I did something very similar with one of my Howa mini’s in 7.62x39. I made a 30 caliber bushing for a 6.5 Grendel reamer and reamed the 7.62 chamber with it to just clean up to the neck. I bought a clearance 6.5 Grendel sizer reamer and it was easy enough to make a resize die for the .311 Grendel version. I had the same thought of getting the 125gr bullets into the 2600+ range to increase performance on medium game in expansion window and increased range. My results were similar to yours.
