I need a little help. I changed the barrel on my AR from a .223 to a .300 AAC. I test fired it tonight and it ejects the spent shell cleanly but does not load the next shell. Not sure where to start, any ideas would be appreciated.

Either ammo, spring, or faulty gas system. More info needed. Give us something to work with other than it don't work.

Mag spring sounds like the offender here. What mags are you using? feed ramp clean and lined up?

First I would try a different mag.

Next I would think gas system. Maybe gas block alignment or leak.

Next I would be thinking a lighter buffer/spring.

Next I would be thinking its weak ammo.

It is a brand new PMAG, first time used. The shells feed fine when you pull the charging handle back and release it. The fact that the shell ejects leads me to think gas system not pushing the bolt back far enough. I put a new gas block on the barrel and used the original gas tube. Does the gas tuibe have any seals on it like orings? The gun fed good with the oiginal barrel so I am not thinking springs. It does not even move the next round.

No gas tubes dont have orings.

I would still try a different mag just to rule that out since its so simple to do.

My best guess is that the gas block is not lined up over the gas port all the way causing a slight blockage. Is this a pinned gas block or a clamp on or set screw type?

Sounds like a gas problem, pull the gas block and make sure the hole in the barrel is clear, and it lines up with the gas block hole. Measure the distance of the gas block hole from the end of the gas block, MOST are setup so they don't set up flush with the step in the barrel. ALL of our blocks, we got in DO NOT seat against the barrel flange. My .02 worth.

How short is the barrel? the shortier the less blow back

It is a 16" barrel. I think I may have just found the issue. I bought 8 boxes of factory Remington ammo at Academy the other day. I think it is subsonic. I just looked the code number up on the Remington web site and it says the velocity is 1016 ft per second while all the other loads are over 2000. I guess I will be trying to trade it with someone who wants the slow stuff. Does the subsonic ammo normally cycle in an AR?

You have a carbine or pistol gas system?

It is carbine length. Can anyone tell me if the subsonic hould cycle OK normally? I know in my .308 it will not.

Subsonic 300 BLK will not cycle in a carbine length gas system. You have to shoot supersonic loads in a 16" carbine length barrel.

What if you run an adjustable gas block? I have no clue just asking.

It sounds like your rifle is short stroking.

If you replaced the barrel or built a new upper for your AR yourself, it is most likely that you have the gas port and the gas block misaligned. A surprisingly small misalignment can have a large impact on the function of your rifle. I would start there.

Another possibility may be that you need a lighter buffer or you need to trim your buffer spring, but thoroughly satisfy yourself that your gas system is perfectly aligned before you do anything that costs money or permanently alters components.

EDIT: NEVERMIND, NEVERMIND, NEVERMIND. ALWAYS READ ALL THE WAY THROUGH A THREAD BEFORE COMMENTING.

I just saw that you were shooting subs. They work a lot better if you run suppressed (more back pressure). I would still verify that the gas system is perfectly aligned, then I would consider installing the lightest buffer I could find and possibly a low-mass bolt carrier. OR, I would either (i) get happy with working the charging handle or (ii) go to supers.

Thanks guys.

Correct me if I'm wrong, but a pistol lenght gas sytem is designed to function with both. Trade for some supersonic ammo, and your problem should go away. (or just sell it now at a profit and scroung around for desent supersonic ammo till this craze dies down.

I had the same problem and was told this by the gun shop

I hate those crappy Remi's but I learned if I push the ammo forward in the mag, I get just enough gas to cycle next round. I also have a bunch of boxes of PNW Subsonics that cycle perfectly...