i'm going from a 20" barrel to a 16". also going from a .223 chamber to a 5.56. how much velocity can i expect to lose. lets say my velocity is 3600 fps out of a 20" barrel chambered in .223. can anyone give me a ball park figure on this ?

Time to get out the chronograph and not guess!

You are normally going to lose about 200 fps. The 55 gr. factory Winchester ammunition will make 3200 in a 20" and about 3000 in a 16". Going back and forth from .223 to 5.56 is going to affect pressures a little, and will have some affect on velocity as well - but that part of it will be minimal.

thanks Roskoe