I've been using Kolar full-length sub-gauge tubes for years. I've got a set of 20 ga. Chambermates that I sometimes use in my Fox SxS for skeet. They do work very well. I've even run a few 100's with them in the Fox.
With the shorty inserts like Chambermates, the consensus seems to be that stepping down one gauge (12 to 20) is fine, with consistent velocities and little difference in patterns from the host gun/chokes. Down two gauges may be OK, not always great. (I landed a deal on a set of 12 to 28's that were too inconsistent, and I sold them.) Stepping down three gauges (12 to .410) is just too much.
Independent tests have shown that with the shorter .410 tubes in a 12 ga. gun, velocities can be very erratic, ranging from 800-900 fps to almost 1200 fps from shell to shell. Bloopers are common.
Stands to reason since it would be pretty much impossible to get a good, consistent seal with a .410 wad in a 12 ga. barrel. Most agree that longer tubes are necessary to allow for a more complete burn of the .410 powders.
