The penny drop slot or penny cartridge is a section of this instructable that requires the most precision. The Penny slot allows the player to drop several pennies (in my case 6 Pennies) that will automatically reload in the chamber when the gun is cocked. To do this, the slot that the pennies are inserted into must be precisely the size of the width of a Penny such that a penny will drop without getting stuck but not so loose that the pennies can jam in the cartridge. In addition, the cartridge will have to be placed in such a position that the gun's sliding hammer can enter the rear of the cartridge without pennies falling out and exit the front of the cartridge to shoot the penny out. The penny must be held upright in this firing chamber and prevent multiple pennies from falling into the chamber. If any of the measurements on this are off by even a millimeter, the gun's sliding hammer could potentially hit the cartridge and break it off of the gun.
One of the critical parts is the hole for the sliding hammer to enter and exit. For my particular cartridge, I cut the rear hole too large, so I had to glue an additional small piece of wood over this opening such that a coin could not fall out but the sliding hammer could enter. The hole on the other side must be large enough for a penny to exit easily.
The cartridge is done in walls are done in layers so that the penny is held upright in the firing chamber, but there is more room for the sliding hammer to enter this section. If the walls are too far apart, the pennies could jam in the firing chamber.
An additional complexity to the position of the firing chamber is that the stick that makes up the sliding hammer must completely pass through the firing chamber. The stick then blocks pennies from dropping down until the gun is cocked back. If the sliding hammer's stick is too short, pennies will fall out of the front of the chamber before the trap door closes upon the gun cocking.