I know from the regimental number that he was Union since the Missouri Confederate regiments only went through the 16th.

Are you certain he was captured at Butler, Missouri or was it in Butler County, MO? The 1860 Missouri census shows a John Casinger as head of a large household in Butler County. He was born KY, husband of Mary born MO, father of William, Nancy G., Francis, Martin, and James--all born MO. The other three Casinger households in Missouri were in a different part of the state and were not headed by a man named "John."

The Union military did not send Prisoners of War to Benton Barracks normally, but there was a military prison at Alton, Illinois.

This is a delicate subject, but I'm starting to think John Casinger was apprehended for desertion at or near his home, based solely on the little information you provided.

The cards for the Union 24th Infantry Regiment at the Missouri Secretary of State website includes one for a Niles Cassinger of Company G who was enlisted and mustered into the regiment in November 1861 at a place called Camp Weston and later transferred to Company D. That is the only record I can find that is similar to the name you provided. Bear in mind that these records contain lots of errors and omissions.

Bruce Nichols