I load 20.5 grains of H110 in Federal Nickel cases with a WLP primer under a 300 grain Cast Performance WFNGC for 1220 fps from my 4 inch 629 Mountain Gun. I doubt this is the limit but with a very light gun it is a handful and will accomplish everything I could need in New York State.
Stranger, I load 21.5 grs. W296 powder in a win. case, CCI #350 primer over a NEI #295.429GC 295 gr. cast Keith-style bullet. It goes 1300 fps out of my #29 with a 6" bbl. Very accurate,too. 1-1.5" groups at 25 yards.....
Your 629 will digest a load using the .44-300g WFNGC using 21.0g H110/WLPP/Any brass for as long as you can shoot it! This load while being warm enough to get H110 into it's efficiency burning curve, is still fairly mild, and won't stretch the limitations of strength of your Smith at all! It has proven to be a very accurate load in most guns tested, and requires no pampering to get 1" 25 ydard groups out of most guns.
Yes, you can safely go beyond this level with your Smith, but this loading threshold will allow almost unlimited use of this heavy load in your revolver without the worries of loosening it up.
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