Applejack is a Mosin-Nagant: rugged, dependable, simple, and with a stock that matches either her coat or her mane.
Fluttershy is a suppressed (not ported as in the picture) Ruger Charger: very quiet, hardly any recoil, and used only for plinking.
Pinkie Pie is an AR-15 with enough tactical rails to mount
|Like this, but in pink.|
Rainbow Dash is the aforementioned AA-12.
Rarity would be an $800,000 VO Falcon, because if you're going to shoot, do it with style, darling.
Twilight Sparkle is an M40A5, because it's the rifle of Marine snipers and obviously requires tons of training, including memorizing lots of facts about ballistics.