My Marlin 32-20 shoots so well with a 120 plain base and 5 gr of Unique that I don’t use anything else.
5.0 grains of Unique is a close parallel to that 5.5 grains of Herco that is doing decent work in the 32/20 wheelguns, which is very close to the Waters Pet Load of 6.0 x SR-4756. If your results with the Marlin are like mine, the 4756 load gives about 1200-1225 FPS in my 20" 94CCL and 900-925 in 5" revolvers. It is a very pleasant load in a rifle, and doesn't shred small critters like the same-weight bullets do when launched at 1800 FPS. I think we are saying the same thing in the languages of differing powders.
And now for a bit more swamp gas concerning the 32/20 WCF in both rifles and wheelguns. Some of you may be familiar with the work done a few years back by the late John Kort (AKA "W30WCF") concerning a smokeless substitute for black powder in the 44/40 WCF using the old Lyman #427098 as its slug for cowboy match shooting. #427098 (and #401043 and #311008) all lack a dedicated crimp groove, and their usual anchoring got done with a roll crimp around the bullet's ogive origin. That works fine in revolvers, but can fall short in a levergun's tubular magazine. If the bullet base is not supported by a firm powder column beneath it, the bullet is prone to "telescope" back inside the case as it shunts its way toward the cartridge lifter and the rifle's chamber. Most smokeless powder loads meant for Colt SA and Win '73 rifles take up a small portion of the cavernous 44/40 case's internal volume. What to do?
My route to success in 44/40 WCF involved use of the SAECO #446 flatnose plainbase of about 200 grains in heft, which has a nice crimp groove that enables a cartridge OAL that meshes with my old '73 Winchester's lifter and with the Pasta Colta SA's chamber lengths. Life is good, IOW.
John K had other ideas. He liked that OEM bullet #427098, the same way 1911A1 addicts like Lyman #452374. He messed around with high-bulk rifle fuels a bit, and came up with RL-7 as his answer to the powder column height/pressure yield 'ideal' in the 44/40. Depending upon cases used, that meant 24.0-25.0 grains. I tried this mode CAUTIOUSLY, a friend with a Win 92 clone from South America ("El Tigre"??) ran them without issues, the brass showed evidence of soft pressures and velocities were in the 1150-1175 ballpark--just like John K and his buddies had experienced. OK! I ran some through my '73 carbine and the Uberti, and the loads behaved themselves well in both arms. In the revolver, there were a few 'zombie' unburned powder granules, but in the rifle tube the grittiness did not show up. Fair enough, it works--and solves the problem of bullet telescoping in #427098-loaded smokeless cartridges in the Win '73 and Colt SA without stressing the mechanisms.
Not one to leave well-enough alone, I scrounged a couple hundred #311008 to try in my 32/20 arms. Half of the 44/40's RL-7 load (25.0 grains) is 12.5 grains, so I tried that powder weight in Starline 32/20 brass......and it was a bit too much of a good thing. I recalled that IME 25.0 grains in W-W and R-P 44/40 cases gave a slight amount of compression to the powder column of RL-7, but to get the same column height in Starline brass required a grain less fuel. I cut the powder weight back to 12.0 grains and got a snug slight column compression. NICE.
To the desert I went with the ammo--Marlin 94CCL--Colt Bisley--and S&W M&P. Long story made short--the velocities in the 20" Marlin ran in the 1100-1125 FPS realm, the Colt SA got a bit over 800 and the S&W ran about 30 FPS faster on average. SDs were small, which is NOT USUALLY a feature of smokeless loads in 32/20 arms. Accuracy was decent, but not the best seen from any of the arms. Close, though. Call it "Acceptable". Primers were Rem #6-1/2 small rifle, which seem to equate to small pistol magnum brisance. Remington ad copy says they are optimized for cases of this size. Groups shrunk across the board when I started using them in 25/20 and 32/20 rifle apps.
The down sides--in revolvers, this powder is pretty slow-for-application (Well, DUHHH......). It leaves some grit in its wake. The Colt SA system can work around that quite well. The swing-out S&W could NOT. Grit got under the extractor star, even though I took pains to make sure the muzzle was straight up when I thumbed down the extractor rod. For sport shooting, not a real issue--in a defensive scenario,
NO BUENO.
Powder cost is another factor--assuming that a pound of RL-7, 2400, Herco, and Unique are priced close to one another, using twice the powder weight per charge can start getting spendy. My view--unless you absolutely MUST USE bullets without a crimp groove, the RL-7 Answer may not be your best route.