Hi Rat,
Yes you are correct in that their were horns of the type you mentioned but also British light Infantry were issued horns to load powder and ball from the pouch when not using cartridges. Also they were issued to Ferguson's riflemen, Simcoes Queens Rangers. They used the only two British rifles used during the war. The model 1776 rifle and the Ferguson rifle. Checkout DeWitt Bailey's book on British small arms in North America. He has a few pictures of light infantry horns along with a wealth of information about the rifles and smooth bores used by the British here in North America.
Regards,
Peter