I figured I would share this for anyone else experiencing problems with a HFC M9. As far as I can tell, this will ONLY WORK WITH THE HFC CLONE.
I've had this gun for a few years, and the topspin on it does not work correctly. Never has, but I found a neat little fix to make it work. Hope this helps someone else out!
Hop-Up (or Top Spin, which is what they call it) is not working correctly. Even at 350 fps, the bbs will simply drop off with what appears to be no backspin at all.
First, I opened the inner barrel assembly and put about 2 layers of teflon tape around the base of the rubber hop-up to seal it correctly to the inner barrel.
Next, adjust the topspin. After a little digging, I found a useful video by Evike below:Evike Video
You will notice one problem while watching this video. The HFC version does not have a screw like the version in the video does. This screw sits on the top of the hop up bucking to put pressure on the rubber piece, which in turn gives creates a top spin on the BB. The rubber slot on the side does nothing to affect this (as I have found).
Using some simple jury rigging skills... I fashioned a piece of plastic to insert into this hole on the top. Then I taped over it with 1 layer of electrical tape. Anything more than 1 layer will prevent you from inserting the full assembly back into the outer barrel sleeve.
This took me a few tries to get it right. You'll have to keep shaving off a little more of the plastic piece each time until your bb's have the correct spin on them. Take your time with this and get it right, because after you do this you will never have to adjust it again!
Hopefully someone else can learn from this!