If you kept the powder charge down to the levels that were used in the Rifled Muskets that shot them it would be safe. That powder load was 60-65 grains of 2Fg powder.
That said Minie' Bullets don't come in just any size.
They are usually about .58 caliber.
That means you would need a 23 guage smoothbore to get any kind of good fit that would even begin to shoot accurately.
In a 20 guage a .58 cal Minie' would be about .035 (1/32") loose and you'd need a mighty big hammer to get one started into a 28 guage's .550 diameter bore.
There are a few .54 caliber Minie' bullets that could work in a 28 guage smooth bore.
Getting back to the powder load, a 7/8 oz shot load in a 20 guage is about right. 7/8 ounces of shot equals 383 grains of lead.
Most .58 cal Minie' bullets weigh between 320 and 570 grains. As you can see, a heavy Minie' bullet can weigh more than the 7/8 ounces so to be on the safe side, do not try to increase the powder load.
As for accuracy, as the others have said, if your talking "minute of barn" you'd do all right.
If your talking minute of pie plate at 50 yards your probably out of luck.
Needless to say, if we're talking about a 300-600 grain blob of lead flying at almost 1000 ft/second, anything it hits is in deep trouble.