Spring binding is when either the spring sticks to the spring guide for some weird reason, or your spring is too long for the gearbox. This was the most random of my suggestions, though—I generally don’t expect this issue.
Midcap syndrome can be checked for by: using a hicap, trying different mags, and just trying to shoot the last few round of a mag.
I have to say, I think it almost has to be BBs causing a compression block. It’s possible someone smarter than myself will swoop in with the answer, but I don’t see why you’d only hear the sound when actually firing unless it was related to jamming. Both of your barrels are normal—I trust the Lambda to be the right inner bore, and the Krytac is actually a 6.05mm, so that leaves your hop unit, hop rubber, and potentially your tensioner and nozzle. The best way to try and diagnose a jam ime is to just get inside the thing and take a look, searching for wear, damage, and other indicators.
One note is that your Maxx nozzle is actually shorter than standard M4, which could possibly be related. Obviously every gun is different, but stock M4 is 21.4mm, and I tend to find works well.
What model of gun are you using?
