I told you I was getting a bit airgun crazy again, didn't I?
Five or six years ago I bought a Slavia 631 spring piston air rifle at the Rickreal flea market. I only gave $40 for it, as I am a shrewd negotiator. It came with a Simmons scope but no iron sights.
The rifle looks too short because a couple of inches of barrel is missing. The previous owner must have had a problem removing the front sight., or dented it, or something. So he removed an inch or two...
Really, it's just too short.
I turned some random aluminum alloy down at the end on the South Bend and drilled it to fit the barrel. I was going to bore it exactly to size, but the drill I used cut just right at the diameter I was aiming for (which is technically a mistake, but it actually didn't matter).
Turning the OD (it's scrap so it was dinged and had spots of oxidation), grasped by the short section I turned previously and supported by the live center so as not to stress the thin tube section it's chucked by.
I moved the work over to the Jet 920, which has a DRO for cutting the grooves.
I drilled and tapped for some #10-32 set screws on the underside, but I should have used something like a 4-48, as I only got about two threads engagement, and the setscrews had to be ground down to about 1/8" of length. Poor planning just like the drilling, but it was a "for fun" project so I just winged it and it holds fine.
Now the rifle looks pretty, which was the whole point. I took it out and shot it a bit and the muzzle brake makes it easier to use, the longer barrel giving more leverage when cocking.
Here's the .pdf of the Slavia 631 manual
A Czech airgun forum
A Slavia 631 Tuneup