Howdy
To properly answer this question, first you have to define which lever gun.
The really old Winchester designs, such as the 1860 Henry, 1866, and 1873 feed a round straight into the chamber. These rifles use a carrier or cartridge elevator which rises straight up from the lowered position. It really is like an elevator riding up an elevator shaft. The cartridge feeds straight out of the magazine, and then once the carrier reaches the top of its stroke the cartridge is pushed straight into the chamber just like a torpedo being pushed into the torpedo tube of a submarine. This type of action allows the cartridge to feed straight into the chamber with the least hangups.
Other lever guns, such as a Winchester 1892 and 1894, Marlin 1894, and Henry Big Boy employ a tilting carrier. The carrier tilts and the cartridge slides up into the chamber much like a round in a 1911 riding up the feed ramp. This type of action can be more troublesome with some bullet shapes, because any sharp edge or shoulder may catch on the edge of the chamber as it enters the chamber.
Lastly, you have to define the cartridge itself. All the old WCF cartridges; 44-40, 38-40, 32-20 and 25-20 had a tapered or bottleneck shape. That taper helps the round enter the chamber a bit more easily than a straight round such as 45 Colt or 38 Special/357 Mag. It is no accident that the first WCF cartridge, the 44-40 had a taper to it. Lined up like a torpedo on the carrier of the 1873 Winchester it slid directly into the chamber with almost any bullet shape. I have always suspected that Winchester chose the shape of the 44-40 after their experience with the straight 44 Henry Rimfire cartridge in the Henry and 1866 Winchester.
Getting back to the original question, I have a Marlin 1894 chambered for 38Sp/357 Mag. It has no problem feeding Semi-Wadcutter bullets up the slanted carrier. The nose of the bullet strikes the top of the chamber and tilts into the chamber without allowing the shoulder of the bullet to catch on the edge of the chamber. However not all such Marlins can feed this bullet shape successfully.
In Cowboy Action shooting the most common bullet shape is a Round Nosed Flat Point. This will bullet will feed well in just about any rifle and any caliber. Next most common is probably the Truncated Cone bullet shape, which also feeds beautifully. Neither of these bullets shapes has a hard edge to catch on the chamber.
I only feed Round Nose Flat Point bullets in my 44-40 and 38-40 Henry and 1873 lever guns. They feed fine, just like a torpedo entering a torpedo tube.