I field stripped my MP 12 to give it a once over before taking to the range for the first time. Upon reassembly, I had to remove the upper again as I felt some resistance, I did not use any force once I notice this. After removing the upper a very small retaining ring/clip that is open on one side fell out. I have no idea where it goes or how it was knocked loose. It’s not in the owner’s manual and cannot find any field strip videos that reference it. Any help would be appreciated! (photo attached) I think it goes on the trigger pin?

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Welcome aboard!

I've never even seen an M&P12, but that a snap E-ring designed to keep pins in place. Look for a pin with groove cut near the end.

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At one time, S&W provided detailed schematics for their firearms to show all the parts and their locations but they seem to have abandoned that practice, perhaps at the recommendation of some over zealous lawyer. That's too bad but with any kind of luck, another owner will be along to provide info on where that snap ring goes.

The M&P 12 is an interesting firearm!

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Could it be the retaining clip for the push-out takedown pin?

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