rp85: Marlin barrels are known to be oversized and it is not uncommon to encounter a a 44 mag or 444 Marlin to wear a barrel with a .431 - .433 diameter. You should slug your barrel first to determine the actual diameter and then I strongly recommend you bore lap/fire lap the barrel and use gas checked slugs; the accuracy will improve noticeably.
Your rifle may shoot jacketed bullets with good accuracy but lapping the barrel to remove obstructions, burrs or roughness left by or due to factory machining will enable you to shoot cst bullets with (usually) better than jacketed bullet performance.
I have 3 444 Marlin rifles. Two of the 3 wore micro groove barrels. Jacketed bullet accuracy was passable, about what many would consider 'good enough' for a lever action, but cast bullet accuracy, even gas checked accuracy, was near abysmal.
I fire lapped the bores according to Veral Smith's process and the improvements in accuracy nearly floored me. I lapped my bores to .432. I used .430 jacketed and .433 cast. The jacketed bullets grouped under 2", most near to 1.5", but the gas checked cast bullets, those weighing 300 gn and those weighing 335 gn grouped in less than 1", time and time again. My 26" Montana Rifleman barreled Winchester Big Bore in 444 pushes the 335 gn slug at close to 2400'/sec; the 20" micro groove Winchester Big Bore pushes the 300 gn slug at 2300'/sec. Both drop 5 rounds into 1" at 100 yd, and it is due to fire lapping the bores I firmly believe.
I would drop down to Veral Smith's forum and cruise through the pages until you find the posts on bore lapping/fire lapping and then go to the Beartooth Bullet website and read what Marshall Stanton has done with bore lapping. He quotes Veral Smith quite frequently. HTH.