I had the barrel relined on the one 27-S that I restored last year with the scope. The bore was completely gone. My local gunsmith and I have a good relationship ship. I help him when he needs help (like I install Jard match triggers in his customer's AR's and other things) and he does work for me at very reasonable prices. He relined my barrel and shorten it to 19", I paid for the liner and the barrel drill which he kept, that was my payment.
In the scoped Marlin 27-S, I tried a bunch of loads combinations. I settled on a load with Sierra 75 grain JFP behind 11.2 grain of H4198. It shoots about 2" group at 100 yards, 1635 fps. I've tried 86 grain JSP, but the only load that grouped 2 to 3 inches was 14 grains of W748, about 1400 to 1500 fps. While that load was good, it increased the cost per round to reload. Haven't found a cast bullet load that grouped well in it. Most likely I needed to lower the charges. I've used RL-7, IMR4227; I've always used rifle powders in rifles even though most .25-20 loads use pistol powders and lower charges. The rifle powders shoot the same velocities, but have lower pressures.
In the one that I just restored, the barrel has rifling, but it is pitted and rough. I haven't decided yet whether to reline the barrel. Just for fun, I've been shooting 88 grain GC hard cast bullets from an NOE four bullet mold, behind 10.8 grains of H4198. Groups run about 6" at 100 yards with a peep sight, a lot of that is me as at 65 years my eye sight isn't what it once was. Haven't shot iron sight in years. At 60 yards I hit a 7" flywheel with every shot. But since I plan to shoot only cast bullets with it, I'm going to slow it down to 1100 to 1200 fps and see if that helps my groups. Need to find different powders to do that. I just cast several hundred 70 grain bullets out of a Lyman 257420 GC mold. Haven't shot any of them out of this gun, yet. I also have a old 85 grain Ideal mold.
These Marlin 27 pumps are just plain fun to shoot, slim, fast and light. The .25-20 round is also fun to shoot. Exceeds .22 Magnum energy and is quieter and cheaper reload than buying .22 LRs.