I am using them against a flat base. The weight of the bullet and the force driven up the bore flatten the sub base dome some what and the skirt holds the gas behind the sub base... Gas pressure appears to be great enough that the dome is never fully collapsed...

Here is a picture of new and recovered sub bases...



You can see they are somewhat compressed but do not loose their form completely.

I even use them under 50 cal bullets shooting them sabotlessly...