Up the barrel of a Decommissioned gun.

https://lemmy.sdf.org/post/703424

Up the barrel of a Decommissioned gun. - SDF Chatter

I’ve been looking at this for 30 minutes expecting to see james bond
dont look in the barrel it could shoot you in the eyr
Well, not when you look into the breech.
That's very interesting; it seems to have a progressive twist rate. I assume that this is from a large bore armament of some kind? I believe that gain twist is sometimes (usually?) used on artillery. (See: https://www.youtube.com/watch?v=KI-_CtmOzFs)
Ask Ian: Progressive Twist (Gain Twist) in Small Arms?

YouTube
Rifiling can’t have too tight of a twist or it will cause overheating, jams, and other negative effects on the rounds. The same twist is (at slightly different profiles) used on most modern firearms and artillery.
I see rats in a sewer.
Rocks and pebbles. Not entirely sure how they got there.