An adapter insert for the Rival D.Va blaster that should allow a fairly air tight fit for your choice of barrel material. 21.75mm OD, 16.2mm ID, the slots are designed around #13 orings but should work with any oring greated than 11/16" ID and less than 3/32" thick. Not tested yet. Should be able to just shove it down the barrel until it hits the grate in front of the plunger. Should also work with the Kronos. Probably works better with the AR removed.