MSH: I don't have access to the statistics, but illegal mods definitely trump outright illegal arms. The other big area for enforcement (in fact, probably the primary one) is straw purchasing...legal purchasers buying legal firearms, but doing so to sell/give to illegal purchasers (convicted felons/non-citizens). When you have a little old lady from Pasadena buying two AR-15s a month and paying in cash, something's probably not what it seems. Or a store that just seems to conveniently "lose" 10% of their inventory each year.
Favorite thing I ever shot was a Mauser Parabellum 9mm. Smooth as silk, almost no recoil at all. Slimline barrel, and the one I was using was silvered. A true gentleman's weapon.
Most awesome thing I ever shot was a
LeMat (modern replica, not an original).
EDIT: Okay, runner-up most awesome was a friend's smoothbore flintlock musket. (He was a Revolutionary War re-enactor). Made his own powder charges and showed me how to rip open the charge with your teeth, load the charge, load the wadding, load the pan, take aim (that part is really more for form than anything), then pull the trigger and go "HOLY SHIT" as you proceed to set off an earth-shattering kaboom. Even with nothing but powder and wadding in the barrel, you can shred foliage for about 10 yards in front of you if you shoot it into the woods.