. Pull it off and check it out. 14.5" middys are real ammo sensitive, but a 16" should be ok. Try Federal ammo. I thought American Eagle was GTG??

Wolf ammo is filled with sand and ground up pieces of the Soviet navy. It may have clogged up the gas tube.

I know, I know, the pressure in the gas tube should blow away anything in there, but I've seen it with my own eyes. That Wolf gunk won't clog it, just reduce it's ID enough for problems.
The .223 spec American Eagle did not cycle correctly even a single time.

The "Wolf" in question is not steel cased Russian stuff, it is "Wolf Gold", Taiwanese made 5.56 M193 with sealed primers and clean, consistent performance at an affordable price. I love the stuff, it just happens to be imported under the "Wolf" name which I like because Wolf is a big supporter of Project Appleseed.
I did order a new gas block to try, just as soon as I can get the flash suppressor off.
It sounds like you are describing a short-stroke/gas leak problem; but you say the gas block is tight with no leakage.

Have you checked the gas key? That is a common source of gas leakage that people sometimes miss. Gas tube obstruction is rare but it can happen with some foreign ammo and is worth ruling out. If you can rule out not enough gas, then that pretty much leaves a timing issue - maybe rough chamber?

Have you tried swapping the entire bolt carrier group to see if that fixes it?
If it were me, I'd inspect the gas block carefully. If all looked good there, I'd open the port up .010 or .015 and see what happens. I've never had to mess with ports on any of my own, but did have to do that with a buddy's rifle after installing a new barrel.

Pretty difficult to "over-gas" an AR, anyway. It can make the recoil impulse sharper, but unless you see signs of one battering the buffer bumper, it's GTG. For reliability's sake, I'd rather be a little over than a little under.
I swapped out the block for a new one, we'll see if it works better.

I have tried multiple different BCGs, same result.
Gas key is tight and I had the same problem using bolts from different, otherwise flawlessly functioning guns.
Well, we are down to gas tube, gas block, port size or something in the chamber dimensions/roughness. The fact that it runs with 5.56 tends to suggest that it isn't the chamber though.
I am with BR's suggestions in the last post. If the key is tight, you need to check for blockages and or small gas port..
Thanks for the help guys. I inspected the tube carefully, it seems fine. I just put on a new block that seems to fit better; if that doesn't fix it I am going to find a way to measure the port precisely and either make it larger myself or get it done professionally.

I have read somethings about 16" midlengths needing some break in time with 5.56 before they will run .223 100%, but I have to believe that failing to cycle properly -every- single .223 round after almost 200 5.56 has to be more than a "break in" issue.
I have read somethings about 16" midlengths needing some break in time with 5.56 before they will run .223 100%

That isn't "breaking in" so much as it is gas port erosion. The port starts out too small for .223 to function well; but as unburnt powder erodes the gas port and more gas is allowed in, it starts to run .223 OK for awhile. You'll get a long period where both work well and then you start showing problems with 5.56 instead and have to increase your buffer weight.

It will take a few thousand rounds to open up the port that way :)
if that doesn't fix it I am going to find a way to measure the port precisely and either make it larger myself or get it done professionally.

#1-60 Wire gauge drill set

That'll get ya measured within 0.003", and also give you what you need to make it bigger.

For more precise measurement, you'd need gauge pins. But they don't have other uses like a drill set, and you don't need to be that exact anyway. If it runs 5.56 fine but not .223, I'd open it up ~.005 to start; it's not way under gassed.
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