T/C makes three that I know of as I have all three. One is almost flat, one has a small concave section but a large enough flat ring around the concave section to support pellets, and a completely concave one that has a very small ring around the concave section. My other rifles seem to usually have concave breechplugs except for the new Knight KRB which has a slightly coned breechplug. I suspect this may be to help reduce blowback as there seems to be little or no blowback on the KRB. I think the large concave plugs were for loose powder and when the pellets arrived the plugs started to change for them.