20g to 410g adapter question


Why is the external surface of 20-410 adapter is strangely shaped? Here is a 12-20 adapter with smooth outer surface for comparison.


Lighter weight?
Less area to get fouled up and bind it in the bore?


Mainly done to reduce the contact area in the chamber so that the fit is easier, basically what Jestertoo said. Can’t be done with bigger sizes as there is not the wall thickness to support the pressure from the fired cartridge. Some were trialed that I know of and they swelled between the landing areas and the cartridge cases stuck in them and were almost impossible to extract.