WE M4 GBBR - Trademark lower doesn't fit blank upper, is that normal?
 

Hi,

I have a factory open-bolt m4 with a blank body and a spare receiver set which seems to be a v1 Colt trademarked, originally close bolt.

The uppers and lowers don't fit when switched between sets. Is this normal or do I have a (or both) misshappen receivers. Is there some modification I can make to make them fit?

Thanks for any advice.

Happened to me too.

I have a WE M4 (no trades), then I bought a new WE upper to make a CQB build. I found out after trying to reassemble it that it was all out of tolerance. Nothing fit well into it, and still needs modding to fit properly. All the pins wouldn't fit properly from one upper to the next.

So I would chalk this up to poor tolerances from the WE factory. You may need to sand and/or grind one piece so that it fits into the other.

its tolerances, some of my uppers and lowers just wouldnt work together, but fine with others

Thanks for the replies guys. Glad to hear I'm not crazy haha :)

Each batch of WE have different specs over the years. This has been verified by Angry Guns.

So as an example, if you have a older variant of then WE M4 body, it will not be on spec with one you picked up recently.

Cheers! Confirmed by Angry Gun :)

I guess moral of the story for me is if I want a lower and interchangable uppers I should buy multiple uppers and a single lower at the same time :P

Not that it's easy to come by lowers here!

I had the same issue trying to fit a Colt traded lower onto a new WE M4 I purchased last year.

I just had to slowly file away some material on the holes of the lower receiver. It fits now but it's tight enough that I need to use a punch to take it apart.

Yeah, that's how the upper that came with the traded set fits with my blank lower, funnily enough. I need to tap the rear pin to get it out.