The Mosin receiver is pretty soft for a military rifle but given the circumstances of the time (needing millions of them quickly) they are still holding up well 70+ years later. This receiver measured Rc32~34 in several placed before I started so my choice of material was easy, I used hot rolled steel for ease of machining and weldability. The bridge doesn't get any strain or pressure, it's just a vertical spacer for the pic rail. Lots of grade 8 bolts have been used for bolt handles on many surplus rifles, I just made my own.
This is still one of my favorite shooters. It's a bit heavy but superbly accurate with handloads. I got an Arisaka receiver in a box of parts a while back, I've been thinking of building it back up in 54r as well just to have 3 oddball rifles.