Because of complaints about cartridge cases becoming stuck in chambers, a trap was added to the butt that included a "headless case extractor."

A lighter carbine load featuring a 400-grain bullet and 45 to 50 grains of black powder was also issued and, as might be expected, the velocity and muzzle energy were somewhat reduced. Too, the old rack-and pinion extractor was found to be lacking, and a new ejector, which involved a small spring-loaded stud set in the breech just behind the chamber, was adopted. The block itself was also slightly modified by employing coil springs for both the firing pin and latch.

Dubbed the Model 1866, 25,000 of these rifles were turned out at Springfield using Model 1863 rifle-muskets as platforms. In 1867, a shorter Cadet Rifle appeared but still no cavalry carbines—that role being handled by the thousands of Spencer repeaters still in the system and Sharps percussion carbines converted to .50-70.

Even though the 1866 was considered a successful arm, some modifications were deemed to be in order. The extractor still left something to be desired, and a new version was designed that involved a spring-loaded claw set into the receiver at the hinge of the breechblock. Too, it was decided to shorten the gun’s barrel and to adopt an en-block receiver that could be screwed onto the rear of the barrel. Other mods involved a new long-range rear sight and larger cam latch.

In 1870, the first carbine version of the Allin conversion appeared. It too used bits and pieces of Civil War muskets, but the barrel was shortened to 22 inches, the half stock fitted with only a single barrel band, and a bar and ring attached to the left side of the stock opposite the lock to enable the use of a carbine sling. Only 341 of these guns were made, making them among the rarest and most desirable of U.S. martial arms today.

Even during the heyday of the .50-70, it was recognized that .45 caliber was ballistically a better choice. Most Europeans were adopting “smallbore” .45 or 11mm loads, and it was finally decided that Uncle Sam should follow suit.

In 1873, a new model of Trapdoor (so-named because of the appearance of the breechblock) was introduced. Unlike its predecessors, it was not made from altered muskets but was fabricated completely from new. Also unlike the earlier guns, it was not finished bright but had blued metal parts with a case-hardened tang and breech and black oil-quenched breechblock.

The carbine rear sight was graduated to 1,300 yards—100 yards more than the rifle. The carbine wasn’t more accurate, of course—the sight radius just allowed more graduations.

Many of the parts were the same as those of the 1870 models, so Springfield was able to use some of the tooling for components on the older guns. The gun was offered in rifle, cadet rifle and carbine versions.

The .45 caliber cartridge, for which the guns were chambered, employed the Benet internal priming system and a drawn case with heavy copper content. Bullet weight was 405 grains, and the powder charge was 70 grains for the rifle (muzzle velocity 1,350 fps; muzzle energy 1,590 ft.-lbs.) and 55 grains for the more diminutive carbine.

In 1882, a 500-grain bullet was adopted for the infantry rifle. At the same time, the Benet priming was replaced with the Boxer external style.