3 lessons to be learned here IMO
while we can blame "Mike" for not knowing and should try to educate him...i have a feeling he had some help in his "selection" of ammo.
i've been in store where someone will come to the counter and ask for "9mm ammo" and be handed any box that says "9mm" on it. i've even seen a seller tell a customer that .380 is "the same thing". the funniest is watching a clerk hand .380 ammo to someone wanting .38 spl..."it'll fit". actually 9x17mm and 9x18mm ammo will fit and fire in a 9x19mm handgun
just as likely is the situation that someone "gave him" some 9mm ammo they had laying around
i don't think a switch in caliber will help, he just need to be educated in recognition of the correct ammo.
...
I think 9M's thoughts above are close, if not, the exact problem "to begin with"..
Lesson 1. Never assume the_clerk is handing you the correct ammo. "Verify before you buy" and "any chance" of getting 9mm Luger for 9mm short/380acp or visa verse is confirmed, corrected, verified, at the store, not when the need arises and one pulls the trigger and clk or one of several other possibilities, one shot, not enough/correct bullet charge to cycle out the first fired round and load the next correct size/powered, or incorrect, underpowered, bullet i.e short 9mm/380acp in 9mm gun or, somehow putting 9mm in a 380's mag but..
Lesson 2. Having an "unloaded/chambered" SD/HD gun when the need arises and then "racking in" the first shot only to have the gun jam one way or another, and there ya are..
Lesson 3. Keep one's HD/SD gun loaded/one in the tube, always.
Guns only go bang when one's trigger finger pulls the trigger, so to avoid finding a problem when you need that first shot, I would suggest, highly, to Mike, from now on keep one in the tube/chambered, that way he'll not discover "a problem" i.e jam at the moment of need, rather, at the moment of no-need and be able to correct it without his life being on the line, one way or another, with no bang, FTFC, jam, one underpowered shot only, with gun not cycling out the spent casing nor loading the next bullet.
~ Verifying ~ goes a long ways, in so many ways.
OMMV,
Ls