I load 7.5 grains of Unique or 6.0 grains of 231 for my S&W 57 using 210 SWCs. Don't have a Marlin in .41 Mag. but have done enough work with the .44 Mag to tell you that you are doomed to frustration unless your bullets are oversized for caliber and you keep plain base cast down under 1200 fps.
thanks for the info.for my tracker 41 mag best groups so far 15.0 grs 2400 with my 210 cast(412).I just heard that with the micro-groove they must be over sized to shoot good.hope they work out as I don't want to be limited to jacketed bullets in the marlin.
I just started experimenting with bullets cast from an LBT mould. They come out at 225 grains and I've been sizing them to .411. (My 1894 FG bore's groove diameter is .4106.) Loaded over 17.5 grains of Lil'gun, they chronograph at 1670 fps and the first five shot group I fired at 100 yards went in less than 2.75". I fired one through a 10" log in front of a steel plate bullet trap at 50 yards. The only deformation of the slug was an angled flat on the front where it contacted the steel plate. I heat treated them by water dropping them right from the mould and they register 19 BHN. The .44 mag has some great capabilities but I really can't imagine this load playing second fiddle to a .44 in any way. I haven't tried it in a revolver yet.
I have been using Unique almost exclusively for my .41 Mag loads and most of my other pistol loads as well for 15 years. I usually load around 9.0 grains under a 215 commercially cast SWC bullet and a large pistol primer. I have gone up to 11.0 Unique on a few loads but there were some signs of excessive pressure so I would not recommend that. I load for 4 different .41 pistols, I have not yet loaded any cast bullets for my Marlin Lever gun.
One of the things I use to help the longevity of my brass and get more consistent velocity is use a taper crimp die instead of the normal roll crimp. I prefer a heavy crimp and the roll crimp was tearing the crap out of my brass. I use taper crimps on everything I load now except my big bore rifle rounds.
My experiments with 2400 were dismal, even in my 6.5" Smith, although my recent loads with WW296 show some promise, even in my Marlin, under 210 and 220 jacketed bullets.
We had fun with the 41 and lead (used OrgnTr off the shelf .411" 215 swcl's) - as has been pointed out elsewhere: generally with pb bullets, one has to keep the velocity down in order to get reasonable groups... but for whatever reason our 1894 wanted the opposite, ie, the harder one drove the bullets, the better they shot. Our best loads were essentially full house jacketed loads, eg, 17gr H110 running 1410fps driving 10 at 50y into .96" (the best we saw), or 17gr h2400 at 1456fps etc. If you're interested, we have all our 41 mag data on our website (url below, look under levergun studies/data).