couple things pete: can you confirm that you used .44 mag in one of these? is that the highest pressure round you have used? also, what are the inserts made out of? i.e., is scratching/scraping the inside of your barrel a concern? and how well do they fit in the bore? is it a very tight fit? or is their "play"?
at this point, my thinking is this.
pete was able to safely use inserts for rounds up to .44 magnum. the max pressure there is 36 KSI. .30-30 is slightly more than that, so if I were to do this i would have to be prepared to destroy the action of the gun (as pointed out by zippy13) and possibly injure myself.
The accuracy, as noted by pete, is decent at best. I think the inserts might be capable of good accuracy if mounted on something sturdy, but the fact that they are inside a tube which may or may not fit them perfectly raises serious accuracy concerns.
another concern, as noted by rugerismisticness, is that the sudden sideways movement of expanding gas as it exits the end of the insert but is still within the shotgun barrel will cause erosion. my concern wouldnt be erosion, but LOTS of carbon deposits to clean.
basically, what i would end up with is a cool toy. not something useful. im thinking i will call the manufacturers and talk to them for a little while, and unless i get extremely positive vibes from them, i think it would be a no-go. maybe get an insert in .22 hornet for general plinking fun.
recoiljunky: you should definitely do that. if you get the longer insert, i would be interested in the insert's tightness in the bore of your gun and its accuracy.