M6 ammo;
As far as I am aware (all that I have seen), ammo provided (issued)for the M6 = Alum. cased .410, #6 shot, Brass cased .22 Hornet 45 grn? round nose fmj., with std. military head stamps.
Depending, on the kit, could have been deployed with or without "extra ammo". (Have seen "brown box" .22H., but never less than comm. equiv. 50 rnd. box, .410 that I've seen has all been single rnds.)

The USAF was at least somewhat impressed with the .22H, as they also issued a colapsable stocked bolt action (the model# escapes me ?) But haven't issued (to my knoledge, which ma be stale) .22lr, or .22mag, the later has similar balistics to the hornet+ cheaper, lighter ??????

.22Hornet has some problems, as others have mentioned, and I agree with Chris about the K Hornet (Kilbourne,wildcat chamber),but while superior in all respects, does require handloading, and as he mentioned, do use a small pistol primer.

As to combo guns in general- hey great idea, but in practice not all that great... To each his own.