Failure to feed....Why?.

If one mag works fine and another does not, is it possible it's a bad mag/spring causing the issue?

Many decades ago, the USMC taught me the seven phases of weapon functioning. "Feeding" was defined as the placing of the next loaded cartridge in position to be moved by the bolt or slide into the chamber. Then come chambering. Sounds like you are talking about 'feed' in that sense.
Failure to correctly position the 'next' round (round elect?) would seem to be a problem of either spring pressure or deformation of the magazine body. (This would indicate if the problem is with one magazine only, or with any and all magazines.) One should inspect the magazine to determine proper spring tension, and determine if deformation is the cause. In the Marine Corps, the solution was to replace the faulty magazine. Not having tax money in civilian life, one might carefully remove the deformation or replace the spring.

However, if this is recurring problem and with all magazines, the bolt may not be recoiling far enough to clear the rear of the case. If that is the problem, then cleaning the gas tube to insure proper functioning or cleaning and lubricating the bolt carrier and inner 'tube' of upper receiver may work.

Sometimes the mag release can do funny things. It's worth a try to loosen it one or two turns and test again. As long as the release lever doesn't fall out of it's slot when you depress the release button, it's all good.

I would add a turn (clockwise) to the catch but I’m curious about the ammo. You should also check for gas leakage around the block and do the standard bolt check for rings. Then if no problems, look up the correct way to lubricate the bolt and test again with other ammo.

Given that most uppers are over gassed and you are using a light buffer it should be capable of cycling in-spec .223 My next inclination would be to swap magazines then the catch. I would change one variable at a time to ensure isolating the problem; improper presentation of mag, bad mag, underlubricated, leaky bolt, misaligned gas block, or weak ammo.

These have already been mentioned, but If the bolt locks back after last round, then I would look harder at the feed ramps since you said it does it with all mags.
If the bolt does not lock back, I would look harder at the gas block alignment. I had a Ruger AR that the gas block would creep around. It might shoot a mag fine, then stop cycling, then start working again. Drove me nuts until I figured it out.

OP, what ammo are you shooting?

I personally use actual 5.56 ammo for the first 100 or so rounds with each new rifle no matter if I built or bought it. And I'll agree with others.

Check that the bolt locks back 100% of the time on an empty magazine
Check the gas block that it is installed correctly and is not leaking
Check the gas rings on the bolt
Check the feed ramps.

One thing that gets overlooked at times is the gas key on the BCG and the gas tube alignment. If the key is loose or crooked or the gas tube is crooked, that can lead to issues too.

I purchased a complete upper with misaligned GB before, was a pain but easy fix. Speaking of uppers (and lowers for that matter) I’m not sure if S&W is forging their own or not. If “A” marked, it’s an Anchor Harvey, they supply a lot of raw forged uppers.

“A” visible top center of pic. I forget who’s upper this one was, probably an Aero.

Is the bolt picking up the next round or the bolt carrier?

If it's the bolt carrier then your rifle is short stroking.

Install a new Colt extractor spring.

338reddog said:

I am using Federal 5.56 brass case fmj. Bolt is not locking back. I took the gas tube off and realigned it. I have not checked the gas key but I did use BCG from my other AR.
I am leaning towards gas block

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It does sound like a gas issue and an AGB might allow you to "tune" the gas better. that's where I would throw my money having seen AGBs improve reliability in my AR10. And when you're adjusting the gas on an FN FAL, you open it up until it won't cycle and then fire a single round from a magazine loaded with one round and then you start closing it off, firing another single round the same way, close it off some more, fire another single round, close it off a little more, fire another single round, close it off and so on and so forth until you have given it enough gas to hold the bolt back after firing a single shot. then close it another couple clicks just for good measure and fire off a magazine and make sure the bolt is going all the way back and picking up each new round from the magazine. An AGB on an AR10 or AR15 would be basically the same thing. That's what I think but I'm no gun smith so...

338reddog said:

My AR upper is a Frankenstein. The upper receiver is supposed to be a S&W but seems to have similar markings to Aero. The bolt is Areo and the barrel is a ? 1:7 Wylde chamber. Seems to be very accurate.
Here is the issue( most of the time) it is a single shot. The bolt starts to strip cartridge from magazine but overrides it. The other day I ran a full mag no issues second mag every round became a single shot. Not sure if I need more lube (type?) or more or less gas or buffer. I’m using carbine buffer. Carbine gas tube.
I have tried on another one of lowers, same issue, but both have same buffer weight.same problem
My Anderson upper runs just fine
Jeff

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Hello Jeff i have a few questions for you.
What’s the barrel length?
What’s the buffer weight?

it sounds like the gun is short stroking, before you start playing with the gas block let’s do something,

1. disassemble the gun clean everything and lube the heck out of it, i mean everything that moves with oil, not grease. 0W-20 or 5W-20 motor oil works fine. Including the charging handle channel and bcg.

