Empty a few semi-auto clipsmagazines as fast as you can. You might expect to hear a regular rhythm constrained by the weapon's upper limit, but you'll actually hear something pretty irregular, including some very fast spurts. There almost seems to be a sweet spot, meaning it's less about how fast you click than about whether you click at just the right times.
Given that caveat, here's information on the MK11, SVD, and SKS. I don't co-op, so I haven't unlocked the M39. For a semi-realistic number rather than an optimal (impossible) one, I emptied 10 clipsmagazines for each weapon, timing with a stopwatch, and averaged the time needed... i.e., there's lots of room for observation error.