@ 09:43 am (GMT)
Nathan FosterOne more thing Mike, being a very old rifle, chances are that the throat is getting somewhat worn now. best to give it a quick birthday (polish) before going further ahead.
Once the throats become worn, it can pay to use a projectile with a longer bearing surface. In a worn tang safety model with 24" barrel, properly bedded, lugs in full contact, mag box floated, a 100 grain BTSP may hit its sweet spot at velocities of around 3300fps or higher (especially with Superformance powder). It pays to work right across the velocity spectrum to see what it likes. If nothing pops, go to the 120gr Speer BTSP.
But all attempts towards accuracy may be moot if the bedding and other factors have not been addressed. The usual story for the M77, most never reaching their potential simply because folk don't put the time into them.
There is another recent thread on this forum of which I spoke about loads, dies and common concentricity issues in the .25 bore. I'll bump it to the top now so that you can easily find it.