As noted before a lot of soldiers were captured and released almost immediately. Most quickly found their units. Some chose this time to get out of the war. The went home, to California, some got lost and were killed by bandets. Some became bandets and deserters. Some Union parole records are great. They say where captured, when, and where he was in prison. Some prisoners were moved great distances to be paroled. Some were released where they were captured. New Orleans, Vicksburg, and several places on the east coast were trade points. So many soldiers for an officer and one for one enlisted. My GGF when captured at Hatchie Bridge with a broken arm was moved to the Hospital in Iuka. About 60 miles. In a few days he was paroled and he walked south to Grenada where the 6th Texas was waiting for their horses.
Muster rolls are great, but they are not perfect for many reasons. A muster roll for a new company is found at the county when the unit was formed, and will usually not match the the muster when the unit reaches Arkansas. Men will join along the way and men will drop out. Next comes spelling. My GGF and his brother had 13 variations for their names. Going through the records, I decreased the roster of the 6th Texas by eleven names. I also found one record that belonged in the other brothers records. Records were missing.
But in going through records and muster rolls I discovered that Company I of the 6th Texas and Company H of the 9th Texas were Sharpshootong companies and spent the summer of 1862 training as sharpshooters and skirmishers. Then the two companies were joined as a battalion and then joined with Ras Stirman's 1st Arkansas Sharpshooter battalion to form and Arkansas regiment under Colonel Phifer's Brigade at Corinth II. I have not discovered how they were employed at Corinth other than Peter Cozzins saying they were on Phifer's left as an Infantry regiment. If that is true they were miss used. Even so, they made it into the middle of Corinth and almost over ran Rosecran's headquarters. Had they been reinforced by Cabel's Brigade as planned, Corinth might have been a victory and the 27th would have been heros as part of Cabel's Brigade.
It takes a lot of soldiers records to tell a story. Iuka was described by records which tell of soldiers KIA or wounded. Going through these records I verified the soldiers of the 27th buried at Oak Grove Mass grave in Iuka. Though not perfect, it allows us to believe that soldiers by name are buried in that grave and families will know wher kin are probably buried.
Pay must have also been bi-monthly, but quickly came to a halt. Ross and other commanders are described as paying soldiers from their own funds so that soldiers could get shoes and cloths. The raid at Holly Springs allowed the brigade to have complete union uniforms to replace their tattered clothes. The people at home sent things as we do today. But money was becomming scarce and the brigade farther from home. Sometimes records from one bi-month corrected the previous month, with no explanation. Reading records is difficult. Pinmanship contributed to good records or made bad records. Spelling was a problem and out right mistakes by the copiers was evident. I correct my own mistakes, and wonder how they did all that they have done. I pray for people to point out my mistakes.
Soldiers did go home. Some units called it desertion, while others covered it. Soldiers would go and then show up when spring came. Furlows Were given, but not often. Don't worry about assumptions. We fill in the space and correct it when better data is a vailable. The story of my GGF's life has been a correct and fill in process. I wish that my parents and their parents had done a bio. Genealogy would have been much easier.
Generally speaking the 27th was assigned to Whitfield's Brigade in December of 1862 for the raid on Holly Springs. Colonel Griffith was the acting commander in Ross and Whitfield's absence. During the next year the brigade was Whitfield's but Ross was the Ranking Officer when Whitfield was not there. Colonel Griffith had gone back to Texas sick. He later recovered and made General in the TST. When General Johnson took over the Army and S.D. Lee became the Corps commander the brigade was split in half for a while and operated with infantry brigades in central Mississippi in fromt of General Sherman. Finally Lee corrected this because it was causing problems. Ross took command in December of 1863.
May 17 is a specific date that is not tied to a battle. From May 5th to 9th the reginment skirmished along the Big Sandy Creek, MS. On 9/10 thery were at Utica MS. Then July 8/9 they were at Clinton and Jackson, MS. What is missing is the weather in that time frame and other tasks not related to combat given to the 27th. Your kin may have been assigned to smuggle ammunition and food into Vicksburg, or there may have been covert activities going on. A lot of soldiers of several units were captured about that time. They could have been out foraging for the Brigade and Ran into a Union trap. Like I say, an Assumption but corolated to fact. But you must state it is and assumption.
From what I see, your kin returned to the 27th in July of 1863 and was with the unit until the end in may of 1865. This is similar to my kins records. You are correct they were not at Champion Hill. The retreat from the big Black occurred when Vicksburg fell and Grant and Sherman could look elsewhere gor Action. General Johnson pulled out to consolidate his forces. He also had to wait till the paroles of the Vicksburg soldiers occurred. Had the Union kept Pemberton's forces. Sherman would have been in Atlanta much sooner.
On the September 1st 1862 battle at Britton's Lane I have nothing, so needs more research. At my site I have a list of the battles of the 27th that came from several sources that are noted there. I hope all this is helping.