7x57 is the cartridge I've seen the most improvement. Loaded long, with Rl17 and the 150 Ablr, I get a solid 2900 fps out of my modern M70 featherweight.
I use a QLS holster with the Safariland DFA. It drops the holster to right below my pack waistbelt so I still have the pistol on when I drop my pack. Works well for me.
https://safariland.com/products/model-6004-25-single-strap-leg-shroud-with-drop-flex-adapter-dfa-23116
Since you bell the case to insert the bullet, you want to make sure the taper crimp removes that belling. Sharp edges can cause feeding issues. Too much crimp can bulge the case. What kind of bullets are you using? If they are cast, bullet lube smearing on the case can cause problems. So can...