Front ignition has been experimented with modern guns - easier than with a muzzlelaoder as the powder charge and ignition is more easily controled.
Much advantage has been claimed for frontal ignition. Experiments with both chronographs and pressure measuring machines were also made for middle, rear as in normal and front. The last data I've seen on this, done in the 90's, was inconclusive- sometimes better, sometimes the same, somethings not as good for frontal - ergo, the same for rear ignition.
Balisticians say that pressure works equally in all directions. Logic would suggest that more comes out the vent than is projected against the barrel beside the vent or the ball or bullet, yet they say no. Good enough for me. THEY have the machines & can test pressure against the barrel any where they want, breech, sides, bottom, top, etc. Where pressure can be effected, is straight back, but it takes a container like a ctg. case to effect this & one of special shape. In a ML, there is no case, therefore pressure works in all directions equally- according to what passes for modern science on that subject, today.