WE M4 bolt won't lock open on last round

[GHammer80]

I know there is a switch on the magazine that can allow this happen depending on it being on or off. I've tried it on and off multiple times after emptying 10+ rounds. The bolt always goes to the closed position on the last round, anyone know why this happens? This is a new gun so many I'm missing something.
Thanks

[GHammer80]

It locked back once today, but seems to be doing it inconsistently.

[technosnob]

You may want to start here....

http://www.gasguns.info/forum/viewtopic.php?f=43&t=2605

This forum is a good source for all things GBBR

[ShelledPants]

Ghammer, it can be a number of things.

Things to do. Follow my guide in the bottom of my signature and clean the gun. If things aren't running smooth and lubed, then something may be dragging just long enough to stop the catch from working.

Is the magazine fully gassed? Sometimes, if it's cool enough, or if the gun isn't gassed fully, you will not lock the bolt back.

Check your white spacer on the bolt. if it's moved from ANYWHERE except the rear of the bolt... then you can get weird problems (starts with the bolt not locking home, and can cause things like no fires, and trigger group lock ups.

Best of luck,
Tom

[GHammer80]

Thanks guys!

[Qlong]

I just brazed on a thin sheet of steel, about 0.75ish millimeters. It was shaped into an "L" and brazed at the lock. It has been locking 100% after 10+ mags. The extra bit of material helps as more surface area of the lock contacts the bolt.

EDIT: I also have the shims on the lower.