The Winchester 1866 was chambered for the .44 Henry rimfire cartridge, like the Henry rifle before it. Before too long, however, centerfire ammunition began to take over as the best and most common type of cartridge. The Model 1866 continued to sell for decades, but some buyers wanted to use centerfire ammunition in them. A centerfire version of the .44 Henry was developed, and conversions of the 1866 were done by both the Winchester factory and professional gunsmiths. Mechanically, the change was quite simple; just replacing the two rimfire firing pins with a single central one, and replacing the bolt face with an insert with a hole for that new firing pin.

Winchester factory example are typically found in the 140,000 serial number range and later. The best-documented batch was a sale of 1,020 centerfire 1866 rifles to Brazil in 1891, with numbers in the 167,000-169,000 range.