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Gunmane

AK Gearbox and hop-up unit moving up when magazines inserted?

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So my friends, I've come across another snag in my journey to try and restore this accursed LCT RPK of mine.  Today the lonex hop up chamber arrived from airsoft gi, and I got set to work getting it together with the stock barrel and modify flat hop set up that I had previously in it.  Got the barrel in, got the front end back on, seated it all, put the magwell spacer back in, and went to test magazines to see if they would fit.

I saw that the hopup unit and gearbox were tilting up whenever a cyma rpk midcap, lct magazine, cyma/MAG/rando 74 style mag, etc. was locked in (with some not being able to lock in randomly at times as I kept redoing everything).  I thought maybe the magwell spacer was causing it, so took that out, tried again.  Same thing.  Thought maybe the front portion is tilted off somehow but nope, straight (at least as it could be) and solid.

Now, the lonex hop up unit should not be to blame, as it is the same length bb feed nozzle wise as the lct or any other chambers I have laying about (I measured everything).  The height the magazines are pushing it up to will bring the gearbox out of alignment with it, and if I move the gearbox up it doesn't seat right and the internal selector spacer will not line up with hole for the external bits to go together with it.

Checked a few of my other replicas and they either have none or very little movement at all.

As of this moment, I have it somewhat stable enough to where two of the cyma 45rounder style midcaps fit, one with no visible movement and one with slight, two don't fit at all including the one that just arrived with the hop-up unit (yet it will seat in my cyma ak104/5 whilst the older other one won't) and I suspect maybe it is just my bad luck with magazines that are improperly made and won't fit anything right (and it doesn't seem to be a case of the front lug or rear one being too large, they are all the same size (some of the fronts metal lugs taken off and compared).  Right miffed if that is the case.

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I have a problem with my own LCT RPK that might or might not be related to this. My gun is chipping BBs when firing, causing the exterior film to foul the inner barrel. After some shots I can see little white portions of them flying out of the barrel, and when that happens BBs start flying in every direction.

Now, in your case, the gearbox itself shouldn't be moving at all. The external selector lever and the grip's screw should keep it absolutely rock solid. I never reassemble with the mag in, so I don't really understand why you say "if I move the gearbox up it doesn't seat right and the internal selector spacer will not line up with hole for the external bits to go together with it." All those parts should have been assembled before you try to insert any mags in. Align the selector hole, place the lever and tighten it, place the pistol grip and tighten it as well. The gearbox should be solidly seating at the bottom of the body, and hopefully aligned with the rest.

What happens to me is that the nozzle is actually aiming a bit low, like half a degree, so I tried placing a chock under the front of the gearbox. That way, when I look through the hole in the body for outer barrel, I see the nozzle centered and 
concentric. I use a ProWin that attaches itself to the gearbox, so that's a plus for stiffness.
This morning it was still chipping BBs, though...

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Aaaaand I put the gun on the floor and when I looked at it from above it seemed to me that stock-body-barrel weren't drawing a straight line. I think this gun was, in fact, ever so slightly bent around the hop area. I've more or less straightened it, but I have yet to see if that was the problem. I wouldn't be surprised if it was.

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