We used to do a lot of these and had specially profiled tubes made to a heavier profile to the backends . Standard 20 bore tubes may be OK if its a reasonably light weight gun ,needs to be measured up . As said you may need a new loop but will also need to have the extractors built up or new fitted . Most that we did turned out OK but there were some that really needed to be re-forended .AS to proof pressures you are still going to have high back pressure so its a matter of deciding if the action and lumps will stand it .
Dont have the up to date pressures to hand but think its the same for 16 as 20 .
