Ejector
Thank you for the guidance on this subject. I think I might buy the parts needed and keep them in separate plastic bags so I know what's what. I will finally achieve my dream of shooting all these HKs on FA from the MP5-40, HK51, HK53, HK91, MM23e ... I've only got the MP5-9mm and HK21E (Reg Rec) to rock so far.It's going to be a bit confusing as you come up to speed but it's allot simpler than it sounds. I went thru this learning curve a few years ago. Assuming your sear is in a pack now, it was set up for either 9MM ( or 40/10 mm - all pistol caliber is the same), .223 or .308. When you move between these calibers, you move the pack to the new hosts trigger housing and put in the right ejector. 1 push pin, takes 2-3 minutes top while at the range. Lots of easy videos on this - no disconnector or or other issues. I always bring 2-3 hosts when I shoot and am popping the pack in and out in the lane.
There are 3-4 different strength hammer springs you can use in your pack. The heavier springs are designed for larger caliber - most of us put in a heavier spring and it runs with all ammo - might slow pistol caliber cycle rate down a bit but that's the only issue I've ever heard of. Little more work but you can easily swap hammer springs too - depends on the hammer design but once you see it done - nothing to it.
The retimed hammer is something else. The aftermarket sear you spent so much money on isn't exactly like the original (unless you've got something called a DLO box). This difference creates a slight change in geometry. I ran my sear pack with the original HK hammer for 9 months and it is not a problem nor to my knowledge is it dangerous. It's just that because of the change in geometry, the HK hammer doesn't lock as far back when riding on the auto sear so you can get light primer strikes in certain guns. I got them on a PTR 32P where the primer wasn't being hit hard enough despite the strongest hammer spring. You can buy a retimed hammer for your pack (easy to install yourself) and it is designed to "cock" fully back. That's it - nothing to worry about - lots to learn as you started. Good luck. Matt