Look for Vent liners here on the board. You definitely need one with the barrel you are getting.
You at one time asked about drilling the vent perpendicular or at an angle. Drill your vent at an angle forward. The will allow the gimlet to pierce forward into your cartridge. If it the vent hole is straight up and down there is a chance if the foil at the base of the cartridge is a bit thick the gimlet will pierce the base wad and miss the charge.
The angle forward will not effect how you slap hammer works. The hammer if you are using power stud blanks need to hit on the rim any way. For musket caps the angle will not be sufficient to cause problems.
Buy the way friction primers are readily available by mail order and should work well with your gun. If properly constructed you should be able to use friction primers or power stud blanks with you vent.
Use the google query format shred provided to search the board: site:www.gboreloaded.com/forums vent liner
You will get a bunch of hits including some on the muzzleloader forums but also on the Cannon board.
In the K.I.S.S. series of mortars I have made I have drilled a hole down the center of a bolt or socket head cap screw to make a liner.
Here is the Golf ball mortar vent.


Here is the vent on the K.I.S.S. the can pop can mortar.

A sequence on making the vent liner from a socket head capscrew.



This is comparing the socket head cap screw for the pop can mortar and screw to be used in the bowling ball mortar.
Here is a drawing for putting in the vent in for the bowling ball mortar. This drawing needs modified to indicate that uper part of the vent hole needs clearance drilll for the un thread section of the screw body.

Here is vent detail for a pressure bottle mortar.

This is one way to it. Others, I hope, should be along and share how the did theirs.
What is neat about this board, we have lots of ideas. We are glad to share. No one idea may appeal to you. But how someone did something may trigger a thought in your mind how solve a problem. You may take part of how some one did something and part of how some one else did the same thing and come up with your own idea. What ever you do-share.