the "ratings" on safes are for unscoped long guns. like a shotgun collection or something. or lever guns with just irons.
so if you need to house 15 long guns with optics, you're gonna be tighter than a bugs borthole in a "30 gun" rated safe, especially if a selection of them ar AR style rifles, or chassis style rifles with their necessarily higher optics mounts, as well as pistol grips.
this is coming from someone who owns a "27-30 gun" safe - 30x20x56" - ive got a couple unscoped long guns in there (a single shot 22, couple shotty's, that kinda thing) and several less than your 15 scoped rifles, a couple loose scoped uppers and she's packed. like i have to move multiple things outa the way if i wanna get to other things - unless what i'm grabbing happens to be in the very front - level of packed.
i dont even think i can squeeze an extra upper in there between them.
if you need a good visualization..

i got a screaming deal on it used, so i cant complain, but i'm really wishing i would have gotten something closer to a 40x24.
so yea... go big. you wont regret it.