Going further on my statement, not all ACM P90 hicaps are made equal either. I have two ACM P90 hicaps. Both are fairly old (if it makes any difference). One of them, just to see how well it worked one time, I dumped a mostly full (probably around 250-280 rounds) magazine in one or two long bursts on a single wind. I then proceeded to do the same with my TM hicap as well with similar results. The other guy that was standing around told me I was going to strip my piston. Fortunately, it hasn't stripped yet. I can't recall if the other ACM hicap works as well, nor can I actually remember which of the two I did that mag dump with.
Or, another time during a game, I had my 3 P90 hicaps on me. I was firing, and the BBs jammed up in the magazine, causing the magazine to completely unwind. I quickly changed my mag, continued to fire, and the second one did a similar thing shortly after. I changed mags again, and if I recall, I had forgotten to wind that mag, so I just held back and re-wound up all my mags.
What would be nice is if there were some easy way to rig up a P90 hicap so that it had a similar mechanism to those flash mags. That way, you wouldn't need to worry about when it unwinds itself or needs to be wound up again for whatever reason without having to take it out of the gun. The other solution is to do what I've mostly done (though I haven't been out to play since I did so), and switch to the MAG midcaps. No more issues with unwinding mags, physically they feel more durable, and you're only losing about 30-40% of your capacity, that is if the mag feeds properly in your gun...
As for fixes for the hicaps, is it just that it can't unload all the BBs in a full wind, or is the loading wheel skipping BBs every now and then, meaning that your magazine is essentially unwinding half of the time without actually loading a BB? There are some possible fixes for that second issue floating around, and for the first issue, it would be more of a matter of fiddling with the magazine to see if you can't get it to store more energy in the winding spring. It's hard to diagnose without fiddling with it, getting a better understanding of what it's not doing correctly.