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#4 Buck
Mossy 500, 18.5", full stock
If you're worried about overall length w/full stock (tight quarters and/or flagging), give "short-stocking" a try. Learn a bit about it here:
https://www.youtube.com/watch?v=PCqJIW4_K7s
I can't stress this enough -- practice, practice, practice -- even if it's just dry-firing. I make this part of every range day in a tactical bay. I practice this with both shotgun and AR.