One of the many difficulties encountered by both sides at the battle of Shiloh was the lack of standardization of small arms. Many units had men armed with such a variety of rifles and muskets that it was nearly impossible to keep them supplied with appropriate ammunition. In the aftermath of the battle, Grant took steps to work toward standardizing ammo at least within regiments.
From the Official Record:
GENERAL ORDERS, HDQRS. ARMY OF THE TENNESSEE,
No. 41. Pittsburg Landing, April 21, 1862.
Companies and regiments having a variety of caliber of arms will exchange and transfer from one company to another, so as to secure but one caliber in a company. This is highly essential to convenience in issuing ammunition.
Where necessary, arms may be returned to the ordnance officer on the steamer Rockett and an exchange effected there.
This matter should receive the immediate attention of division commanders.
By order of Major General U. S. Grant:
JNumbers A. RAWLINS,