Shot my new-to me(read used) acr yesterday. Experienced roughly 1/5 rounds failing to eject. Almost 100% of these was the fired case stuck in action, often riding up and over the next round. The ejection pattern was slightly erratic as well.

Tried it with three different types of ammo - Mild 55 sp handloads, american eagle and finally hornady superformance. All three experienced similar failure rates.

Rifle looks to have fired around 1000 rounds before me, was rebarreled with a green mountain blank to 18.5" (Canadian minimum length for non-restricted)

It sounds overgassed or not assembled correctly by the previous owner.  Who did the barrel work? A machine shop or an actual builder?

Try this.  Switch it to s mode and see if it runs.  If so it’s overgassed

If it was consistent ejection I’d say replace the brass deflector with a v2 and do the ejector and extractor springs

Thanks Bigdog - it was what I'd consider a combination of builder and machine shop - they do a lot of work on all different types of firearms, but i'm pretty sure they do know their way around the acr as they do a fair number of them.

overgassing was my first thought, brass deflector, ejector/extractor is my hope..

I'll give the "s" setting a go and see what happens.

Any update OP?

Mine did the same thing when over gassed.

I haven't been able to get out a fire out sadly.

after doing some digging and it seems these "builders" may have just used ar barrels with too large of gas ports.

I've since parted ways with the rifle to a new owner, who is well aware of the issues and will give it the time of day.