After 2 years and between 20,000 and 30,000 rounds, the ol' Marlin Cowboy started breaking. I had spares for everything, so no long term down time existed. I would suggest you have the following spares at least, and if the gun's getting up in rounds, replace the * parts (part numbers are for .45 Colt Marlin Cowboy Limited. #s may vary with caliber and model):
Extractor with spring -514569 (the spring will break *314695)
Firing pin, front -*414299
Firing pin spring-*401295
If you use the new one piece firing pins, all firing pin spares may be dispensed with.
Ejector with spring-514268 (Spring had sprung, and was ejecting in front of gun. Now hits me on right shoulder (left-handed shooting). For a right hander would mean putting brass behind shooter to right instead of out of bounds.
Ejector spring - *401294
As a spare you'll want the ejector with spring. But you can just replace the spring. Takes more time, not something you'd want to do at the unloading table.
Carrier Assembly-514961 -- when it fails, send it to Marlin for a rebuild, gratis. But keep a spare. Replacing it and the * parts annually before a big match is probably not a bad idea. Coyote Cap has a modified carrier assembly with innards that can be rebuilt. When he does action jobs on Marlins (very reasonable by the way) he modifies the carrier assembly to eliminate a rare malfunction as well. If you're a serious competitor, sending him your Marlin is probably a good idea.
Note that this is after a lot of rounds. The Marlin is the class of the field regardless of price, I think. Just keep the gun clean, the screws tight and go over the above parts every 10,000 rounds or so.
After shooting, remove lever and bolt. REMOVE AND KEEP SAFE THE EJECTOR! You won't believe how many shooters have gotten to the line and realized they didn't have an ejector in the gun the hard way!
Clean the bore from the chamber. I use a Bore Snake. Since I shoot Clean Shot, I spray Break Free CLP on the Bore Snake. Usually one pass is enough with Clean Shot. I'll spray out the chamber with Windex/Vinegar (not ammonia.) Then I'll air-dry with compressed air. If things look dirty, I'll spray Gun Scrubber or Brake Cleaner. Dry with compressed air or hair dryer. If you're using real BP use Ballistol./water (10/90%). Make sure you have it thoroughly dry. I stopped using Ballistol because of a lever rusted shut and considerable internal rust. This doesn't happen with Break Free, and Clean Shot doesn't REQUIRE Ballistol like real BP does. Reassemble. Test cycling with dummy ammunition! This prevents the "forgotten extractor" embarassment.
Then, using an appropriate gunsmith's screwdriver for
each screw, tighten every screw on the weapon. Wipe rag over exterior
using Ballistol (smokey), or Break Free (smokeless). You can also spray
with Sheath. The weapon should have a light coat of oil.
No longer than 1,000 round intervals: Disassemble weapon. (Leave Carrier Assembly assembled. Bolt sometimes needs to be disassembled to clean out extractor hole and to check and clean extractor.) Clean the parts of the action completely. They develop a "gunk" coating. The magazine spring has a tendency to rust. I coat it with a light coat of RIG grease. Clean out the mag spring tube using patches. Oil everything lightly with your magic oil of choice. Reassemble. Test cycling with dummy ammunition. Tighten the srews. Coat weapon with light coat of oil.
For Winter Range
The dirt is so bad there you want as dry a weapon as you can get away with. Keep it covered with a gun sock of some sort when not shooting.
I found a set of instructions that sound as good as you're going to get for home gunsmithing. If you're not going to send it to Coyote Cap, this is as tuned as you'll get. I haven't done all of it, and mine's smooth, but it has thousands of rounds through it.
The Marlin Model 1894
Getting Started in Cowboy Action Shooting, Page 1b