Anything short of the real rifle really isn't satisfactory.

A .22LR AR15 will let you practice the manual of arms and if shot consistently at 50 feet can be accurate. The trajectory doesn't match 5.556mm in the least so changing shooting distances requires drastically different hold-over/under than the full caliber weapon.

If you want to practice clearing stoppages like failures to feed and stove-piped cases a .22LR carbine is the say to go. I personally don't think they're reliable enough for more than an informal 1-day training course. Clearing those stoppages is different than clearing 5.56mm stoppages. We have to induce stoppages during training with even low-end 5.56mm carbines.

As a post-training tool the M&P15-22 is pretty nice but still requires a dedicated range or rural property. For the city or suburbs a high quality airsoft carbine like the KWA LM4 PTR works well. This is a gas blow back (GBB) gun that's surprisingly realistic with good accuracy with the right BBs. The gun itself costs as much as the M&P15-22 but the cost of ammo is very low. I can shoot off my patio deck in the suburbs without a change of clothes or driving to the range. 9-inch sheet metal E-silhouettes are meat on the table out to 25 yards. Front sight. Trigger press. And when bored or frustrated switching the selector to AUTO can be therapeutic.

The subcaliber replacement bolt carrier devices work after a fashion if you have a 1/12 barrel. 1/9 sometimes works. 1/7 will frequently distort the soft lead bullet to the point it's wildly inaccurate. We used these back in the M16A1 days but dropped the devices when the M16A2 was adopted due to this.

I sold both my M&P15-22 and .22LR bolt group and bought the LM4 PTR.

-- Chuck