Taking the concept one step further would be a cannon. I built one of these when I was in high school. Used a 4" steel bar that was about 24" long. Bored a 2" hole in it. No rifling. Made a mold for casting a 1.9" lead ball. I would expect the pressures, even with a 1/4 can of black powder, were quite low.

However, the trend toward smaller bullets at higher velocities and pressures in undeniable. Mainly for long range trajectory and, to a lesser, degree, recoil I'm personally thinking about hunting with a 200 grain Shockwave in 2006. This .40 caliber bullet has a high BC and is going over 2300 comfortably; and could represent an honest 250 yard load. I like the big lead conicals but they start dropping off pretty fast beyond 150 yards.