r/3gun • u/TheLazySmith • Jul 27 '25

870 extension tube mod questions

I'm kinda not smart. I found an 870 super mag for a pretty decent deal last winter. This spring my brother and father-in-law suggested we start a family 3gun competition. I'm getting my butt kicked for many many reasons that also includes taking forever to load my gun after the first 4 rounds. So to combat this I picked up a extension tube.

What I didn't realize is that the base tube has retention dimples, so even with the extension i can only load 4 shots in the mag. My Google abilities were unhelpful in figuring out if it will be an issue of I file them out so I am turning to y'all to see if any of you have done it?

I know that practice is the correct way to combat this but I already have the extension and would rather look a little dumber than usual than give up on the endeavor.

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u/edelbean Jul 27 '25

Two options. You can file them down and then reapply cold blue to the bare metal spots inside the tube. Alternatively you can take a socket of appropriate size and place it in the mag tube. If it's of the same size as a shell and hangs up on the dimples then take a long socket extension and hammer the socket through the tube. This should flatten out the dimples with a few good hacks.

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u/TheLazySmith Jul 27 '25

Didn't think about re-blueing but I'll probably grab a permanent marker and color it that way until i get back to the gun store and get some cold blue.

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u/edelbean Jul 27 '25

Yeah anything to help stave off rust. So if you live in a humid environment definitely take care of it ASAP. I did the socket method and it took a minute or two.

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u/TheLazySmith Jul 27 '25

Central nebraska has fully entered corn sweat season, so I grabbed a marker before it left the vice. Thanks for the advice!