I bought an Anderson lower.  I have read about some mags not fitting correctly in the mag well so I decided to check mine.  Sure enough the Pmag didn't drop free, a tight fit.  The military surplus one was fine.  I checked the inside of the mag well, and sure enough there is a lip of sorts inside.  Could I just grind it down a bit with my Dremel and use aluma black to touch it up?  Just a bit of a shave down for the larger plastic mags to fit better?

No issue with that.  I have a couple cmmg lowers that were advertised as tight.  Sand me down.

unless you're very adept with a dremel tool use a file and sandpaper (wrapped around the file so you keep it flat.) or you're gonna gouge the crap out of your lower when the drum,wire wheel runs away from you.

Nothing can take a job from almost perfect to total trash faster than a dremel.

No dremmel. Just use a metal file, a flat and a triangle to clean it up.

Only the Dremel will guarantee a diploma from the WECSOG.

In true arfcom style.... Dremel it.

Flute the barrel while your at it

Seriously. Just use a file and you will be fine

I had a Del-Ton lower that was like that.  Just use a flat file and sand paper until you relieve it enough for them to fit.

I have decided not to modify anything.... GI mags work fine, so I will probably just stick with them.

Think about this...

Every time the metal parts slide past each other they "modify" themselves a little bit.

If you know where the issue is and exactly how to remedy it, there is no downside.

Step away from the dremel tool!

I just bought a matched set upper/lower from a known manufacturer that was Cerakoted FDE.  Most mags were fine but M2 windowed PMAGs would not drop free.  Unfortunately, M2 windowed are my competition mags, and I've got more invested in that one magazine type than in the lower.  So I decided to open the mag well a bit.

I started by applying blackboard chalk to one of the M2 windowed mags.  Inserted and removed the mag a few times, then I could see inside the mag well where the chalk had rubbed of (high spots).  I slowly started removing material with a series of fine-toothed files, making sure I was able to get into the corners as well.  Re-chalk, re-mark, repeat.  It took a while but now all of my mags work.

It happens.  When the broaching tool wears the hole gets smaller on each gun run.  I sent the manufacturer a message with the issue I encountered, how I addressed it, and a suggestion to perhaps test their lowers with a M2 windowed PMAG occasionally in the production run.  Haven't heard back from them yet.

I always get grief when I say this, but why not sand down the Pmags a little since GI mads work in the lower? A PMag, even more than a metal mag, is a disposable item.

When broaching lowers and the broach wears, P mags stop dropping free LONG before GI mags.

From my standpoint, still having memories of the AWB, I want my lowers to work with all common mags.  And for me at least, lowers are cheap enough right now that I've got far more invested in PMAGs than I have in a stripped lower.

Must be a problem with earlier Anderson lowers. The AR I just put together does not have that problem.
V

Maybe you need a Pmag fix.  Their mags are usually too fat especially when loaded and the lips spreading the body.  Don't start grinding on your lower.  That's stupid.  Use aluminum mags that drop free under their own weight.

I worked on/filed down a tight mag that wouldn't drop easily.
I would not file down or grind on a receiver over a mag, besides plastic is easier than metal to work on.
Good Luck!