2. Make sure your buffer and buffer spring is carbine length, a rifle length spring in a carbine tube will/can short stroke an ar15. OIL the buffer, Spring and the inside of the buffer tube. Check the buffer detent cam move up and down freely. And that the gas tube is properly installed with the detent.

3. Put some Oil/wd40 inside the magazine body and spring.

4. Put a round in the chamber and fire it. Does it eject it? Yes/No?

if Yes, put a round in the mag and shoot it, does it lock back? Yes/No?

if yes, everything should be fine, if no, you have a gas problem.

remove the gas block clean the port of the barrel and gas block and reinstall everything back.

@338reddog, if Mist says replace the extractor spring, do that. I almost replied the same as it’s a recurring theme but I was reading it as a new build and Wylde chamber. Take his advice and don’t chase your tail.

I had a similar issue with a new AR I received in trade for 2 cases of 9mm ammo! I followed all the above recommendations chasing what was thought to be short stroking.

The solution turned out to be so simple I'm almost embarrassed to mention it! Some manufacturers ship their BCG's with a little "O" ring around the extractor spring. Somebody's idea of reliability enhancement. It isn't necessary! Removing the "O" ring solved the problem and the gun has been 100% since.
Smiles,

Ah. My suggestion was based on this statement-

338reddog said:

The bolt starts to strip from magazine but overrides it.

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Which is a completely different problem from-

338reddog said:

The round in magazine is not being stripped. Bolt closes on empty chamber.

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Your AR is short stroking. Gas drive has a problem with leaking/blockage/weak ammo.

Start with a lock back check using full power ammo. Use an H or H2 buffer and a new action spring.

PS- Don’t be surprised if your extractor spring is bad.

The extra power extractor spring for ok Colt or Sprinco provides more force to control the round and needs no O-ring to interfere with it. Many a bolt comes with a standard 4-coil which is probably fine until your round count increases to the point it isn’t.

For such an inexpensive part that plays a critical role, having the best quality spring made is a no-brainer. Lots of issues solved with a cheap solution.

338reddog said:

Not sure if the bottom of the bolt is dragging the round slightly out of the magazine. My explanation is probably not great as I’m new to this platform. The round maybe moves forward a 1/4” out of the magazine. Will do as suggested. I will take some pictures if problem persists. Do you think the o-ring on extractor spring could cause a issue?

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I understand about being new. I've been there myself and if I want to keep increasing my knowledge, I must remain new.

Knowing the correct language when asking for and giving help is essential for clear communications. But the only way to learn it is to jump into the conversation with both feet. You're off to a good start.

The bolt not catching the rim and closing on an empty chamber is a sign of short stroking.

The bolt pushing the cartridge partially out of the mag and riding up over the base of the cartridge (Bolt Over Base) and jamming the cartridge in the action is a symptom of weak extraction. What happens is the extractor loses control of the empty and the ejector cannot eject the case. For the ejector to work, the extractor has to hold the case head firmly against the bolt face and compress the ejector. The loose case interferes with the BCG as it attempts to feed a fresh round from the mag and lock into battery.

The only way the bolt can push the cartridge out of the magazine enough to jam it into the action is if the bolt travels rearward enough to catch the rim. The loose empty impedes the feeding of the fresh cartridge causing a bolt over base malfunction.

If the bolt doesn't travel far enough to catch the rim, it'll simply glide along the cartridge without pushing it out. There is like a ridge on the bottom of the BCG that keeps the round pressed down until there's enough rearward travel for the bolt to catch the rim. Therefore, short stroking is unlikely to cause a bolt over base.

O rings were used because the original extractor spring used in the M4 soon proved to be inadequate. Colt redesigned the spring, but the military refused to approve it because that meant adding another number to the system. While the "Big Army" dragged their feet in approving the much needed new spring, Crane came up with the addition of the O ring as an interim solution. It was used long enough it picked up a good bit of institutional inertia. Extractor springs that need an O ring to function reliably need to be replaced.

In my experience, there is only one extractor springs that can be trusted- Colt. I've had only one Colt spring fail and that's because I damaged it during installation.

I've never used Sprinco springs, but they come highly recommended by the late Will Larson, who was a very knowledgeable and experienced armorer. I had the privilege of attending one of his AR armorer's courses.

Getting back to extractor springs- Only one maker of ARs I know of uses Colt springs. That's Colt. The only two makers of ARs that I know of uses Sprinco extractor springs. That's Sionics and SOLGW. The extractor spring of nearly every other brand is suspect and should be replaced. (I don't include KAC in this category. I don't know what spring they use, but I've never seen or heard of KAC ARs having problems with extraction.)

Using an O ring with a Colt or Sprinco spring can exert too much pressure on the extractor impeding it from snapping over the rim of the cartridge.

You don't need an o-ring in a semi-auto carbine. The o-ring was added to address problems caused by the higher cyclic rate of the M4 (over the M16) and the higher chamber pressures during extraction as a result of the shorter carbine length gas inpingement system.