Re: 22LR conversion!!
The definition of "acceptable" is the issue here. If you're talking about a true training rifle for a dedicated HP-XC competitor, the drop-in conversion units are not acceptable. For this sort of application, or to use them in any sort of smallbore competition, you'll need a dedicated upper, and one that's properly built at that. Derrick Martin, Frank White and a couple others will set one up nicely. Again, for purposes of just plinking or familiarization with an AR platform, the drop-ins are great.
With good ammo, my Accuracy Speaks upper, as well as every one I've seen from Compass Lake (Frank White) will hold x-ring on an A-36 50 yard smallbore target. The drop-ins, on the other hand, would be hard pressed to hold 10 ring, and most I've worked with would be doing good to have held the 8 ring. It's all a matter of degrees.