Fwoosh instead of boom.
Others noted that the model engines may or may not have a bore, and that bores may or may not be round. To that I'll point out that some of the bores aren't actually cylindrical, either. Various tapers are used, too.
It's to maximize thrust, certainly, but that's not just burn rate.
You have to toss mass away from (as in separate and remove, creating a reactive force) or smack mass into the mass you want to move. Gases wandering about don't help, excepting as they increase the efficiency of the parts that are. (or happen to hit the mass appropriately)
Thus, the other biggie is shaping the flume. With a naked burning cylinder end, you do get thrust, but the lateral expansion is almost utterly wasted. With burning from the center out (or plugged with appropriate nozzle) that lateral expansion increases the pressure of the main body of the flume and, thus, the thrust. (Any nozzle used needs to be fitted to this purpose correctly or it will get popped right off.)
Check out "shaped charge" for further edification. See also why guns have a chamber. These are all linked.
(Ed.: My friend pointed out that insulation against the burn is quite important, as well, to prevent the destruction of the shaft and also reduce the benefit of the nozzle.)