I guess I was being vauge; I find that levels of realism increase when one introduces a wider assortment of firearms or ammuntion in a logical manner. Only in very rare circumstances should any PC be able to have access to the exact gun and ammo they want.
In a military or law-enforcement game, the player's weapon will be issued to them, so will their ammo. D20M includes a good sidebar on dealing with requestioning of supplies; although, Spycraft does handle this mechanic better.
In a game where the PCs are just civilians, well, access to weapons and esoteric ammo is even more difficult. Gun laws vary widly, I know, but with the exception of handguns, sporting rifles and non-combat shotguns, the PCs will have had to worked very hard to get access to any sort of military grade weaponry.
That all said, with regards to variant ammo, I tedn to control access to it even tighter than I control access to weapons. It will be a very, very rare occassion for a PC to get access to the equivilent of a "Vindicater Minigun with Explosive APDS" in my campaign.
is it your experience that variant ammo rules result in this realistic equipping occuring?
Yes, and no, it is my experience that sound rules for variant ammo adds realism to the game, when that variant ammo is introduced in a logical, true-to-life (or at least, true-to-genre) and above all balanced manner.
Sneak a peak at Ultramodern Firearms next time your in your local hobby dunge- er, shop. The Ammo chapter is short, and the author's intro argues my point much better than I can.