If you'll excuse the fact that I'm not shooting them in a Marlin, I have many loads to offer, with trials of literally dozens of powders.
Many of these loads were developed in a "levergun" chamber that the TC custom shop seems determined to afflict on the purchaser. This includes an overlong neck and no throat, a difficult combination to work with when it comes to cast bullets.
I'm assuming you have a somewhat slower twist Marlin, and cannot take some of the longer bullets a faster twist rate would make possible at low velocity.
If you decide on a bullet weight, your next decision is just how fast to shoot it. Reason being that a high velocity load for pests that does not consider saving meat will have a much higher point of impact than a 1100-1300 fps "small game load."
Once you've decided on weight, generally 60, 75, or 86 grain jacketed if that's you're thing, I would suggest a powder that mostly fills the case if you're deciding on a varmint load ('Lilgun powder is an exception to this). In this area velocities of 2300 fps (60 grain) are possible, with 1900 to 2000 fps being reasonable for the 86 and 75 grain bullets.
These loads are NOT for rabbits you want to eat.
If choosing a suitable case filling powder, IMR or H4198 are good choices. 13 grains of these powders yields about 2000 fps with the 75 grain Speer, perhaps the slowest expanding of the jacketed bullets, but expand at these velocities it will. Consider this charge the normal maximum with the 86 grain Remington flatpoint at over 1900 fps, with good killing power on about everything smaller than deer.
With the 60 grain Hornady (improved due to longer bearing surface) a near full case of 15 grains IMR 4198 gets over 2300 fps and will remove large chunks of your target. Not a small game load! Wonderful for lightning quick killing effect. The skiving and shape of the Hornady 60 flatpoint allow improved accuracy and expansion characteristics over the old shape they used to sell. The light weight and shape mean it is best employed within about 150 yards, with 100 to 125 yards being the limit for called predators, which encompasses almost all opportunities.
A milder load of 11.5 grains H4198 gets about 1720 fps with the 86 grain Remington.
10 grains 2400 gets 2200 with the 60 Hornady SP.
Reloder 7 is of the same approximate burning rate as the 4198's and gets similar velocities. Behind a hard cast Lyman 257420, 13 grains Reloder 7 produces 1950 fps and is a good killing load with less pelt damage.
However, 13 grains Reloder 7 is a bit slower than a similar amount of 4198, and use of this charge behind the 75 grain Speer gets a mild 1680 fps. Jacking up the charge to 14 grains produces a hair below 1900 fps.
12.2 grains Reloder 7 produces 1700 fps with the Remington 86 grain SP.
If using jacketed bullets for a small game load at lower velocites, likely the Speer 75 grain softpoint has an edge on the other two bullets, as it has a flat point of larger diameter, and killing effect is excellent even though the bullet does not expand at lower speed.
In the typical levergun chamber and throat, the 75 Speer is my choice as likely to be "most accurate". However, accuracy is quite usable with the other two jacketed bullets. The Speer has a broad range due to its construction and shape.
If this bullet is used for small game, I suggest speeds of from 1100 to 1300 fps as being most suitable, and would name my candidate powder as W231. Start at 3.5 grains for a mild pop and velocity in the usable range suggested. It meters well, burns clean, and is reasonably position insensitive.
The very best employment of the 25-20, however, is likely with the cast bullet. I cast over five different designs for the 25-20, but if you're not a caster, Meister makes an excellent 85 grain flatpoint, being similar looking to the Lyman 257312 save it does not have a gascheck. With this Meister bullet, I suggest a charge of around 3 grains W231 to obtain about 1100 to 1130 fps and good killing effect with little noise. Also suitable for around 1050 to 1100 fps is 3 to 3.2 grains Universal Clays. Unique at 3 grains is also suitable, but charges this small with the flake powders often do not meter properly from fixed cavity measures (like the Lee Pro Auto Disk) and must be weighed. To do otherwise with any flake powder is to risk squibs from the flakes bridging in the cavity in weights of 3.5 grains and lower. Such also applies to Clays (not Universal Clays), Red Dot, 700X, etc. - in other words, any flake powder of relatively large granulation.
