I had fun with my last cannon project, so I thought I'd try one that is a little more complicated.
As with my previous models, this isn't any particular historic gun more an amalgamation of artillery I've seen in pictures and on TV. I just enjoy figuring out how I can make a printable model so that I can interact with the mechanism.
In addition to the printed pieces, you will need a spring from a pen to raise the breech. I also made a push rod out of a bent paperclip to eject the casing.
I had to print a few versions to get the tolerances right, in the end I was able to get the block to slide easily as long as I got rid of all of the support plastic in the breech cleaned out (on my printer, with my settings, etc.). You may need to sand the block and lever to get the correct fit. I also found graphite lubricant helped in some of the early models.