standard issue for older class SSN security
On my first boat, a 637 class, the small arms locker had 3 flavors inside....
M-14, 45 year old .45 ACP government model, and 870's with pistol grip and folding stock.
Our range master (the Torpedoman Chief) instructed all the topside sentries to shoot it from the hip and we shot silhouettes at ~10 yards and by the time you emptied the shotgun, the targets always seemed to be peppered up pretty good.
The Officer of the Deck could issue the M-14 or the 870, but the 870 was always preferred as it was a lot easier to climb up and down ladders and go through water tight doors and hatches with.
We were always in a protected port with security provided by the shore command at the lowest alert level, so the darned things were carried empty, no matter what it was.
Yes, small arms training aboard ship left much to be desired, but at least I never had any of my guys "overachieve" and shoot themselves in the foot.
We were VERY proficient with all the weapons that had a 21" caliber, however