Well the non-shrouded BCG is very much mill spec,,the newer design of hammers work well with them,,the older style hammers with a notch up front,,will not..best way to tell if yours is non-shrouded,,if you can see 1/2 inch or so of the back of your firing pin,,its un shrouded...I was just making sure everyone got the heads up about it,,so noone blamed it on the Plum Crazy design,,,because a few may be building "beater" rifles with them and possibly using that type of bolt(most of the uppers in the 1980s used these soley) hope this helps......X