After Federal forces occupied Fort Smith in September 1863, they began to recruit local civilians to serve as auxiliaries. My understanding is that some companies of Home Guards were formed (as a dismounted militia), as was a mounted force known as "rangers." I've never seen them referred to as a battalion, but that doesn't mean it didn't happen.

It may be that your ancestor served in one of these ranger companies; that makes sense, seeing as he's from Sebastian County.

A good source of information about the rangers is the "Fort Smith New Era," a Unionist paper published in Fort Smith starting in late 1863. Vickie Betts has transcribed several articles from this paper on her web page; microfilm copies are also available at the Arkansas History Commission.

The following is from Ms. Betts' page:

FORT SMITH NEW ERA, May 14, 1864, p. 2, c. 1

The Rangers.

Captain Turner's company of Rangers are doing good work, ferreting out and killing bushwhackers. They are on the move all the time and render the government efficient service, as well as defend their own homes. They are in earnest, and not only know how to hunt, fight and whip bushwhackers, but know how to treat them when they happen to take prisoners.

On the morning of the 5th inst., Sergt. Sails and I. Duncan, of Capt. Turner's company, were taken by the bushwhackers and after being kept for two or three hours Duncan was released, but Sails was stripped of all his clothing but his drawers and was then brutally shot. He was a worthy young man, a good soldier, and his loss is deeply felt by the company.

On the night of the 6th, while camped near Smedley's Mill, Capt. Turner and 25 men were attacked by 54 bushwhackers, under Lafayette Glass. The Rangers repulsed them twice, killing three of the rebels and wounding seven, among whom was Glass, who is mortally wounded. Three of the Rangers were wounded, but not mortally.

Hope this helps!