Hey guys:
I hope I'm not beating a dead horse here, I couldn't find another thread here that covered this, so here goes:

I'm looking at rebuilding my current carbine upper with at least a mid length gas check. I like the 16" but I'm trying to decide if I should go with a true mid length with an 18" barrel.
What are the advantages of a mid length over a carbine? Does it extend the range out much beyond 500 meters?
It seems like most military balistics charts, etc. are based on the M16 or the M4. I haven't really found anything on the mid length "recce" guns, so any info would be appreciated.