I have S&W, Ruger and a Marlin 336 in .44 Magnum. Like yours, my Marlin will give excellent accuracy ~1.5" with selected, gas checked cast bullets [Lyman's copy of the Thompson designed 429215gc & 429244gc] but only if dia. is kept to 0.432" or more. Using moderate loads of Unique, Herco and 2400, with velocities not exceeding ~1100 fps, I've shot 0.432" dia cast bullets through both manuf's. guns. Accuracy is down around 1.5" at 25 yds with no sign of over pressure nor visible damage to the guns. Again....with moderate loads.
Throats on the Rugers [a trio of .44 special Flat Top BH's, and a pair of .44 Magnum BH's] run 0.431". Sizing my cast bullets to 0.430" vs. 0.432" makes no discernible difference in group size, nor bbl./throat leading.
The throats on my 2 sons' M29's, as well as my M69, M629, and a M29 Classic pre-lock all run 0.430".
Alloy for the above is straight wheel weight with about 2% tin added for mold fill out. Leading is non-existent as you'd expect with the annealed gas check.
Additionally, I have used and like Lyman's 429421 copy of Keith's design though it drops from the mold at 0.430"...too small for the Marlin, but works well in the handguns. Too, Mihec's excellent 432256 copy of the old H&G 503 design does equally well in my short guns but gave up trying to get a non-GC'd bullet to shoot in the Marlin.
Pressure levels are impossible to estimate without the necessary equipment, alloy content, lube type, friction surface area, & bore condition; hence my self-imposed limit of mid-level loads. They work for me and my guns and I reserve factory jacketed bullets for full house magnum loads & hunting.
Without knowing the condition of your pre-29, I can't recommend a diameter...and you'll have to slug the throats to get the information needed. I'll repeat that 0.432" works for me, but I'd size for 0.431" were I only loading for the revolvers.
YMMV, Best regards, Rod
