I've seen this question pop up on the forums now and then, so I figured I would post my findings.

The chamber in the breechplug of my .54 Lyman Great Plains flintlock is 1 & 1/8 inch deep with a diameter of approximately .36" and holds 25 grains of GOEX FFFg as measured in my TC U-View powder measure.

The chambers in the breechplugs of my several Green Mountain percussion barrels are 15/16" deep with a diameter of approximately .4" and hold 30 grains of GOEX FFFg as measured in my TC U-View powder measure.