I know of two ways to vent a percussion drum, but the only reason to do so is to prevent hammer blow back due to a weak mainspring you can't replace.
One way is to drill a very small hole( 1/64") at an angle in front of the nipple, into the drum, so that gas is vented forward and upward. This is fine if you always hold the forestock several inches from the trigger guard. However, if you move your hand back during off-hand shooting- say, when shooting uphill-- you risk having your fingers not only burned, but flesh blown off, by that hot gas.
The other way to vent the drum, is to drill that very small hole through the middle of your Clean out Screw. I did that with a foreign made gun that was discontinued. I could not replace the spring( I looked hard). The hammer would lift up enough, even with modest powder charges, to let the cap come off the nipple. I change nipples, etc. and nothing else helped. Today, You can buy the " Hot Shot Nipple, that has a hole drilled through the top sides of the nipple, to vent gas to not only prevent blow back, but to free the percussion cap from its grip on the nipple, to make for easier removal.
The hole in the clean-out screw vents gas to the right side of the gun, like a vent hole in a flintlock. You have to warn people not to stand close on the side of your gun when you are shooting. I have seen debris spit as far as 6 feet on one occasion from that hole in my clean out screw. That is the downside.
I did this work on my gun about a year before all the after market nipples, Like the HotShot, from Uncle Mikes, and the " Spitfire", from ? , and the Ampco bronze alloy nipples came on the market. I didn't know about Deer Creek having parts for a lot of CVA and other European made guns, so I would check with them for a replacement spring. And, I have since learned some other tricks to strengthen springs. For that reason, I would not vent my drum again- certainly not to prevent blow back of the hammer.
Venting to improve accuracy sounds like an interesting idea, but I would want to see the before and after result on my chronograph, and on paper, before I considered venting my drum permanently. Replacement clean out screws are available, so it would not be a costly series of tests to do.
Oh, I chose the 1/64" bit to drill the hole through the center of my clean out screw largely because it just fit into the screw slot of the clean out screw- TOTALLY ARBITRARY! But it worked. I was doing a lot of trick shooting for my club at demonstrations with that gun in those days, and I got the peace of mind knowing that a percussion cap was not coming back at me, or sailing off into the crowd of on-lookers.
:thumbsup: