No, there's not a punch-line to follow, like "fill ‘em with lead & use ‘em as vice-blocks" or "crush ‘em & use ‘em for paperweights."  I believe I may have stumbled onto a real, constructive, shooting-related use for them.

I recently built an AR  in .21 Genghis (5.45x39mm).  I was told to use standard (5.56/.223) 30-round magazines WITHOUT anti-tilt followers (capacity would - understandably - be fewer than 30 rounds).  Well, being an avid follower of this site & having read the mag-FAQ, etc., all of my USGI 30-rounders have anti-tilt followers of either the green or (more recently) orange variety.  I was considering un-updating (de-updating?) a couple for use with the 5.45 rounds when it hit me: one of the bad qualities of the USA mags is their prone-to-tilt followers.  The other bad qualities of the USA mags wouldn't be very important to me in this application, i.e., I wouldn't lose any sleep over their lack of durability & reliability because I won't shoot this rifle near as much as I do my .223 ARs (at least this has been the case with my 7.62x39mm ARs) & I don't intend to deploy this weapon in any my-life-depends-on-it situations.

Like many here, I have numerous of these heretofore irredeemable magazines lying about from the "good ole days" when (1) I could buy them new for $5 each, (2) I hadn't discovered arfcom yet, & (3) I just didn't know any better.  So the other day I loaded a couple of them as full of 5.45x39mm FMJ as my thumbs could manage (25 rounds, IIRC) & gave it a shot.  And it shot.  The first time, the last time, & every time in between.  It fed, fired, & ejected every round from both magazines & even locked open the bolt on both when empty!  I fired the first 7 or 8 rounds of the first mag slowly & deliberately, then rapid, three-round strings until empty.  I blasted through the second mag as fast as I could pull the trigger.  Nary a hiccup from the ostensibly heinous, not-worth-the-five-dollars, crappy-spray-painted-thin-walled-cheap-steel, might-as-well-toss-them-in-the-drink, 30-round USA Brand AR-15 Magazines.

Granted, I must admit:

(1) Two magazines worth of shooting does not a test make - I plan to dig out all of the USAs I have left (20 or so) from storage & load & test them multiple times...

(2) It is possible that the two I could put my hands on the other day (the ones that weren't in storage) were in some way superior to their average counterparts...

(3) Very few people here have any interest in .21 Genghis &, accordingly, few will have much interest in this topic - but wouldn't it be great to know that all those USA mags out there are at least good for SOMETHING?!?!

I will update this when I have a chance to test this "working theory" more thoroughly...

(edited to correct brain-fart - of course y'all are right & they are steel...)

My shitty USA mags are made of steel + everything else you just said.

Cool.  I have 10 of these POS mags, but sounds like you've found a good use for them.  Mine are made of steel.  I have been looking for a good home for mine since they are tight in my magwell and simply aren't reliable.  I feel bad just throwing them away.  If you want these, IM me and I'll mail them to you.  I'll pay the shipping.  I'm serious.  At least someone may get some use out of them.

Nick. Did you go the Alexander Arms route on the 5.45 or a custom job? thanks

Yes & no - I bought the AA upper from ILoveToShoot here on the board & then built it (the complete rifle) the way I wanted, i.e., post-ban M4gery.  The AA "factory" .21 Genghis mag (profile of a USGI 20-rounder) just didn't look right with that configuration, which led me to the above experiment...

FWIW, I like the caliber & two of my AKs are chambered for it.