On the VFC model, the black clamp on the side pops out all the way..you slide it on the flash hider and then push it back down.
On the G&P one, it doesnt work like that. Just like you said, it only moves a few mm. When you move the little latch on the outside of the suppressor, you will notice the ring on the inside move making the diameter bigger. You then slide over the flash hider. You must have the flash hider with the little notch carved out. The pin on the inside tip of the suppressor will sit in the notch. You then slide the quick release latch back. This causes the inner ring of the suppressor to close and clamp itself around the last ring of the flash hider. Between clamping on the ring, and the notch, your suppressor will stay put.
MODT - Magnus Operator Development Team - tu fui ego eris