graywolf, there is actually quite a bit of room for the rounds to move front to rear in the mag. you can test this yourself.
load up a mag and slap the rounds to the rear. did you hear a sound? slap it again. no sound, right? now slap the rounds to the front of the mag. you'll hear the same sound as the rounds seat on the front wall of the mag. slap it to the front again and no sound.
as far as rolling, i was always aware that the rounds could and would roll, but it never concerned me because they are cylindrical. i had never thought that rolling could be a cause for concern. i'll have to investigate.
a glock mag spring is so stiff as to actually prevent any movement. (in my experience the spring is sufficeintly strong to actually prevent loading to capacity without the aid of a mag loader) the steyr's spring is not.
as far as the recoil, everything is moving together, having been subjected to the same force. the gun/magazine/rounds are all moving as a result of recoil because of the direct mechanical connection provided by the mag spring pushing all the rounds up against the slide.
also, load up a mag. tap it to the rear and chamber a round. pull the mag out. you'll notice that the top round in the mag is no longer against the rear of the mag. the rails on the bottom of the slide actually pull this round out a little bit as the slide comes into battery.
another test. load up a mag and tap the rounds to the rear. holding the mag in one hand and using the tip of your thumb, slowly push the top round out. you will see that it comes straight out until the rim gets close to the end of the feedlips. at this point the neck is clearing the front of the magazine. notice how the round droops a little bit.
now, same thing but slap the rounds to the front of the mag. notice how when you push the round out it slowly heads upwards. it's this part where the round heads upwards that i believe leads to trouble.
keep in mind too that the issue we're avoiding is jamming. this jam is cause by the rim of the spent casing contacting the neck of the round in the mag. obviously, the more variables we can stack in the favor of putting the round in the mag in the path of the spent casing's rearward travel, the worse off we are.
now, we can set up the unideal circumstance. let's put all the rounds against the front wall. when you chamber a round, you drag the round in the magazine a little farther forward. just enough so that when the spent casing comes flying back, the rim hits the neck of the round in the mag which stops the spent from extracting fully. but the slide keeps going and you get a double feed.
alternatively, if you have a super strong extractor spring, the extractor won't give up it's grip on the rim as easily and could actually pull the spent even though it contacted the neck of the round in the mag.
i realize we're talking about ten thousandths here or 20 thousandths there. but the reality is that if i don't tap my mags, mine will jam about 2x in three mags. if i do tap them, it is impossible to induce malfunction.
i've also seen ten thousandths here and a couple of degrees there lead to all manner of trigger reset issues.
this is my theory. proven by test in my own experience.
if a gun feeds reliably without tapping the mags, excellent. i still maintain that tapping mags to the rear certainly can't hurt anything. and in many instances this action can cure a feeding ailment. it's standard protocol for many auto-loading weapons of both hand and long variety.
