Buffer and buffer spring recommendations

Not trying to start another "my 14.5 middy is short-stroking" thread, but…..

I have a 14.5 midlength bcm upper that was running tula .223 very reliably but I recently went from a fsc556 to a mb556 and a NIB BCG vs. a "regular" BCG. Now the rifle won't cycle the under-powered tula. I now that the buffer and the spring are usually the first things to look at, but they are both carbine so I'm not sure what other options I have there. The gas rings on the bolt are the standard three ring set where as my "old" BCG had a one piece gas ring. Could that be the issue? I haven't had a chance to try the rifle with my old BCG. The muzzle device can't be the issue right? since the round has already gone past the gas port….I would think that would clear the mb556 from being the culprit.

My only other thought is that the low profile gas block was not aligned right during the new rail install, but that work was done by a reputable gunsmith. Am I missing something?

So, the rifle cycles 5.56 just fine, but since I have a lot of tula, it would be nice to run it in not "bolt action" mode. Thanks for your help!!