It's been documented that the M16 tended to jam frequently in Vietnam. Subsequently they provided the cleaning kit and made it compulsory to clean weapons when standing down. Certain parts when clogged with m&d or ammo powder was the cause of it. Comparing to the AK47, they left it in the m&d for days, and it still fired without jamming after that.
I don't count the number of rounds I've fired on HK's range of weapons, but I would like to think I've enough experiences with them, and I stand firm on my preferences and comments on them.
As for the AK, I've used it a couple of times, simple and reliable but didn't like it much, sure there are people who swear by it, but not me. And this would be because [FONT=Geneva, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif]optics require special adapters[/FONT][/FONT] and I don't like the safety [FONT=Geneva, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif]mechanism.[/FONT][/FONT]
M16s are great out of the box and in a range, most civilian owners would not have any issues with them, especially for the later versions.
However, my experience with them in the field are less then desirable. Mostly due to issues with feeding, failure to extract, rim-sheer and choking of the gas tube but this could be due partly to the environments where I was deployed and the rounds I was getting for them.
It would be interesting to think that mag springs play no part in feeding the weapon. Weak springs could cause a jam.
If barrel length has nothing to do with FIBUA, try bringing a Barrett into a building next time. However, you're right to say the length of the whole weapon plays a part.
I don't have the measurements of all 3 weapons at the back of my head and my experiences with the SAR-21 has been limited to the range. Didn't know the had a 640mm version, didn't see it back then and certainly didn't have the chance to use it.
But having said that, there is only that much of length you can trim from your butt stock before the weapon starts to feel funny in your hands. Ultimately, there is a length on your butt stock you need - or are used to - which helps you operate your weapon with greater efficiency and accuracy.
I'm tall with long arms, and would prefer my butt stock fully extended mostly, hence a shorter barrel is my option and preference.