John,
I hope this news will not upset you too much, but your ancestor John W. Barker from central Arkansas is not the John Barker who rode with Quantrill. I know this because Quantrill's John Barker was one of the 40 or so guerrillas Quantrill picked and took with him to Kentucky in December 1864. This I obtained from John McCorkle's memoir penned by O. S. Barton after the war called "Three Years With Quantrill." McCorkle also wrote on page 140-2 of his book that John Barker was killed with three comrades in January or February 1865 near Harrodsburg, KY. This John Barker rode with the west-central Missouri guerrillas starting in 1862 when he was part of a local band formed from southern neighbors in the Cass, Jackson, and Johnson County, Missouri area, according to Rose Mary Lankford's "The Encyclopedia of Quantrill's Guerrillas," Evening Shade, Arkansas: published by the author, pp. 8-9.
I wonder if your John Barker rode with one of the guerrilla bands near his home in central Arkansas. You may wish to examine that possibility with local county records or with Arkansas Civil War records.
Bruce Nichols