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Just the hopup rubber? or the hopup rubber on the inner barrel in the hopup unit?
If it's the latter, the only thing stopping a BB from rolling down the barrel is if the hopup is on enough so that there is a bump blocking the path of the BB.
There's some areas where you MIGHT get an air leak with swapping out the hopup rubber.
- the inner barrel is moving back and forth in the hopup...thus moving the rubber back and forth...when it's away from the nozzle even just a bit, you'll get a leak. Either swap the rubber, swap the hopup or use a bit of teflon tape (thin plubmers tape for sealing screw fittings) so that the inner barrel/rubber is good and snug in the the hopup unit (I.e. so it doesn't move at all)
- the lips (the very edge of the hopup rubber) aren't positioned nicely and thus aren't making a full seal with the tip of the nozzle. Again, you'll get a leak. Pull the barrel and rubber from the hopup, reposition and try again. The rubber lips should be even all the way around in the hopup unit.
- it's impossible to set the hopup without a bit of distance to test...but with the hopup off you should see any (or very little) rubber protruding into the inner barrel...and as you turn the hopup on, you should see a nice even bump protrude more and more. Set it half way to start with and then BB's won't roll out the barrel when you're test firing.
You could also try to reseat your mechbox in the box...sometimes they can be torqued a bit and essentially the nozzle gets pulled away from the hopup. A good example of this is with an m4 where the stock pipe screws into the spring guide. Some guys really crank on the stock screw and it essentially pulls the whole mechbox back a bit.
Best of luck,
Tys
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