For higher velocities of around 1280 to 1300 fps with the Meister bullet, try 5.5 grains 2400 or 6 to 6.5 grains H or IMR 4227. These last two are my universal "accuracy loads" as they have shot well in two different barrels with three different throat configurations. Such loads are mild and are about the slowest powders reliable in truly cold weather of 10 degrees or less.
For higher plainbase velocities, with the Meister try 9.5 grains 4198 for just under 1500 fps. This is the highest velocity in which accuracy is reliable. Higher velocities start to throw fliers and enlarge groups with the plainbase bullet. If cold weather use is contemplated, use dacron properly to position the charge. Less powder is burned completely compared to the faster powders, but accuracy is the result due to the lower pressures. I would not use the load in extremely low temperatures.
If you have a slow twist Marlin and the 1100 fps Meister loads do not shoot (it's a relatively long bullet) try the 2400 and 4227 loads, as the additional speed aids stability in slow twist rifles.
For small game loads, I consider gascheck bullets a needless expense. The plainbase bullet is better here.
The finest bullet for small game use, in my opinion, is the RCBS 85 grain Cowboy flatpoint. The large flat gives excellent killing effect even at mild 1100 fps speeds. Pair this bullet with 3.0 W231 or 5.5 2400 or 6.0 to 6.5 grains H or IMR 4227 for excellent accuracy. The last two powders get 1300 fps or thereabouts and have a bit of an accuracy edge on the W231 load, but I am able to produce about 0.8 inch groups at fifty yards on demand with the W231 load. The two slower powders average closer to 0.6 inch at that range. That's for five shot groups.
There's a trick to loading the RCBS bullet. It was originally intended for black powder, which is why the lube grooves are so large. If filled with lube, the bullet is overlubricated, and fliers and enlarged groups result. If the lube grooves are not filled, with the higher pressure fast powders the lube grooves collapse to some degree, unbalance the bullet, and affect accuracy if the bullet is cast of 12 BHN wheelweights or softer.
The solution is to cast of wheelweights, quench to harden (I obtain around BHN 27) and lube lightly with Lee Liquid Alox. If used with the W231, 2400, or 4227 loads mentioned above, I predict you will be pleased. Accuracy is excellent and killing power the same, with only mild meat damage.
If you don't mind weighing charges, 6.0 grains IMR 4759 (the occasional run, purpose built cast bullet and reduced load powder) is outstanding and produces 1350 fps, which approximates the jacketed factory load.
If you want a higher velocity load that is easy on pelts but still has good killing power, try the Lyman 257420 cast of a high tin/antimony alloy and load behind it 13 grains H4198 for 1950 fps. 8.5 grains H4227 (IMR really is the "old" H4227, so the same charge is appropriate) gets 1680 fps for a midrange type load with elevated killing power. However, 4227 is rather position sensitive, and if you've a reason to employ dacron, here's your excuse.
This is not a knock on 4227 - many powders are surprisingly position sensitive in the .25-20 case with the exception of the fast pistol/shotgun powders. Remedy is to go to the case fillers, as mentioned, like 4198, or the thoughtful use of dacron with certain loads using cast bullets. Dacron reduces gascutting on the plainbase bullet, eliminates powder position sensitivity when appropriate, and ensures that leading is reduced to the point that the rifle can be shot indefinitely without accuracy changes due to fouling.
A mild load of 3.0 grains W231 behind the Lyman bullet gets 1250 fps, sounds only a little louder than a .22, and gets decent groups at 50 yards. A good small game load for those wanting to settle on the universally usable cast gascheck bullet for all lead bullet use.
