The Howard design had a tubular receiver which was virtually an extension of the barrel.
Swinging the lever forward opens the breech by drawing back the breech block/plug allowing the cartridge to be inserted from the bottom. The reciprocating breech block was locked by a toggle system, an internal striker being cocked automatically as the bolt ran back. The standard cartridge used was an extra long (1 1/2" in. case) 44 calibre rim fire, known as the 44 Howard Extra Long.
The Thunderbolt was an unsuccessful entrant not only in the US Army breechloading rifle trials of 1865 but also in those convened by the Adjutant General of the State of New York in April 1867. Neither board liked the absence of an external hammer.
The civilian market didn’t fare well either. All together, there appear to be less than 1,700 Thunderbolts manufactured from 1866 to 1870.
The sporting rifle was chambered in virtually any rimfire cartridge from .44 Short to .44 Extra Long. A 54-bore shotgun version was also made in small numbers. A few of these Thunderbolts rifles found their way west in the late 1860s, but were never popular.