For general plinking, 6.0gr to 7.0gr of most of the faster burning pistol powders with bullets ranging from 180gr to 260gr. Last year I was looking for plinking loads for a 6" bbl'd beater 629 truck gun. I cast my own bullets & had several different fast burning pistol powders lays around. Wanted a load that would do 1 1/2" or less @ 25yds. It didn't take long to find a couple plinking loads that fit the 1 1/2" criteria.
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Clays, american select & bullseye did extremely well and clays was top dog. No need to burn 18.0gr to 20.0gr of 2400 in a plinking load when loads with 1/3 of the powder will produce accurate loads that will put a pretty good thump on anything it hits. You'll easily get 100fps+ out of the long tube on that rifle compared to the 6" (allaint) or 8" (hodgdon) reloading data.
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Those were shot using a rest and a 4x scope on that 6" bbl'd 629, I'm sure you could easily tighten those groups up using a rifle. Years ago (90's) I had a 1895 marlin in 44mag, it was a hoot to plink with. I gave it a steady diet of 240gr cast bullets (keith) and any fast burning powder I could get my hands on.
Enjoy your new range toy & look up some of the cowboy action reloading data. That's what other people with the same equipment are using for plinking/playing/competitions.