It's a tossup for calibre or ctg. length, either .45/90, ie: 2.4" case, or the .45/70, ie: 2.1" case. The .303 is listed for 3.075" COL. I assume the Enfield magazine will allow that. The 2.4", .45/90 has a smaller advantage over the 2.1", .45/70 in speed than you might think.

The overall length of both rounds can be the same 3.075"- given appropriate bullets - one merely adjusts the seating depth to take advantage of the mag length. It's as easy as that, IF the chamber (and bullet) allows it. Of course, the .45/90 case will seat a 400gr. bullet to a length greater than/will a .45/70 case. The .45/70 will be maxed out with a jacketed 400gr. around 2.90", whereas the .45/90 will seat it to the full length of the magazine - a whole 1/10" more. This might mean only 75fps higher speed in the long run.

Without any leade (freebore) you cannot seat the bullet out that far and you end up with a 2.55" OAL for the .45/70, or 2.85" for the .45/90. Obviously, the 2.4" case will exceed the 2.1" for power in this shape chamber.

Put a 3/10" leade in the .45/70 chamber against a no-leade .45/90 chamber and the .45/70's OAL will be 2.85" making both rounds identical as to powder capacity and loading/velocity, etc. A 3/8" leade on the .45/70 chamber will allow a bullet length of 2.925" - getting close to absolute max - if a long-nosed bullet is used. Bullets of 430gr. or heavier might be necesary to require that long an overall length.

How the chamber is cut (design of the reamer) and what bullet is desired, will determine which ctg. is best for a given action.

The #4 action will allow virtually any 'strong action' loading if in good shape, while the #3 will be best suited to being loaded as if it was a Marlin - ie: 43,000PSI max average pressure - except - The #3 (if chambered with a 2/10" or 3/10" leade for it) will allow longer oal than a lever gun, which increases it's capacity to that of a longer case, in conjuction with the length of the freebore/leade. Some call that the throat - that is incorrect - as the throat is the tapered-height (from goove depth to the full bore diameter.

The leade/freebore is the parallel section that mostly controls the ctg's. seating depth. The angle of the throat in conjuction with the angle of the bullet's ogive also controls the seating depth - to a much lesser degree than leade/freebore.

If it was my rifle to re-barrel, I'd probably go with a .458 2" (not the .450 Marlin) with a 3/8" leade/freebore. Such is the reamer I have currently. A Lee 'Factory Crimp Die' will allow crimping any jacketed bullet - anywhere - it does not need a crimp groove. The die will make one while crimping the bullet in place. With all straight sided cases, crimping helps with ignition and ultimately, with accuracy because of it.

The reason for the .458 2" - any magnum brass can be used to make your brass. You are not confined to Hornady brass. To turning of rims and no extractor work.

.350 Rem mag, .338 WM and .458 WM need no reaming nor turning.

I've got a whack of data (35 years of it) to help out with that if desired. It also works in .45/70's in stronger actions. Not all is heavy loaded, but that's where this round shines.