AvidFlyer.

Following up on Ben's comments to thicken the rim, I have done that for a couple of carttriidges. It is fairly simple. I insert a dowel into the case mouth and place the rim on a steel surface. I use the flat on a vice. Tap the rim with a hammer, stricking a forward blow. A forward blow pushes the outer edges towards the case mouth. It really doesn't take much effort to thicken the rim.

I have also taken piano wire and wrapped it around a steel dowel or bolt and made steel rings that can be slipped over the mouth and down to the rim. I use wire thick enough so that one ring will provide the right headspace.

Have fun shooting your new double.