I googled “357 magnum GURPS lever gaming ballistic” because Doug always has the coolest worked stats:
.357 stats , scroll down to the barrel length chart.
Sounds like his research indicated the .357 above 16” didn’t really gain in damage, so 5d+2 pi seems good.
Other stats i found on Academy Sports for the 1873 Winchester that they sell in .357/.38 Special; it’s one with a 24” barrel.
Product length (in.): 43
Barrel length (in.): 24
Magazine capacity: 14+1
Product weight: 8 pounds
Give it Acc of regular Lever rifle Acc 5 and bulk -5; then probably Recoil 3 and RoF2..
Range maybe 700/3000 looking at the ‘94 Winchester .30-30 since it’s smokeless powder too.
If you’re reloading Black Powder, check out the rules in AtE for damage reduction, but I’d list the BP .357 load at 350/2200 just guessing it between the .44-40 and the .45-70