Jeff, I have heard alot of bad things about the Mini-14. General feeling among many is it is a POS that jams alot.
As for the Glock or 870, both are proven guns. But, they fall into two very different categories.
Not to push it on a Newbie, but for taking out alot of perps fast in close quarters, you might consider a Class 3. Class 3 includes: Short Barreled Rifles, Sawed-Off Shotguns, Machine Guns, Silencers, and AOWs (Assorted Mix Of Less Regulated Short Rifles, Sawed-Off Shotguns, and Novelty Guns Such As Pen, Briefcase, Wallet, and Ring Guns.)
Machine Guns would be best, but a Reg. Rec. M-16 runs $8500+. An M60 runs $15000+. The cheapest are Uzis and MAC-10s. An Uzi SMG can be had for around $3000. MAC-10s are sometimes as low as $1000.
The other choice is an AOW Shotgun. You can get a Factory Pistol Grip Shotgun (Must Have Never Had A Stock) Now, File A Form 1, and wait to receive it back. Then saw off the Barrel to whatever length you feel like. Short Rifles can be good. YOu could go the CAR-15 Route. File A Form 1, then when you get it back approved, add a 7.5" - 11.5" Upper to the lower receiver and voila a legally reg. SBR.
Outside of Class 3, you can get an M4gery with 16" Barrel and a Reflex Sight. This allows quick aquistition of the target. 9mm Tactical Carbines also hold their own. The peep sights are great for drop the Perp at up to 100 Yards quickly. These include Hi-Point 9mms which run $169 - $219, Uzis which vary considerably, and MP-5s which the cheapest to build run $900+.
Pistols are great for Close Encounters where quick firepower is needed. They can be stored loaded unlike most rifles since the hammer can be decocked.