I have a 1952 Marlin 336SC that I shoot cast bullets exclusively in. My preferred load at the moment is the RCBS 200gr FN GC with 13grs SR4756 . I have killed deer with this load and shot rams at 500 meters.
I'm shooting plainbased as well as gaschecked bullets, at a variety of velocities, from mild to full factory duplication, sometimes plus a bit.
Cast of wheelweights plus a little tin, the RCBS 200 FN GC is a good grouper at full .35 Remington velocity. Try 33 to 35 grains IMR 3031 for starters, and you will find that this load does very well on deer. This will get between 1950 and 2050 fps depending upon charge and temperature.
Plainbase bullets usually require lower velocities for accuracy than gaschecked bullets, and it is here that lighter charges of fast burning powder are most appropriate. It is also often beneficial to try dacron fiber filler between powder and bullet to reduce gascutting and prevent variations in velocity from extremes of powder position in unfilled cases. If you haven't used it before, get some instruction on its use before trying it. This is to be used with the lighter loads, and there is no reason to use it in full power loads using gaschecked bullets with powders that nearly fill the case.
If possible, try to get bullets for the lower velocity loads on the large side, diameterwise. This is often needed not only to fill a bore than may be on the larger side, but even more so to fill an oversized chamber neck, reducing some of the misalignment that may occur with the cast bullet when it enters the rifling. This is necessary for many .35 Marlins because of the short to nonexistent throat they have. For full loads using wheelweights and the RCBS mould, obturation may take care of a bullet that is a bit undersized. You may find that if you try to drive this bullet too fast accuracy falls off. You may need to experiment with the light loads to determine if they shoot better seated lightly into the rifling (don't overdo it or you'll pull the bullet if you eject an unfired round).
Keep the plainbase velocities from 12-1400 fps for the most part. Loads that are much faster (usually) start to need a gascheck. There really is no benefit to shooting the RCBS 200 faster than 2200 fps, and accuracy is often better nearer 2000 fps. Mine weigh 214 grains when cast of wheelweights and with gascheck and lubricant applied.
A number of designs intended for the .38 Special or .357 revolvers may be used in the .35 Remington with success, provided they cast to the correct diameter. It is possible to get moulds that cast to this larger diameter from Lee (ask me how). Sometimes you've just got to try them and see if they work.
I too am new to shooting cast bullets in my Marlin 336RC - .35. I have some experience both shooting and hunting with cb's in the .348 calibre, but these were commercial cast bullets and for this reason I had postponed making the committment to casting my own for the .35.
I knew all about the "fact" that the Micro-Groove barrels wouldn't shoot cast bullets worth a darn, unless you could produce an oversized bullet of at least .002 over groove dia.. But being bull-headed I just went ahead, and with an RCBS 200-35FN mold that dropped as cast at .358 I began my first attempt at accuracy with cast bullets in my 1957 MG rifle.
The bullet, alloyed of wheel-weight, pure lead and additional Tin had cast at 210 grains, and was lubed with a hard lube that was rated for over 2000 fps. I had heat-treated my bullets after casting using the oven method and quenching in cold water, and this had resulted in a harder bulet of around 25 BHN. I had received advice that the MG barrel would do better with a harder bullet at the velocities I wanted for hunting. My initial thinking was to try to produce an accurate load and then later if practical, lower the hardness level for hunting purpose. But first to try for accuracy.
My favorite powder is H4895 because of its consistency and stability with both position and temperature variables. Also H4895 provides me with higher density loads in the .35 Remington cartridge, and I have come to believe that 100% loading density is a real plus to attaining best accuracy. I decided to build up using one grain graduations and the limit would be when the .35 case would not hold more. I knew that this would be a gradual start and I would be finishing with a full-power load, while not exceeding 2200 fps. (the same velocity for my 200 grain Jacketed bullets.
I always use a good chronograph, and consider load-building a waste of time without one. I should also mention that my sights are Iron. I shoot with a Lyman #2 Tang sight and have a Marble's gold bead front. Normally I would improvise a shooting bench with sand-bags to rest the rifle on, but the winter snows prevented me from getting up to my usual shooting range. I had to drive a spike into the side of a tree to rest the rifle on and and shoot standing up. I was simply too anxious to try these bullets out to wait for spring to arrive! My targets were set up on the side of another tree at a range of 50 yards.
The first load was 36 grains, and I had alloted 4 rounds at each graduation as I worked up. As I progressed through 37-38-39-40 grains I was getting around 2 inch groups which are quite typical for me with jacketed bullets (I'm not a great shooter). The velocity had started at 1700 fps. with the initial 36 grain load and now at 40 was 2000 fps. The next group at 41 grains opened up to 4 inches, and I thought then that I had reached the velocity limit for a cast bullet in my .35.
If one studies the shooting of cast bullets to any degree, you will find that around the 2000 fps. mark is a critical point at which accuracy fails or requires more corrective measures. Increasing bullet dia., changing to a different lube, and of course trying another powder sometimes helps, but often this velocity limit is the end of the road for accuracy hopes. I was aware of this, expecting it and actually not displeased with the performance I obtained. As I waited for the barrel to cool, I had a serious talk with myself. It went something like this: "Okay, you've probably had it as far as accuracy is concerned and you've only got one more set of cartridges at 42 grains left. It's very likely this next group will be even worse, but if your going to prove that out definatively, you have to shoot your best anyway."
I fired off the next four rounds very deliberately and slowly, and didn't look critically at the target. Then I put my binoculars up and took a peek. I had shot the best group ever in my life! It was a 5/8" group that could be covered with a nickel. Everything had come together, barrel harmonics, load, everything. I had a .35 Remington cast bullet load that delivered 210 grains at 2150 fps. and with accuracy that was better than I had ever hoped for.
I have since verified the accuracy of this load, and am very pleased with my first efforts with shooting my own cast bullets. This has been a long-winded post, but the point is that the Marlin 336-.35 combination works, Micro-Groove or not and those interested in shooting cast bullets can take their Marlins seriously. I have more to learn and try, but I've got my hunting load and things can only get better.