"What are they currently chambered for? I thought that the MR-73 was chambered in .357 Magnum, if that's the case then you may find the accuracy to be rather lackluster with .38 Super as it typically uses .355-.356 bullets wheras the .38 Special and .357 Magnum use .357-.358 bullets so unless the barrel is to be replaced you'll be shooting undersize bullets. Other than perhaps the ability to use moonclips, I don't really see the point in converting the revolver as .357 Magnum can be had more powerful and less expensive than .38 Super. If the moonclip thing is what you're after, why not simply have the face of the cylinder machined to accept .357 Magnums with moonclips? This is a common modification on S&W and Ruger Revolvers."
They are originally chambered for .357 Magnum but a spare cylinder had been made in 9x19mm. For this reason the first MR 73 until 1983 had a .354 barrel.
then it became a .356 barrel.
I have made many tests with various revolvers using modified cylinders ( Ruger SP 101, SW 60, RMR Ruger Manurhin F1, SW 586, Diamondback) or original 9 mm cylinders when they existed (Ruger Speed Six, SW 940, Manurhin) and found out that usually accuracy is good. Bullet expansion under the gasses pressures is enough to alleviate even a difference in diameter.
I found similar results when I developped two ammunitions for a French Ministry of Interior service. I used a complete equipment in a tunnel to test many loads using .308 to .311 bullets in a Soviet SVD, a Russian MN 1891 and a Romanian PSL. All parameters were analysed, speed, pressure, accuracy, repeatability of results, wear and tear.
I obtained CIP agreement for two cartridges in 7,62 x 51 and 7.62 x 54 R.
I find .357 Magnum is an overkill for my use.
I always had a revolver and a pistol in same caliber, I did start this trend in 1980 when I had a Colt Governement and a Colt Python in 38 Super , I later had a SIG and a Manurhin in 9mm and finally a SW 610 Classic revolver, a Bren Ten , a Colt Delta Elite, a Colt Delta Gold Cup and two Glock pistols 21 and 29 in 10 mm.
I am preparing a MR 73 and a high capacity pistol in 38 Super Comp (rimless 38 Super)