hey all i have an armalite mid length.
i am considering getting a bcm bolt group.
my question is can i just drop it in or do i have to have head space checked?
i really don’t know much about head space so any info would be great.
thanks in advance 
Try your search fu…this has been covered very well several times in the past and a large amount of info is available on the entire subject of headspace and upgrading the BCG.
IMHO the BCM BCG is a excellent upgrade for the Armalite and have done so on my remaining Armalites as well…I follow Iraqgunz philosophy that every AR should be setup as a defensive weapon first and target gun etc. second.So I prefer to have the more durable and reliable parts in place regardless of intended purpose.
From personal experience I would also recommend a BCM or LMT charging handle as they are better quality,and replace the buffer/action spring with a BCM,LMT or Colt unit.
To answer your question,yes its recommended by most here to verify headspace using the USGI Field headspace gauge just to be on the safe side.