GOLDEN EAGLE MC6637M MK16 9" KEYMOD
Will update this as I get time to work on it
Initial build quality is good, well weighted and balanced.
Magazines are a little tight but also don’t drop out freely if not seated properly which is probably a good thing.
Need to grow some hardened gels as my usual go to, X force pinkies, are a bit too soft and tend to break.
Hop-up is one of those LDT slim ones like what was on the MP5’s. Barrel looks to be brass and doesn’t look to be easily replaced, has a narrowed end that the bucking goes onto and a couple notches cut out that the “t-piece” clamps onto to stop it from rotating. Barrel itself is quite short so would probably rather lengthen that and use a muzzle hop-up.
The mag that came with it and the extra mag I got have different surface finishes, one is more smooth but the other has a bit more of a texture to it.
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Brass barrels are a PITA as they take a LOT of special care and maintenance to maintain their polished internal finishes. 
Do they work, yes …are they solid, yes… do they improve performance over standard barrels…yes.
But unless constantly maintained, they will turn to shit REALLY quickly. 
I have always recommended Stainless Steel over Brass or Alloy barrels, as they are the only material that can hold their highly polished internals with the most longevity over other materials. 
Ask me how I know 
End of day it’s what you can get too. 
Pulled the barrel out again today to see about fitting a longer one.
Used my better verniers, barrel is actually 7.6id.
Breach side ended up just being a notch cut into the barrel that a lip inside the bucking sits in to hold it in place, so pretty much anyone with access to a lathe could carve a notch into a standard barrel and file down two grooves either side for the locating tabs on the t-piece and it should fit no problem.
I had a length of DB 8.5od barrel left I thought might fit like the AAP does but unfortunately not, but I’ve printed out a spacer that should work.
Reassembled the bloody thing while it was printing for some reason and cbf pulling it apart again tonight.
Also replaced the buttpad with a MFT stock for more comfort. That pad has a little flick on the bottom and man does it sit in the wrong spot on your shoulder.
