Gary may have a different answer than mine, But my understanding was that the selection was made more on the basis of field craft and knowledge of skirmishing tactics and dedication rather than just shooting ability. This does not mean that shooting ability wasn't an important consideration, but shooting ability wasn't the only overarching consideration. But after 2 years of war most of the men that had survived to that point were good shooters anyway.
It was a dangerous duty and the men would operate pretty much on their own, for the most part, while also working as part of the Picket lines. They wanted people out there who would do the job and didn't want slackers. That is my understanding of the selection process, otherwise they would have simply made the duty a regular duty assignment of the regiments as they had earlier in the war.