Your brass find is one half of the brass 2-piece strap adjuster buckle from a Confederate-imported civil war era British-made S. Issacs Campbell & Co. soldier's knapsack. The other half has a hook which fits into the ring on the half you found. So, search that spot extra-diligently for the other half. It ought to be somewhere nearby. (Unless somebody found it before you got there.)\
The bullet you are asking about is a pre-war version of Sharps Rifle bullet, which civil war bullet collectors call a Multigroove Sharps. Its design was patented before the war by Gomez & Mills. It looks like a .38-caliber one... but I cannot be certain without knowing its precise measurement. (Or... is the 3-groove Minie next to it a .58 or a .69-caliber?)
