Built my 300 blackout SBR and fired my first 20 rounds through it yesterday. Using Remington UMC 120 grain ammo and had about 5 or 6 failure to eject malfunctions in the 20 rounds I fired.  The gun would fire (quite accurately I might add)...the empty casing would be extracted....but would remain loose in the upper receiver....while at the same time the bolt would be cycling a new live round into the chamber.

Everything in the rifle is brand new with the exception of the BCG and charging handle. The BCG is from a department issued Bushmaster and has an unknown round count on it.  I did however clean it quite well before using it. One other issue I had was 2 out of 4 magazines didn't lock back on an empty chamber. Any thoughts? Is it strictly an ejector issue or are there other possibilities? Thanks for any help!

What buffer are you using?

Standard carbine buffer and spring. It came stock in my Windham lower.

The BCG is well lubricated right?

What you are describing can be caused by lack of lubrication.

could also be ejector or ejector spring or the gas block hole may not be lined up well with the hole in the barrel causing an under gas situation

did you put the gas block on the barrel or was it factory built?

if you cycle it  by hand does it eject the round everytime?

I think worn gas rings could fo this too.

Do you have another Bolt or BCG to try in it, or one you could borrow.

Or just change the gas rings and see if that fixes the issue.

I'd try an H2 buffer.   It solved that problem with my sbr.

I built the upper myself using a Spikes upper, Shadow Ops barrel, and a Troy low pro gas block. Obviously I thought it was lined up well when I installed it and the underside of the barrel has an indent for the gas block set screw which I just double checked to make sure it was still tight after firing.  Everything is still solid and seems to be in line the best I can tell.  I just bought a new extractor and extractor spring that I'll be installing in the bolt to see if that takes care of the problem.  

The ejector seems to have the right amount of tension to it. Like mach said, my BCG may have been a little under lubricated. It was slightly shiny in appearance but I wonder if I shouldn't run it a little wet during this break in phase of the rifle.  I could probably borrow a BCG to see if that helps any as well but I don't have another one on hand.  As soon as money allows I will be buying my own BCG and ditching this older department issued Bushmaster BCG.  

I may also try a heavier buffer if none of the above gets it worked out.  I'll try to hand cycle some rounds in and out of the chamber once I get my new extractor and spring installed. Thanks for the help again!

You should always run it wet. Always.

It is a machine, a dirty machine and you need to do two things

1. lubricate the action and reduce friction
2. carry the carbon build up in suspension so it doesn't foul up the action.

My son build a 10.5 SBR. Followed the directions on froglube shot it and put it away.

4 months later we take it to the range, BCG was shiny. It would not cycle one round, not one. The froglube had turned to adhesive and was very sticky. Wipe  bore patch on it and all the white fibers from the patch stuck like glue to the BCG.. Took it hope, got all this shit off. Used real lube like mobil one, took it to the range and never a malfunction.

Always run it wet and if you use something like CLP you will have to keep applying it because it dries up with use and over time.

I like mobile 1, cheap, and stays on forever.