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Posted: 11/13/2025 10:24:51 AM EDT
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First build, using an Aero M4E1 lower with a basic milspec parts kit. After 15 minutes getting frustrated with the front takedown, I finally got it in and went to install my Mag catch/release and immediately ran into an issue. The mag catch will not seat down where it should, seems the last little bit is getting caught up on something, which makes me hopeful it's an issue with the part and not the receiver. I've already ordered another part to see if it's truly a receiver issue or a part issue, so I'm going to try that when it arrives, but if it is a receiver issue, I'm guessing my best option would be to slowly file the mag catch and hit it with some cold blue once it fits? I spent hours last night looking online for people having similar issues and found nothing, so that's part of the reason I'm making this post. But also want to know if my course of action is best or if I should try something else. |
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Originally Posted By mmj1996: I'm guessing my best option would be to slowly file the mag catch and hit it with some cold blue once it fits? That's what I would do. Even if you go crazy with a Dremel and f#(k it up it's just a mag catch. They are cheap and easy to replace. https://www.primaryarms.com/expo-arms-ar15-magazine-catch Which lower parts kit did that come out of? Where the treaded post connects to the tab looks different then a standard mag catch. |
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Came from the below NBS parts kit. Funny enough it came with an A2 grip even though I ordered the one without, and the grip is too small to fit onto my lower as well😂 would have to cut a bit off to get it on, but I'm not worried about that as my B5 fits fine. It did make me think though: these kits are probably thrown together from individual parts, so maybe the wrong piece got sent to me on the mag catch. Will keep y'all updated on the new piece, if that doesn't fit maybe I'll take to LGS before I do any filing to see if any of their spare parts fit. https://ar15discounts.com/products/nbs-mil-spec-lower-parts-kit-minus-fcg-pistol-grip/ |
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Originally Posted By j3_: If you look at the inside of the flat bar is there a small recessed spot and if there is what is it shaped like? I may be able to guess if it does has a mark which dimension might be off by looking at ones I culled out over the years. Towards the back of that bar (the part that would be showing on the outside of the receiver) it almost looks like a near imperceptible raise, which would make sense since it goes in partially and gets stuck. Maybe that's just me seeing what I want to see though https://photobucket.com/share/46175b8b-72ec-48b4-a452-c19e138c2db6 https://photobucket.com/share/1d690ee7-8c9e-4e82-a68a-eaad06becce9 |
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The ones I have that are over on measurement all have a distinct marking in the center of the inside of the bar. Sorry. Hope the new one fixes your problem. Might check the inside edge of the mag catch slot at the mag well opening section for a raised lip along some part of the inside edge of the slot. |
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Yup....that doesn't fit. What I would do is....... Measure the opening of the lower, then find the measurement of a mag release dimension on an AR15 lower manufacturing print. After that, you measure the mag release dimensions. Then you will know if the lower is in spec, and the mag release is out of spec. Or if the lower is out of spec, and the mag release is in spec. 
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Occid Vel Occisus Est !!
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Off the top of my head the mag button hole is .320”x.515” -ish Look into the hole and see if there’s a bulge on the side next to the magwell. Sometimes during the broaching operation, the material gets pushed into the mag button hole 
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