Can a stock lower (5.56) be used with a Oly .45 cal upper? Do I need to change the hammer or other parts?

Heres what I would like to do, I would like to build my first rifle and use the lower to interchang with other uppers. I want to start with a .45 and later buy a .22 (I'm using a Cierner in a Colt now).

The parts I'm looking at are:
Stag lower W/LPK
Ace stock, entry w/1" pad
G27 grip

Would this lower setup work with other configurations or are pistol cal.LPK's differant than 5.56(or .308 for all that)

wadayathink?

A standard 556 lower will work with an Oly 45 upper without changing any fire control parts. 308 will not work with standard lower for many reasons. A pistol lower is the same as a rifle lower, it is just registered as a pistol not a rifle when it is made. A rifle lower can never be made into a pistol, but a pistol lower can be made into a rifle. If I am wrong somebody will correct me.  

Quoted: A rifle lower can never be made into a pistol, but a pistol lower can be made into a rifle. If I am wrong somebody will correct me.

You are 100% correct.