Inside diameter for barrel extension slightly undersized for a tighter fit on the barrel, and a more accurate rifle. On a precision made milspec upper receiver like the BCM4 Upper you purchased, you will need to apply heat with a heat gun to the barrel extension aperture on the receiver. Simply apply heat with a heat gun to the forward end of the upper and the barrel will slip right in. This creates a very tight hold on the barrel and eliminates the need for bedding of any kind. All BCM® complete upper receiver groups are built with the very same upper receiver forgings like the one you purchased.