"Have no background on this conversion but one might think that shooting a round that's .355 out of a bore that's .357 would result in a good bit of blowby past the slug. Don"

Since the Saami specs for land and groove diameter are identical for 9mm, .38Sp, and .357 Mag, it seems to make no difference in blowby. At least it isn't a problem in my three J-frame conversions. Neither is crimp jump.

There are four reasons for doing the conversion
1) most of my other handguns are 9mm
2) 9mm is (was) substantially cheaper than .38Sp and .357 Mag
3) 9mm has considerably more power than .38 Sp due to the 35,000 psi pressure
4) And the most important - as a lark - because I can. I've converted three of them

The only downside is that 147gr 9mm recoil in a 12 ounce J-frame is quite painful.
Also, I'm a couple of months away from 80, and no longer a very good shot. One handed at 50 feet, I'm only getting coke can accuracy. Today, at 50 feet one-handed, I was shooting an inch and a half left and an inch and a half low. Pretty ******, but it was me, not the J-frame.