I just checked mine again. The gas piston is fixed to the bolt carrier such that it wiggles back and forth a little, so alignment is a non-issue with that. The piston
will make an irregular wear pattern inside the gas tube; don't worry about it. Maybe, in a half-million rounds or something, it will wear clear through, and you can replace the gas tube and go on. Don't worry about the paint wearing off.
reinstall the bolt carrier/gas piston assembly and try to slide it back and forth while pushing the hammer down out of the way,
How are you holding the hammer down? Seems impossible to me.
You'll see on the bottom of the bolt carrier a feature that I will call the cocking ramp (don't know if it has a specific name). It rides over the hammer going both ways, and the hammer will try to force the bolt carrier up. Shooting it smooths off any roughness from the factory, but it won't quit trying.
it's hard to charge on a new mag and also hard to pull back to get it off the bolt hold open.
Mine came with a magazine that was hard to install at first, but worked in.
I have to ask, have you cleaned it yet? Start with that and some good oil (plus maybe some grease on the slides, if you like).
These are made to be worked hard and severely abused (for instance, by being in combat). Don't baby it. Yank the bolt back with authority and let it slam. Also, don't expect precision fit or finish; remember, these are disposable battle rifles, and there is virtually
no craftsmanship in them. It's a genius design for cheap, simple manufacture, and made to survive awful conditions. Being a little sloppy in places (or more than a little) seems to help that.
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