According to the best website I know of for identifying "Historical British" Army buttons, your button showing a British Royal crown above a hunting-horn is "a generic pattern of button worn by some Rifle Volunteer units from their formation in 1859." One shown in a photo at that website is listed as a "Rifle Brigade Lead Mount" button.
Go here, and scroll about halfway down the page of photos, looking on the right side: Militarynamedregs
Or, you can try searching through the 37 pages of British Military buttons at the ukdfd website (United Kingdon Detector Finds Database): UK Finds Database - - - UKDFD
I should mention, there is a similar-looking button with the same crown-above-horn emblem but it's surrounded by a wreath.... however, your button has no wreath, so it's not a King's Royal Rifle Corps button.