Tuning Anyone know where I can get…

What a bummer. The oring store can help.
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I found that the exact ones were very difficult to find off the shelf.

So I use these which I got off Amazon:

5mm Silicone O-Ring

The issue with them is that they are just not quite thick enough to get your interference fit of the mag into the breech (you would actually want 5mmx2mm x something like 1.8mm instead of the available 1.5mm). So I used a small dab of a thick adhesive (I used E6000, but any of the 'Shoe Goo' types would work) to hold it in place and lift it just a bit. Works just like the OEM one now, and I never have to worry about another one falling out. Plus I got a whole bag of them now, so I can quickly and easily replace any which get torn up, etc. (but none of them have yet).

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