Chuck, by the time of Chickamauga, the Arkansas regiments in the Army of Tennessee generally discharged only the most severely disabled. If a soldier was disabled, but still able to perform non-field duties, he was retired to the Invalid Corps, where he might be assigned to clerical, provost, or other non-combat duties. Otherwise, if the surgeon determined that he might recover from his wounds in three months or less, he would generally be sent home on convalescent leave. Relatively few of these men were able to rejoin their regiments, however, due to the ever-tightening control of the Mississippi River by the Federals. The records of the 13th Arkansas are pretty murky during this period, due to its field consolidation with (first) the 15th Arkansas, and (second) the 5th Arkansas, but it appears to me that William Sain fell into the latter category.