I went through the first chapter, (the best described of the three battle sequences in the book), and wrote down every instance of weapon use:
"tossed a bomb"
"(atomic rocket launch.) Carries a total of four"
"y-rack lauches two bombs"
"hand flamer in left hand(which implies that there is one on the right, too.)"
"scatter fire pills"
"sets y-rack to automatic, launches a pair of bombs every jump"
"heavy flamer off back, knife beam on wall"
"squaker-bomb on belt"
And that's it. There's no mention of any kind of rifle or cannon at all, just bombs thrown by hand, bombs launched automatically by the suit, three(!) flamers, and a nuclear bazooka. Everything finally clicked when I read this line on page three, one I've probably read dozens of times:
This is just a raid, not a battle. It is a demonstration of firepower and frightfulness. Our mission is to let the enemy know that we could have destroyed their city-but didn't-but that they aren't safe even though we refrain from total bombing. You'll take no prisoners. You'll kill only when you can't help it. But the entire area we hit is to be smashed. I don't want to see any of you loafers back aboard here with unexpended bombs. Get me?
There's no need for a cannon because that's not the kind of job the Mobile Infantry is there to do (explode things and set things on fire). Which means that their equipment is modular, and that they can be outfitted for different missions quickly and easily.