Safe's are like couches, the heavier the better. Sentry ain't so good. Don't forget you will probably have to pay someone to deliver it also. If you can put it in your truck by yourself, you dont want it. Go to a gun store, not a lumber yard for a good safe. $1k can get you a good one, $400 will not.
A safe that holds 12 rifles will hold 12 "standard" rifles. 12 scoped AR-10's won't fit. If that 12 rifle safe includes some of the rifles on the door, the ones on the door must be shallow indeed-no chance of any AR on a door.
This one retails for $1k and stores 18 long guns (barely). Buy at least 50% more space than you currently need. You will buy more guns eventually, and can store your ammo in the meanwhile (good for firemen and humidity).
I like electronic locks b/c the combination can be changed if compromised, can make multiple combinations for other users, and even find out how many times "user" codes have been used. Or even reset all codes using a unique factory master reset code. Most of the better electronic safes use the same locks, you will recognize them once you start looking, and they are top notch.
A good safe manufacturer will give you a free opening service call after a fire, and warranty "some" of the contents against loss. I hear some insurance companies will give you a discount if you have a safe, but State Farm doesnt.
Don't pay for lighting inside the safe. Just buy the battery powered "tap lights" and velcro to the ceiling. You want 10 or 12 gauge steel only. Exterior hinges are fine since the bolts are on both sides of the door anyway. Look for something like "heat-expanding seal around the door seals", and insulation inside. You will need this