I've had my FID for the past 4 years and just turned 21.
Low capacity shotguns = all pumps have no tube limit, and semi's holding 7? or less rounds.
Low capacity rifles = detachable magazines 10 rounds or less. If it is "readily capable of accepting large capacity feeding devices" then it's a no go. Which means almost all popular semi auto rifles.
FID under 18 only allows for transporting firearms. You must be 18 to purchase low capacity rifles/shotguns and ammo. If your buying ammo used in both pistols and rifles (.22LR, 9mm, etc) you need to state that it will be used in a rifle. Since you have to be 21 to buy a handgun you cannot buy ammunition for pistol use until your 21. Dick's, Walmart, and some other shops may not know this that well.
Pump and most semi shotguns are good to go.
Semi automatic rifles are tough since almost all are "readily capable of accepting large capacity feeding devices". If you call around you may be able to buy some unconverted saigas or single stack ak47's but most shops will deny you to play it safe. Ask around some will do it for particular rifles. AR15's, AK's, M1A's, AR10's, yada, yada, yada all have aftermarket mags that hold more than 10 rounds, therefore they are "readily capable of accepting large capacity feeding devices" and are considered high capacity = LTC only.
All bolt action rifles. Now is a perfect time to go to Dick's and buy a $400 Remington 700 and throw it in a chassis. Learn precision shooting. Once you get into long range shooting it can become addiction, like anything else.