Try changing the springs in the mags. those are old magazines, I think mine is from 1968 but the spring is still pretty strong works great in both of my Zenith guns (Z5RS and Z5P).I have an ATI MKE MP5 that Curtis cut and threaded for me years back. It is a solid gun. I filed a paper form 1 and added a suitable number of 922(r) compliance parts. My only compliant about it is that it does not feed reliably from the old straight MP5 mags, even with ball ammo. This is a minor point, but it makes me question the reliability of the gun as a defensive tool. I'll try to post a picture when I get home.
ETA: I should also mention that I only paid $900 for the base gun so there is that.
Here are some photos of my MKE.
agh! Although I'll bet it would have shot very accurately with an A2 attached.Yes, Curtis cuts and threads the barrel w/o removing it - a properly timed 3 lug is installed. This is what they looked like when you bought one back in 2010.
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In the years before this one was SBR'd, she took turns wearing both the A2 and A3 stocks. A real pleasure to shoot. The beauty of the roller-lock design. I still have one that is that butt ugly and unmolested.
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WOW! That's a beautiful piece you have there. I really like the stamping on the magwell, looks much better than the Zenith one. I need to find a forearm like yours.Here are some photos of my MKE.