You are correct in that the infantry could watch the action of the gun crew to see where they were at in loading and firing sequence. Also remember that canister is only effective out to about 300 meters. At this distance the pattern of canister is at maximum dispersion and not as dense. Also, if the infantry was armed with rifles they could start taking down the crew and the horses. Add onto that the terrain. If they terrain was wooded then they could have some cover and concealment in which the designated marksmen could fire from. Also, remember that there was "dead space" in the ground where infantry could lie down or hide. A perfect example of that is the Emmittsburg Road at Gettysburg. Look at the gun line and the range to were the fence line was at. There should have been much more slaughter in front of those guns but many men went to ground right there. It is something that the History Channel show (History Detectives I believe it was) did not take into consideration when they did the whole battlefield calculus on Pickett's Charge.

As far as the machine gun stuff, I believe you'll are referring to "dead space." These are the areas in which a machine gun crew cannot fire directly into and the "beaten zone" or cone of fire of the gun cannot reach. This area is typically covered by another weapons system but the very least it is covered with some type of indirect fire such as the M203 40mm Grenade Launcher, 60mm Mortar, or higher caliber weapon system.

I hope this helps.

Respectfully,

Gerald D. Hodge, Jr.
War Between the States Historian
Historian: 39th Georgia Volunteer Infantry Regiment