I've been reading here a while and I've come across a couple of things I have some questions about... (No I'm not gunna ask how to make an AR full auto) hole"
TIA
91101

Your question on pin holes, The early colt rifles had a larger front take down pin and fire control size.  This was to prevent the use of M-16 parts from working in the rifle.

The sear block is also a colt part, It is a block that is one sided pined into the lower receiver to prevent the use of a Auto sear(Lighting Link).

If your going to buy a Bushmaster, then all of your parts and sizes will be mil-spec(small hole).