Here is the reference that mentions a "Heavy Infantry" regiment armed with Model-1842 muskets. It also mentions a Rifle regiment armed with M-1857 rifles. Another source states this rifle regiment were issued "Harper's Ferry Rifles, so these must have been as you stated from the raid on Harper's ferry.

To honor these men: a history of the Phillips Georgia Legion Infantry Battalion
Page 21
http://books.google.com/books?id=6d5ajjAEi6IC&pg=PA18&dq=camp+mcdonald(Jackson+guards)#v=onepage&q=heavy%20infantry&f=false

Best Regards!
John Kotila