what is a sub-caliber device, and have you used one?

A .22 conversion kit is a "sub-caliber" device and, yes I've used one.

Ceiner 22lr conversion unit for the AR works fine.

Most of the time I have heard the devices that let you shoot (say a 1911) with .22 called a 22 conversion.  But there used to be a device you slid in the bore of a large cal weapon such as a tank or field gun that would allow you shoot 50 cal for training.  

I have also hear a sabot round for .50 called a sub cal device.  Who knows.

I have a sub-caliber 22LR conversion for a FN/FAL that I shoot out of a Springfield Armory SAR48, it shoots good, accuracy at a hundred yard is nothing to write home about. Never a FTF/FTE. It comes with a barrel liner, a new bolt, 2 magazines, and a wooden case. It was originally made by HK for the G1(FAL) rifle. HK also made a version for there G3/HK91, but from what I understand, it won't fit in a polygonal-rifled type barrel HK G3/HK91.