When you look at the ejection port area on your M4, you see the front of the mechbox and cylinder and then the hopup unit in front of it.
The hopup unit has 3 dials/gears on it...the far right top one might be a bit hidden. Only adjust the bottom/left one...it'll turn the others.
Turning clock wise put on more hopup...turning counter clock wise removes hopup.
Hopup is added or removed by lowering (on) or raising (off) a rubber nub that protrudes down into the inner barrel. Imagine a straw...look down it...you see a tube. Now sqeeze/pinch the straw a bit and look down it. There an indent where you're pinching. That's exactly what the hopup unit does...it pushes against a rubber sleeve that sticks into the barrel (there's a cutout in the inner barrel for it to go through).
The rubber is pushed down from the top (12 o'clock position). As a bb pushes against the hopup rubber, air pressure eventually pushes/sqeezes it past the hopup rubber and it picks up backspin. As it travels, that backspin helps keep the bb flying "level"...up to a point.
To adjust your hopup, you need a good bit of space (100ft)...30 ft in your basement won't really be enough...out in the local park is not a good idea.
Start with the hopup off (turned CCW). Hold your rifle level. Take a couple of shots. Your BB probably won't go very far before in just drops. Turn on the hopup...as it takes effect, your shots will go further and further. If you put on too much hopup...you'll notice your BBs go out and then arc upwards (like a jet taking off from a runway). That's too much. Your shot should go out "flat and level" as far as it can without arcing up.
Different hopup rubbers will have different hardnesses...some are soft (and impart more backspin) others are hard (last longer but don't put on as much spin).
Only use good bbs. BBBastard, Madbull, KSC, Excel, etc....don't use the crappy Canadian Tire BBs.
Oh right...forgot the pistol part. Exact same effect...except you use the "key" to turn the adjustment in the barrel CW or CCW. As it turns, it pushes the hopup rubber down or let's it up. If you can get your barrel out of your pistol...you should be able to look down it as you fiddle with the adjustment and see what it's doing in the barrel. With a GBB pistol...you typically need some hopup or else cycling the slide will just knock/croquet the bb out of the chamber before you shoot...it just dribbles out the end.
Last edited by m102404; March 19th, 2009 at 14:56..