.22 conversion units in M16s
If they're doing that as a SUPPLEMENT to the regular training, I think that's fine.
We used (elderly and beat up) M16s with .22 rim fire adaptors to qualify with when I went through Air Force Basic Training in 1980. For a while in the early 90s the ANG was short of 5.56mm ammo to qualify category B & C personnel with the rifle, so we used .22 RFAs. Of course, by then all the RFAs we had were old and beginning to wear out and spare parts were no longer available. The AF wouldn't allow us to use the army converters (which were a different design) for safety reasons. (And I never did know what made them unsafe . . . ) We sent out a teletype to all the CONUS bases (active and guard) that we wanted any .22 RFAs they wanted to part with, and there for a while about every week UPS would show up with another box full of conversion units from some base happy to be rid of them . . . We still had problems, mostly because (1.) the magazine springs were wearing out (2.) the RFAs themselves were wearing out, and (3.) whoever spec'ed out .22 ammo for the government bid at the time got us Eley target velocity ammo rather than something a little hotter. (The Eley worked great in my High Standard target pistol but not so good with the rim fire adaptors).
I have a .22 conversion unit (one of the old ones, made by Bro-Caliber International in 1982) that I had tuned up by John Norrel, and it works pretty good, provided it's kept clean and well lubricated and fed quality ammo.
Specific to military training, the Beretta conversion unit for the M9 is EXCELLENT. There are lots of conversion units for the M1911 pattern pistols available. For the first 7 years of my military career (and about 6-1/2 of my civilian police career) I carried a S&W 15 revolver as a duty gun. (Law Enforcement Specialist in the Security Police). I bought a used S&W 18 .22 Combat Masterpiece as a practice gun, because .22 was lots cheaper than .38. We had sporadic access to the 50 ft indoor range at a nearby ARNG armory where we could only shoot .22s and .38 wadcutters (because the steel in the backstop was soft and the ventilation wasn't very good) and I'd usually shoot about 150 rounds of .22 to warm up and then 50 (or sometimes 100) rounds of the .38 for "real" practice.
As a SUPPLEMENT to training, use of a RELIABLE .22 conversion unit is a fine idea. In my opinion, if you're doing regular marksmanship practice, it's best to use a reduced silhouette with the .22, because shooting on a full-sized target is just way too easy. I certainly wouldn't advocate replacing regular training with the .22, but I find shooting a .22 to be lots more interesting than dry fire . . .
My PD just bought a couple .22 conversion units for the ARs we keep in the squad cars. The intent is to allow us to do remedial training on the indoor range without having to buy specialty ammo.
You can only learn from experience if you pay attention!