here's a trick. Shorten the small return spring (cut it in half then fold over the first rung to use to secure to the unit. That has happen to me where both plungers (outer and inner) release at the same time. With the added stress with the upgraded shooting spring for some reason causes the outer plunger not to release when it is suppose to. I remedied this with either shortening the small release spring thereby creating more force when the bb is set, or putting a stronger spring in (remember there are 2 springs - shooting spring which drive the plunger to propel the bb, and the smaller release spring to set the bb. Hope this works

its always best to have a clean inner barrel for any gun (even these airsoft guns) You can also replace the inner barrel to reduce the amount of blow-by. The Stock HL inner barrel has about a 6.15mm inner diameter. I've been using tight-bore barrels with about 6.03mm or 6.04mm inner diameter. Hugh difference in power

10cm is a good length for an inner barrel

because of the length of the inner barrel the air pressure caused by the airsoft unit can only accelerate the bb if the air pressure behind the bb is greater than the air in front of the bb. the holes allow the air to escape as the bb travels out the barrel. As well, the the bb passed the small hole it allows air to get sucked in. without the hole there would be a vaccum created causing drag on the bb. I've found that all you need is about a 10cm long inner barrel of optimal firing. I replace the stock inner barrel with a cut piece of a tight bore inner barrel. The stock inner barrel inner diameter is about 6.15mm. A tight bore barrel is about 6.03. This reduces blowby (air escaping past the bb when the airsoft unit is fired. A typical stock HL airsoft gun will shoot about 60-90fps. I've had my guns shoot as hight as 235fps

I picked it up on ebay. I'd get a madbull tightbore barrel (6.03m) 650mm length and cut it using a plumbers cutting tool. The barrel is aluminum so its easy to cut. You will have to drill out the breech to 11/32 of an inch (don't go too deep as it could crack it and the you may have problems) As well, you can put spacers in the plunger to preload the spring)