I am fortunate to have very considerable small game and varmint hunting experience with the .25-20. When I say it is the gun everyone should own that has such interest, I'm not kidding, and I'm not overstating things.
It's dirt cheap to shoot, has usable range on the most likely shots we're to make, and is a whole galaxy more effective than the overmatched rimfires when it comes to predators brought in to a call. Killing power can be excellent with cast bullet flatpoints yet with mild pelt damage, or if preferred, higher velocity loads with jacketed or softer (wheelweights) cast gascheck bullets can produce considerable tissue destruction, to the point of tearing large exit holes. The 60 grain Hornady at max speed tends to scatter small critters and tears good sized holes in everything else. The 86 Remington expands faster than the 75 Speer and is in the same category as the 60 Hornady as far as wound channels.
Do not let this fool you into believing a .25-20 loaded with the 75 Speer at 2000 is a slower killer, though, as it works just fine even so and its killing effect cannot be distinguished from the other two. The previously mentioned bullets just make a little more mess.
Flat point cast bullets, even of hard alloy, start to "pucker" the flat point at impact velocities over around 1250 fps. This "puckering" effect dishes the flat point slightly while actually widening the nose flat, a feature that produces the improved killing power (it's good on small game with no increase of nose flat whatsoever, though).
A .25-20 that produces one inch fifty yard groups at 1100 fps is a considerably better killing gun than a one inch 50 yard .22 long rifle, because on many small game species (such as the tough fox squirrel) the RCBS flatpoint has good killing effect even if the bullet strikes a little far back. I've had occasion to note the excellent killing power of the .25-20 many times, and have taken, literally, hundreds of critters with this particular bullet at around 1100-1150 fps, including up to possum, feral cat, squirrel, rabbit, and other such game and pests.
Called coyote have also fallen to the .25-20, and there is no lack of power here as compared to the rimfires. The larger diameter, heavier bullet compares favorably to the .22 Hornet in killing power, and that's stating it conservatively as I also have a .22 K-Hornet. I believe the properly loaded .25-20 is superior in killing power. I also believe John Wooters to be correct when he opined that when the .25-20 was employed within its effective range no .22 centerfire surpassed it in killing effect until you got to the .225 Winchester and .22-250.
With the milder cast bullet loads, small pistol primers are preferred, and are suitable in the higher powered loads as well. Most .25-20 loads, even the full power ones, have less pressure than the 9mm pistol cartridge and their use is quite appropriate. I prefer rifle primers for very cold weather predator calling with powders such as the 4198's, Accurate 1680, Lilgun, and Reloder 7 but use pistol primers for everything else and for warmer weather.
For leverguns, I suggest that a maximum cast bullet diameter is beneficial, and recommend the RCBS for small game use when properly prepared, the Lyman for higher velocity loads. The RCBS bullet should most definitely be hard cast or quenched if smokeless powder use is contemplated.
The .25-20 is my most often fired centerfire rifle caliber, and everyone should own one. They need it - they just don't know it yet. I never knew how badly I needed one until after its purchase. Now, for most of my hunting, I can't get along without it.
My opinion of this cartridge and its utility is so high that I spent over 400 dollars to have a custom chambered OTT Contender barrel made for it to overcome the TC Custom Shop barrel's shortcomings. With a 4X Burris Compact mounted atop its 22 inch barrel, it is in my opinion the ultimate small game and utility cartridge. I can hunt small game like squirrel and rabbit all day, and called predators at dusk.
Same gun for both, with custom loads fitted to it that have points of impact that are not widely divergent despite the differences in velocity. To keep such divergence reasonable, jacketed or cast bullet velocity must not exceed about 1900-1950 fps. For such use I load a 82 grain custom made gascheck bullet with Reloder 7 at 1900 cast of an expanding alloy of wheelweights. These mushroom like a big game bullet for predator calling. For small game use I like the RCBS FP at 1100 to 1300, using W231 (lowest noise) 2400, or 4227.