As noted by several -- the 1st Sharpshooters apparently were not mustered into service until August 1862 which was about 4-5 months after Albert Sidney Johnson was killed at Shilo. There could be several explainations for your ancestor.

1. He may have been with another Texas unit in April 1862 that was serving with Johnson in the Army of Tennessee and possibly later transferred to the 1st Sharpshooters after they crossed the Mississippi sometime in late 1862.

2. Commanding generals of that era had a staff that usually included several guards for protection, as well as dispatch riders to rush messages from the Commander to his Brigade/ Regimental commanders as part of directing the battle (no walkie talkies or field phones at that time). If your ancestor was with another unit, he might have been attached to Johnson's staff in the capacity of guard or dispatch rider. I would imagine that Beauregard, who replaced Johnson had his own staff and may have sent Johnson enlisted staff (guards/dispatch riders) back to their parent units after the battle.

3. Joseph E. Johnston was commanding the Army of Northern Virgina in early 1862 and as I remember was wounded at the Seven Days outside Richmond...he was replaced by Robert E. Lee who remained in command until Appomatox. However, Johnston later assumed command of the Army of Tennessee (I stand to be corrected) either in late 1863 or early 1864. He was relieved by Jefferson Davis in the summer of 1864 (ther was a lot of personal friction between the two) and John B. Hood (who'd earlier commanded the Texas Brigade in the Army of Northern Virginia)assumed command.

4. I believe one of the responses you got back indicated that the 1st Sharpshooters were transferred back to Texas in 1863. If so, it might be possible that before the transfer your ancestor was detached to Joe Johnston's staff as a guard/dispatch rider.

5. Our ancestors, like us today, often had failing memories in their old age and may have puffed their wartime service a little (I was in Vietnam 40 years ago and some of the "war stories" about the Tet Offensive that I've heard from my old buddies...and probably a few of mine...may be a little more 'heroic' than the facts). For example, consider the combat service in Bosnia of a current Presidential candidate! And that's less than 10 years from event to story. Over a period of 40-50 years stories have a way of growing...it's just our human nature to stand tall.

6. What more information do you have on him "..taking the oath..." to the Federal Government? Have you been able to confirm he was captured and a POW? Was it an enlistment oath (i.e. he actually entered into the Union Army as a "galvanized yankee" in order to get out of the hellish prison camps of the era) or was it possibly the oath that was required of former Confederates in restoring their citizenship?

7. I've found that our family legends on my g-grandfather's service were pretty far out as compared to the actual records. After being filtered through 4 generations they only had a slight bit of fact and a lot of speculation. The information on the tombstone would appear to confirm (be some documentation) of his service... i.e., as a "guard" to A.S. Johnson...however, tracking it down might be a chore.

8. If I remember correctly, Johnson was a Texan by choice and was one of the shining stars of the "old Army" (like Lee and others). I'm not sure if he first "offered his sword" to Texas in 1861 after resigning from the U.S. Army. Maybe someone on this page may have information about his status and command between early 1861 and his death in early 1862...i.e., did he have the usual "general's staff" coming out of Texas before commanding the Army of Tennessee that your ancestorm may have served on?

Whatever the final answer, keep on digging...it is often frustrating, sends you up lots of blind alleys, but it's worth it.