Hello, here a few months ago I built my first AR and it has not shot properly sense it was complete. The rifle has a serious short-stroking issue and I have looked at everything to try and correct it, but I have had no luck so far. I'm hoping somebody knows something that I don't and can help me figure out how to fix it. The rifle has a custom lower on it, with a Rock River Arms upper. The upper has a 20" chrome moly barrel (chrome lined) with a low-profile gas block and Rock River BCG. The rifle is chambered in .223 Wylde

Right now the rifle will not properly cycle. After I fire a live round the gun ejects the spent casing with zero issues 100% of the time, but it will not pick up a new cartridge from the magazine. I end up having to cycle the gun by hand to chamber a new round, making this the most useless semi-auto rifle ever made. It also won't lock back on an empty magazine.

I have checked:
- Gas key is properly staked and is not loose
- Gas block has been adjusted to get maximum air flow through it
- Gas tube has no unusual wear
- No obvious gas leaks anywhere in the system
- Gas port is at .09" and is not under-sized
- Rifle is clean with plenty of gun oil, it cycles great by hand
- Ammo doesn't matter, it won't cycle regardless of what brand it is
- Gas rings appear to be functional
- I have shot the upper on a different rifle lower with a different buffer spring, it did not change the rifle's performance
- I cut down my buffer spring (9 coils out of the spring) and got the rifle to shoot ten rounds correctly, but it still did not lock back on an empty mag, and as soon as I switched ammo brands it went back to short stroking.

Total shot in the dark

Is the head spacing correct?

Are the feed ramps correct?

Seems like the only thing you have not checked.

One, never cut buffer springs.

If you cut 9 turns off a spring and it works, something is seriously wrong to begin with.

Sounds like you probably have some mismatched combination of buffer parts like a rifle spring in a carbine system, or an airsoft (short) buffer tube or something like that. Maybe more than one thing going on.

The usual suspect in 99% of cases is a gas block not mounted in the correct location on the barrel. But if the buffer system is messed up who knows.

What buffer tube, buffer, and spring are you using?

Pull the charging handle fully to the rear, and look in the ejection port. How far behind the bolt catch is the bolt face?

iirc a rifle length buffer is 5.15 ounces, if you cut the spring you took weight from buffer system, which makes me wonder if you have correct rifle length and weight buffer.  

By cutting the spring you're trading one problem for another, that's not the answer.

I have not checked those, the upper came completely assembled from Rock River so I ASSUME that those were checked before it shipped out, but who knows.

As far as the buffer kit goes, I used a cheap one that I found online. Truth be told I couldn't tell you what the brand even is; however I know that I ordered the one that I did because it said it was for a 'rifle' length AR. Whether or not it was good quality... it was like $30 bucks...

I'll have to get back with you on the distance from the bolt face to the bolt catch. Right now the upper isn't on the rifle so I'll have to put it back together.

Stock/buffer/buffer tube/buffer spring has nothing to do with what kind or length of upper is on there.
That’s the first tell that things are likely mis-matched.

I don't think head space and feed ramps have anything to do with your problem.

One question that likely will reveal the problem, what stock do you have?

Is your stock a fixed rifle length stock or a telescoping CAR/M4 type stock?  A rifle stock, A1 or A2 length, needs a rifle buffer, receiver extension (buffer tube), and rifle spring.  With a few exceptions, if your stock is an adjustable stock then you need a CAR receiver extension, CAR buffer, and CAR spring.  Your $30 buffer kit is most likely mismatched with your stock.  My guess you are using a rifle buffer and spring in a CAR length receiver extension and stock.

The only thing that makes me question the above is you said you put this upper on another lower and it still short stroked.  Did you use that lower's buffer/spring system or did you put your buffer and spring in that rifle?

Here is a list of common things to check when troubleshooting short stroking.

Possible reasons for too little gas=
Too small of a gas port?
Gas leaking at the gas block/barrel connection.
Gas leaking at the gas block/gas tube connection.
Loose gas key.
Gas block not aligned with the gas port.
Faulty BCG= bolt tail fitment into the carrier is too loose or gas ring vent holes are spaced wrong.

Possible reasons of added friction=
Rough/tight chamber
Buffer or buffer spring too heavy.
Bolt/barrel extension not timed correctly.
Gas key/gas tube not aligned correctly.
Faulty BCG= Overly tight gas rings. Bolt, firing pin or cam pin binding with the carrier for various reasons.
Receiver extension not in a straight line with the receiver.
Receiver misshapen.
Rough or damaged surface inside receiver, receiver extension or on carrier.
Foreign material loose in the carriers path.
Friction from the mag or the next round in the mag on the carrier.
Hammer wedging between the carrier and disco.
Cam pin binding on receiver.
Extractor binding on barrel extension side wall.

Edit= The carriers travel distance can also cause short stroking. A rifle buffer and spring in a carbine tube would be an obvious no no (as mentioned), since the bolt could not travel far enough rearward to pick up rounds from the magazine. Since you can manually load rounds with the charging handle, I assume your bolt is able to travel far enough to pick up rounds though?

When I tried shooting it on a different lower I used the buffer/spring system that was on that lower, not the one off of my rifle.

The stock on my gun is an adjustable one, not a fixed length one. When I ordered the one that I did the description claimed it was for a 'rifle' length gun, but it wouldn't surprise me one bit if it was for a carbine length gun.

Correct, when I cycle the action by hand it has zero issues picking up a new round or locking back on an empty magazine. It is not difficult to cycle the rifle by hand either. Just out of curiosity, do you know what the length of a 'rifle' buffer tube is? The kit that I ordered said it was for a rifle length gun, but I would like to take some measurements and make sure that this really is the case.

A rifle buffer in a carbine tube will not feed from a magazine. A rifle spring in a carbine system will cause short stroking.

Would be easier if op would just post some photos.

A rifle buffer and spring will not allow the bolt to come back far enough to strip a round if installed in a carbine tube. A carbine tube is about 7'' and a rifle tube is about 9.5''. The rifle buffer will be longer because the tube is longer, the springs are also a little different length. You must have installed a carbine buffer if you're able to cycle by hand. The important thing is that the carrier travels far enough but not too far, which is determined by the buffer bottoming out in the tube. I check travel distance by pulling the charging handle rearward until the buffer bottoms out in the buffer tube and then looking at where the front of the bolt stops in the ejection port cut out of the upper receiver. The front of the bolt should stop about 1/8'' before reaching the rear of the port cut out on the upper receiver.

As to your question of measurements=

Spring length non-compressed=
RIFLE: 11 3/4'' minimum to 13 1/2'' maximum
CARBINE: 10 1/16'' minimum to 11 1/4'' maximum.

Buffer length=
Rifle: 5.91''
Carbine: 3.26''

Out of curiosity, have you contacted Rock River about the upper? Since you bought it as a pre-assembled unit, one would think the gas block should be lined up properly and the gas tube to be set correctly.

If it isn't anything with the spring / buffer, it may be something they did at assembly.

The fact that it has issues on 2 seperate lowers with multiple types of ammo makes me think an upper issue of some sort.

I dont know, just my 2 cents.

Well, there’s your problem.  You have a rifle length buffer/spring setup in a CAR stock setup.  I don’t know why it didn’t work on the other lower but that’s the problem with yours.