It looks like a Nepalese copy of an 1864 .577 British Snyder rifle. The Royal Nepalese Armory was emptied some years ago by a consortium of businessmen. These rifles were and are being offered at many sites. The condition of yours and the fact that it includes a sling would indicate to me that this rifle may be part of that collection.
The group behind this also owns IMA, (International Military Antiques) who brought hundreds of antique rifles in. There is a crude but interesting video about this deal titled "Treasure is where you find it"
Here is a link to one site selling these rifles.
http://www.sportsmansguide.com/net/s...274&rp=section