I thought of the 10-to-12 gauge idea when I couldn’t find any 10-ga flares; but couldn’t find them either. A quick specification check tells me why… 10-ga is 0.775" in bore diameter (19.69mm) and 12-ga is 0.729" (18.53 mm). .777 - .729 = 0.046" Divided by 2, and leaving a little room for tolerances, the walls of a 10-to-12 ga adapter would be around 0.020" (0.5 mm), not much more than the neck wall for a .30-'06.
John - are you implying that if I had a 10-gauge shotgun with ammo, that also having this M5 Very Pistol would present a problem?
Thanks all… I’ll keep looking for WW2 / Korean War vintage 10 ga flares. If anyone knows / finds a source, please let me know!