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on PSA's site, they say their 9mm uppers are designed to be used on their 9mm lowers. Does that mean they won't work on the regular 5.56 lower or is that just PSA's way of not guarantying anything?
I just want a 9mm upper I can drop on a 5.56 pistol lower, throw in a mag block and go. No hammer changing, no buffer changes. So most 9mm complete uppers will do this?
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PSA and some other companies have uppers that are specifically made to be used with their dedicated Glock magazine lower receivers. The lower is modified, and the upper has modifications and extra parts to activate the bolt hold-open using the follower of the glock magazine. These will not activate the bolt hold-open if used on a regular lower, and may require fitting to work with a regular lower and magazine block common to the other "Colt pattern" 9mm uppers.
The majority of uppers out there are for the Colt pattern 9mm, which work using a regular lower, and a magzine block for dedicated Colt or modified Uzi magazines, and some other variations. They will drop on to any standard lower.
The Olympic pattern 9mm uses a different mag block, or special magazines, which are different from and not compatible with the Colt pattern magblock. The Olympic 9mm uppers will all drop on to a standard lower.
If your current hammer is notched, you will need to switch that to a non-notched hammer, which will also work for 5.56.
Depending on your current buffer and spring setup, you may need to change buffer weights to get something that will run properly with both the 9mm upper and your 5.56 upper(s) - that's going to be a trial and error thing based on the types of ammo you shoot in 9mm and in 5.56, both.