Looks like your ancestor left the 20th Ark and joined the artillery in Feb 1863. The last muster he shows present with the 20th was June 1862. The 20th Arkanas went on to fight at Corinth MS in Oct 1862 so that would have been the only large scale battle he would have participated in IF he was still in the 20th at that time. THere is just a records gap from June 62 to Feb 63. I do not know much about Marshall's battery to provide any info on activities after that. So, he could have transferred, or he could have been sick or wounded and furloughed and joined up with the battery, or possibly even captured and exchanged and then re-enlisted. So without the documents to state it it remains a mystery.