If the 20ga gun accepts the full length 20-to-410 chambermate inserts then the higher pressure of the 410 cartridge should not be an issue as the Chambermate will contain the peak pressure before it enters the bore of the 20 ga. I believe the answer is true for the old savage FourTenners. This is not true for short cartridge size inserts.
I have noticed on 16 ga guns not all bores and chambers are equal and some, in particularly german guns, are narrower than others. This leads to problems using chambermates in the gun. Of four 16 SxS I have tried to use chambermates with two would not fit all the way in. If you do not already have the chambermates on hand if you can borrow a pair for a test fit before you lay out cash for a pair might be a good idea.
