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Old March 19th, 2009, 13:54   #6
m102404's Avatar
Join Date: Apr 2006
Location: Toronto
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'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 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 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 it takes effect, your shots will go further and further. If you put on too much'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.

Have fun,


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 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 typically need some hopup or else cycling the slide will just knock/croquet the bb out of the chamber before you just dribbles out the end.

Last edited by m102404; March 19th, 2009 at 13:56..
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