I never got around to using LongShot in .40 S&W. I always used powders that could be used for it and 9 x 19mm. 3N37 is very good, but older V-V data is a must. It gave me all the velocity I wanted, so I never got very close to a MAX charge using it. 3N37 and Silhouette are fairly similar powders. I still keep 3N37 around for that reason, as a back-up to Silhouette. I would definitely try N340 in .40 S&W when you can get around to it. I was never much into 180 gr. JHPs and mostly loaded 150 - 165 gr. JHPs and Golden Sabers and 155 - 170 gr. cast SWCs. The best accuracy load I ever produced in .40 was with True Blue, but like 9mm, I use Silhouette for defense loads.
All things considered including the age of the M29, IMR 4227 should work out pretty well for you since you have it and want to use it. The longer barrel will help you get better efficiency and produce less pressure than LongShot. Still though, most of the wear and tear are gonna be caused by physics (recoil) of bullet weight times velocity and the power factor formula can be used if you'd like to make comparisons. BW x V / 1000 = PF.