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"
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).