Quigley Down Under was a fairly good western from 1990 staring Tom Selleck. In the movie he plays Matthew Quigley, an American long distance shooter that heads to Australia to answer a help wanted ad. He used "an experimental weapon with experimental ammo". Here's a quote from the movie describing his 1874 Sharp's Big .50.
“It’s a lever-action, breech loader. Usual barrel length’s thirty inches. This one has an extra four. It’s converted to use a special forty-five caliber, hundred and ten grain metal cartridge, with a five-hundred forty grain paper patch bullet. It’s fitted with double set triggers, and a Vernier sight. It’s marked up to twelve-hundred yards. This one shoots a mite further.
The core rules state that the weapon ranged listed in the gear section are "effective" ranges for the table-top. To get the real world range for a weapon you multiply the brackets by 2 yards and then by 2.5. So the Sharp's Big .50 in the core book (120" max) would have a real world range of 600 yards. About half of what Quigley's rifle could shoot. So here's my version of it:
Modified 1874 Sharp's Big .50 Rifle
Weight: 15 lbs
Notes: AP 2, Snapfire Penalty, Vernier Sight (+2 Shooting at ranges greater than Short)
A few notes about Quigley. I'm sure he would have d12 Shooting and probably the Professional (d12+1) and Expert Edges (d12+2). He'd also have Marksman and Improved Trademark Weapon (Sharp's Big .50). I would allow him to use his +2 bonus from ITW to hit with it in melee combat as well because he does that several times during the movie. He'd also have Rapid Reload which he used frequently through out the movie and Quick Draw which he used in the final scene against Marston. Finally, instead of adjusting the rifle you could use this Edge from Clint Black and just use the normal stats from the core rules for the Sharp's Big .50.
Requirements: Heroic, Marksman, Shooting d10
Your character has an additional Range increment for ranged weapons that is double Long Range. The character cannot move or perform any other actions in the same round he attacks in order to use this Range Increment. In addition, he suffers a -6 Range Modifier penalty to his attack roll.