Shotguns have long been used to guard prisoners, so why not a smooth-bore musket loaded with buck? Easier to unload if one is unable to simply fire the loaded charge and then reload when back on sentry duty...
As for smooth-bores, these were still in some evidence in battles like Gettysburg in July 1863 and Vicksburg, MS.
While far and away the majority of troops in the Federal Army of the Potomac used .58 or .577 cal. rifle muskets, the returns show the following for .69 smooth-bores:
Dean S. Thomas, Ready ... Aim ... Fire! Small Arms ammunition in the Battle of Gettysburg (Thomas Pubns., 2007), pp. 60-67, citing, RG 156 Records of the Office of the Chief of Ordnance, NARA --"Quarterly Summary Statements of Ordnance and Ordnance Stores" 30 June 1863:
88th Pennsylvania [Phil.] (also Enfields)
81st Pennsylvania [Phil.] (also Springfield .58s)
63rd New York--3rd Regt. Irish Brigade [NYC]
69th New York--Fighting 69th/"Faugh a Ballagh!" [NYC]
88th New York--"Mr. Meagher's Own" 2nd Regt. Irish Brigade [Ft. Schuyler]
116th Pennsyvlania [Phil.]
145th Pennsylvania [Erie]
1st Minnesota (also Sharps, Springfield .58s and rifled .69s)
106th Pennsylvania [Phil.](also Springfield .58s, French .58s)
4th Ohio/ OVI [Columbus] (also Springfield .58s, Enfields, and English smoothbores!)
8th Ohio/ OVI "Fightin' Fools" (also Springfield .58s, Enfields)
12th New Jersey--"Buck and Ball" (recorded as making up buckshot cartridges from the buck and ball ammunition before Pickett's charge...)
11th Massachusetts "The Boston Volunteers"
11th New Jersey [Trenton] (also Enfields, Austrian .58s)
12th New Hampshire [Concord] (also Springfield .58s)
7th New Jersey [Trenton] (also Enfields and Springfield .58s)
9th Massachusetts "The Fighting Ninth" [Irish/Boston] (also rifled .69s--in fall '63 got Springfield .58s)
155th Pennsylvania [Pittsburgh]
13th PA Reserves/42nd Regt. "First Rifles" "Bucktails" [Harrisburg] (also Sharps, Enfields)
5th PA Reserves/ 34th Regt. [Harrisburg]
9th PA Reserves/ 38th Regt.[Pittsburgh]
3rd New Jersey (also Springfield .58s)
93rd Pennsylvania [Lebanon] (also .69 rifled muskets)
102nd Pennsylvania [Pittsburgh] (also Springfield .58s)
27th Pennsylvania [Phil.] (also Enfields)