Thank you for the kind words Hobie, but I'm no expert, just a fan.
The only guns they know of for certain Tom used was the 1894 Winchester serial # 82,667 in 30WCF taken from him at the time of his arrest and the Colt SAA he had on him. He also
tried to use a Fabrique Nationale manufactured, semi-automatic pistol serial numbered 56666. This F.N. pistol has a slight resemblance to the Colt 1903 Pocket Hammerless. The pistol is currently in the possession of Richard Proctor, grandson of Under-Sheriff Dick Proctor from whom the little F.N. was forcibly removed by Tom Horn during his failed escape
I'll try and get a better scan later.
I believe the "1876" rumor was started after the McQueen movie (which in the film is said to be a 45/60 but in fact was a 45/75). We also know Tom used a 45/70 at some point in time, from a note related in General Nelson Miles’ "Personal Recollections", Horn once wrote;
“Well a hundred rounds of 45-70 cartridges weigh eleven pounds when you first put them on, and at the end of twenty days, they weigh about as much as a small-sized locomotive”