I'm not here to claim I'm a better man than you, but the differences are more striking than just the different mag and mag "slot" (the part where you insert the mag..).
I don't know if just the configuration of the two rifles is different here, but both grip and rear stock are significantly different.
The first is an earlier model using the common M4-style stock, the second is using their more recently developed proprietary stock.
Also the rear "housing" (can't find a better english word..) of the 417 is longer than the one of the 416.
Housing? EDIT, I know what you're saying now. Yeah, it's a bit longer, but when I looked at the two side by side (in real life) it wasn't readily apparent. The only two clues I had were the magwells (the mag "slot") and the sign under each stating "HK416 (5.56mm)" and "HK417 (7.62mm)" or something to that effect.
To be honest if I actually could model and animate I'd probably even start doing a different rifle than a Hk417 (*cough* L1A1 *cough*) but I cant do this at the moment or in the foreseeable future. Of course it should be possible to take the 416 and mix it up with a straight 20 rounder mag, mess a bit with the code to reflect it's a 7.62 (it is, isn't it?) and call it a day - but to be honest I'm the wrong person to do so. I'm actually bothered the skin on the RO979 states "M4A1" and "property of US government" (or so..) and I personally feel that starting to accept anything but minor inaccuracies is not what I perceive to be the "Inf Spirit".
I just asked because I've been able to get a few more people into Inf (mostly active duty types) but they want more guns. I honestly never noticed most of the problems (never used the M3, nor the UDAR, M60 and M2HB kill fast enough I never saw a difference, etc.)