Help with feed/eject issue

[Armed Civilian]

I have a Stag Arms lower, Core 15 assembled upper (9.5 inch barrel) pistol length gas tube, in AR pistol configuration, a Phase 5 pistol tube with the carbine length buffer spring and standard buffer out of my S&W M&P 15 carbine.

Using Remington 115gr supersonic ammo it will fail to eject and slam the spent casing into the side of the next round as it leaves the magazine and moves into the chamber.
It doesn't do it all the time but it feeds 5 or 6 rounds correctly at best before the failure.

I inspected the extractor in the Core 15 BCG and it has no O ring on the spring so I brought along the BCG from the S&W M&P 15 which does have one. It didn't solve the problem although the problem changed to a failure on every shot and leaving the spent shell in the chamber half way extracted on one occasion and the second time it managed to chamber the fresh cartridge but the bolt went only part way into battery and the spent case was stove pipe.

I tried a few different magazines and different supersonic ammo varietys but they gave no change.

Now here is what I hope is the part that gives you the info you need to help me solve the problem. I ran subsonic 220 gr ammo and it was flawless!

I'm thinking a heavier buffer and/or spring but I'm a total rookie with the AR in general as well as 300blk so I'm hoping for some shared wisdom here to get me to a solution.

There is a slight bit of residue around the gas block area after each shooting session....should there be no sign of gas leakage there? It isn't heavy but it is noticeable if you shine a light down the rail/tube.

Thanks!

[rjacobs]

That setup needs an H2 buffer to slow things down. Dont change from the standard GI spring though.

[300Blk]

I can't tell without having it.

[Armed Civilian]

Thanks for the help.

I put in an H2 buffer and a new carbine spring and the failures have almost stopped completely. I had 3 failures out of 80 rounds of supersonic, 1 with 60 rounds of Remington UMC and 2 with 20 PMW.

With subsonic it still runs flawlessly, although it slowed down the cycle time...at least that's what it feels like.

I forgot my oil so the new carbine spring and H2 buffer went in dry other than residual oil in the tube. I don't know if that would matter.

Should I try an H3 or continue to break it in with the H2? This weapon is only 200-240 rounds old at this point.

Again, thanks for the help!

[rjacobs]

Leave the H2 in there. I never lube my buffers or springs or tubes, generally just leave em dry. Never thought to lube them.

[robrob]

Make sure your extractor has a rubber o-ring on it. If it doesn't install one. Search for o-ring and you'll find the details on the fix. It's an easy fix and will solve most of these issues.

[Armed Civilian]

Make sure your extractor has a rubber o-ring on it. If it doesn't install one. Search for o-ring and you'll find the details on the fix. It's an easy fix and will solve most of these issues.

It doesn't, but as I mentioned, when switching out the bolt carrier group with one that has a bolt with an O ring equipped extractor it didn't solve the issue.

With the H2 buffer and the original non-O ring bolt back in there the failures are greatly reduced.
I'll go ahead and install an O ring in that original bolt anyway and see what it does though.
Thanks