Some civilian model Colt's used larger pins in their lower fire control groups, all others use small pin which is what 99.9 percent of everything you find out there today. I would suggest you stay with a DPMS, Stag, Mega, Bushmaster, or any of the standard lowers. Colt would be a great choice too if you happen to find a small pin model.
Just pay attention when purchasing a fire control group. Everything else is interchangeable. On some calibers you will need to be concerned with bolt face diameter, ie: 223 v 7.62x39 or 6.8 or 450 Bushmaster, etc. I would recommend you get the barrel and bolt from the same supplier.
The only bolt and barrel you cannot purchase from a supplier is the WSSM line of calibers, the reciever requires extra maching and can only be pruchased as a complete upper. Most all other calibers are exempt from this issue that I am aware of.
It may be less expensive to purchase a complete AR rather than build one because of speacial tools needed, however, if you build it, you will have exactly what you want. Then you will build another, then another.
