I saw a story recently about HK "possibly" introducing a semi-auto MP7 pistol for us mere mortals.
This got me thinking about the question of "What is a good, effective, semi-auto PDW?"
(*PDW for this discussion being a larger-size pistol, carbine or SBR capable of holding 20+ rounds, semi-auto only!)
Personally, I just don't "get it" with regards to the ultra-small cartridges like HK's 4.6x30mm or the PS90's 5.7x28mm.
So, for what I'm interested in, what would be the general pro's and con's of the following cartridges, in terms of 10-100 yd. effectiveness, ammo availability and g