Why are you making end caps for your cartridges? They're not supposed to fit flush with the chamber but slightly protrude so they can be cut off when you close the breech leaving the entire back of the cartridge open to ignition.
My understanding is that early Sharps cartridges were made purposefully long, as you describe, and then the block would shear off the tail on the up-stroke, exposing the powder to the flash channel.
This presented some problems, in that invariably some powder came away with the sheared-off tail. This powder sometimes ended up in the action and accumulated under the forestock. After a while, it could explode, splitting the forestock. Or you'd end up with powder and debris on the top of the block, which would need to be dumped/blown off before firing or you could get a little flash in your face.
To solve these problems, a new style of cartridge was designed that fit entirely in the chamber, with the rear of the cartridge flush with the end of the chamber. The cartridges were capped off with a thin fragile skin that the cap blast could rupture on firing and set off the charge.
For N-SSA competition, the latter style of cartridge is desirable because it is easier and faster to load, and most importantly you don't lose any powder due to shearing, so your loads are more consistent and consequently your accuracy is better.
My cartridges are made of printer paper, with the end cap being made of hair curler paper. I will be making a video on their construction soon.
Here is what my cartridge looks like:
Steve