I scanned through the thread, but am unsure.
I bought a used RIA 51782 .40, previous owner opened the mag well up and gave me three MecGar Para 16-40 mags w/ filed Dawson extended basepads that all drop free. I picked up another MG mag and Dawson pad, and some Arredondo springs/followers.
I can't seem to get this new mag tube to drop free with any combo of parts without having to shake the gun or tap the empty mag to get it started. The Dawson pad isn't hitting (confirmed by using a that pad on mags 1-3, and using pads 1-3 on mag 4), I don't think the spring/follower is doing it (again, mags 1-3 drop with Arredondo parts, mag 4 doesn't with guts from 1-3), and the external dims mic out to the same for me, within like 0.001" which to me is just a function of how hard I squeeze the calipers or how square they are on the mag.
Previous owner said he didn't think he did anything special besides filing the Dawson pad until it fit in the mag well, and insert/drop them a bunch.
Any ideas? Should I sand the outside of the tube with 400 grit? Or just insert/pull it a few hundred times? It doesn't look like the mag catch slot needs to be modified at all. This is my first 2011 and first experience with finicky competition type magazines -- I come from the Glock world.
