Okay, as you may have noted from the FedEx thread, the slip-on barrel mount came.
I followed their instructions, which included pushing, with a hammer, the ring as far on the barrel as far as possible. I drilled a barrel sized hole in a 2x4, slipped it over the barrel and used that as a way to really drive the ring as far onto the barrel as possible with the hammer. The set screws are offset and were very pointed, so in theory any attempt turn the ring one way or another is met by resistance from the screw on the the opposite side. Kudos to Brass Stacker for a very neat little product. They have lots of neato products for various platforms.
http://brassstacker.com/
So all that remains is to sight in.
Another one project completed.

Two things I'd never really noticed about the Mosin-Nagant, and I'm sure it may vary depending on model. First there's no safety. Load and shoot. I'm fine with that, just never really noticed before. Secondly I like the finger notch, if that's what it is, behind the trigger guard. A nice touch in shooting comfort. Not something you'd expect to see on an old miltary rifle.