I initially had the spacer installed with a carbine length spring and carbine length buffer. This would not allow the bolt to go back far enough to pick up the round to chamber. Once I removed the spacer it worked correctly. Only problem I have now is the spring doesn't have enough power to fully chamber the round. I fear that if I put a rifle length spring in it that this will not allow the bolt to go back far enough to pick up the round. The UBR 2 manual states that you aren't to run a rifle length spring in it.