Ok fellas. Buffer questions. I've built numerous ARs and always tuned them with an adjustable gas block and was good to go. I have a s&w m&p 10. I put a seekins adjustable gas block on it. So I could tune it to run with a can and not be over gassed. With the screw on the gas block backed all the way out. Allowing as much gas as possible. It still will not lock back on an empty mag. It will lock back when charging handle is pulled to rear. So it's not the mag. However it does eject to 3-4 o'clock. So I'm thinking a lighter buffer but, the factory buffer from s&w is a carbine buffer (2.95 oz) I'm afraid if I go much lighter it will beat its self to death. Should I try a lighter buffer or spring? Or is there another option.

Check your alignment of the gas block over the barrel port, you may need to slide it fore/aft or roll it.

IIRC, when I swapped my factory SW block out, the new block sat a bit further forward than the factory block did.

I had a Noveske barrel with intermediate gas once that was out running the mag catch. I had to add weight to slow it down. Maybe not what you're experiencing but I've yet to see a carbine buffer work in a 308

Was the barrel dimpled for the new gas block?

This, it sounds like you didn't get the gas block aligned.

With the screw backed out (not removed) from that Seekins block, it functions the same as any other gas block. It won't restrict the gas any more than your original gas block did. If you have function problems after installing it but not before, then most likely it's an installation issue.