Hi Warthog,
I also shoot S&W 629 Classics with 6.5" barrels, one with open sights and two with 2X scopes. Found them to be very accurate at 100 yds when allowed to shoot on rifle range targets. Have also shot them a few times at 200+ yds in the field, plinking with safe backstops.
I mostly use 210 gr and 255 gr cast bullets from H&G molds with NRA formula 50% ALOX lube and no leading problems. Occasionally shoot 240 gr Nosler HP jacketed bullets. The 255 gr cast bullet at 1400 fps with W296 has given me best accuracy. Nosler 240 HP at 1400 fps with W296 expands best on game (deer, hogs). The 210 gr bullet is loaded to 1100 fps with AA5 for lighter field load and gives slightly less accuracy than the other two bullets.
If I were going to try shooting out to 300 yds, the 255 H&G with ALOX lube would be my first choice. If I didn't cast/lube my own bullets, I would go with the Nosler 240 jacketed bullet as first choice. NOTE: There is a WIDE variance in quality of commercial cast lead bullets. The choice of alloy used, lubricant, diameter sized, etc. can and does greatly impact results of usage. You may find that some work well and some don't (poor accuracy, severe leading, etc.)
Good shooting and be safe.
LB