Hello all,
I'm having a problem with a mp5/40 clone made by Coharie.
Every time I fire a round the ejector get stuck under the BCG and jams it up.
It will cycle by hand no problem, but when I fire a round it will jam up this way.
What can I do to stop this from happing?

My Coharie 10mm clone did the same thing when I ran it in FA mode. For the most part it ran fine in semi auto.
Hate to tell you this but the only cure I've found is a complete German HK bolt group.

Having said that you can try polishing the bottom of the bolt where it rides over the top round in the mag and I also chamfered the front corners and their trailing edges on the bolt. I cut a bit off the back side of the 90° nose of the ejector lever (where it goes into the bolt)  and welded a little more material on the hump at the rear of the ejector lever. I was trying to get the lever to move up into the channel on the bolt a bit earlier. I did see some (not much) improvement by installing a HK ejector lever.

Good luck, I know it's frustrating, if you were nearby I'd let you trouble shoot with some factory HK parts.

Send it back to Coharie. It might take some time, but they do honor their lifetime guarantee.

Thanks for the info,
I was planning on getting some HK parts anyway

I agree with clicker,

at least thats what I had to do with mine to get it to work.

I ended up having to get a complete bolt/carrier combo. my cohaire parts were so out of spec, that I couldnt re-use any combination of cohaire and HK parts.

after the swap, of bolt/carrier combo, and a swap to a genuine HK ejector, mine is 100%.

also make sure your trigger pack is sitting at the right height on the shelf. I know my 9mm wasnt and it caused almost the same failures.