I know I've seen this posted here, but I can't find the post.

I have mega lowers, which have very tight magwells.  Only 1/4 of my USGI 30's mags will drop free. (all of my 20's, 10's, and orlites drop free no problem)

What's the best way to remedy this, while maintaining full reliability in the magazine?

Thanks,
matt

Quoted: I know I've seen this posted here, but I can't find the post. I have mega lowers, which have very tight magwells.  Only 1/4 of my USGI 30's mags will drop free. (all of my 20's, 10's, and orlites drop free no problem) What's the best way to remedy this, while maintaining full reliability in the magazine? Thanks, matt

Forget trying to get them to drop free and get used to pulling them out when you change them, you can't count on a mag to drop free when it's for real and you have to change mags. Not trying to be a dick or anything but having mags that drop free isn't all that important, it's nice, but not the end all be all.

Buy an in spec lower.

Quoted: Buy an in spec lower.

I'll just ignore the comment from the troll with a colt as his picture.

I think this is what you are looking for. Search  http://www.fulton-armory.com/ for magazine, 3rd hit. BSW

Lifted from the Fulton Armory website:

How do I get my magazines to "Drop-Free"?

by Walt Kuleck

I find that some magazines, particularly 20-rounders, don't "Drop Free" from the mag well when I hit the magazine release. I need quick mag changes for rapid fire stages. What should I do?

As documented in The Black Rifle by Stevens and Ezell, the mag well is one of the more interesting areas of the AR-15-type lower receiver. The mag well early on was not gauged or controlled closely, which led to difficulties when a 30-round magazine was required. Consequently, magazine wells vary from manufacturer to manufacturer.

A quick survey of several AR-15-type rifles, using a Colt 20-round 5.56mm magazine as a gauge, showed that a DPMS lower had the "loosest" fit for the magazine, with Colt, Bushmaster tighter in that order, and the tightest being several Fulton Armory FAR-15 receivers The FAR-15s were tight enough to hold the empty mag in the mag well when the mag release button was pushed. In fact, all but the FAR-15 held the 20-rounder noticeably loosely. In contrast, several 30-round mags, from Adventure Line to Radway Green were all "Drop-Free" in all the tested receivers, including the Fulton Armory receivers.

So what do you do if you have a mag that won't drop free from your lower? Simple. The area of the magazine that provides the tightness of fit is the "shelf" just below the magazine catch aperture in the side of the magazine. Support the magazine with a two-by-four or equivalent, and smartly rap the "shelf" once. Check the magazine. If the mag still doesn't drop free, repeat. Be sure to check after every "rap" so you don't overdo it.

I hope this helps.

Thanks for your very kind business!

Best regards,

Walt Kuleck
Fulton Armory webmaster

Remove the upper and set it aside.  Insert the problem mags one at a time.  With a thin feeler gauge (.020" or so), check around the sides and see if the sides are bulging.  This happens on older mags that have been used a lot.  The sides can be squeezed in gently to fix that.

More often, at least in my experience, the problem is at the front and rear seams.  The will be a slight pucker at the overlaps of the front and rear seams between the spot welds.  

Lightly file the edge of the seams, test the fit and file again if needed.  Refinish with a light shot of moly dry lube.