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I've always thought it could be worked out myself (anything is theoretically possible, right?), if you had the time and money/tools to throw at it.

At a minimum, I would think you would need to plastic weld some new sides onto the USC lower to widen up the recessed mag area, and then open it up to the wider double stack mag width.

There's plenty of used USC lowers out there from people that have done UMP conversions. I'm surprised no one has tried to modify one yet.
 

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You can buy a drum mag that will fit in the USC mag opening, giving you an extra 50 shots.

Other than that, I'm not sure what you could do to the USC lower to make it work. It's all polymer, and the UMP mags are physically wider than the slots indented on the side of the lower. You'd have to re-mold a wider polymer slot that accepts the mag and super-glue it to the existing lower :) and I don't know how well that would work.
 

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...OR, you could fab up some plastic or metal rails on the sides of the front and rear piece of the lower receiver after the middle has been cut out, and use some form of riveting or screws to hold it in place.

I am willing to try this on one of my spare lowers, but I have to find a highcap 25rnd mag cheap before I try it on a whim.
 

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Almost there. it looks like it might need a little more work to open it up completely. Then, all you would need to do is get a sheet of a suitable plastic, maybe 1/8" thick or so, cut it to fit around the sides of the mag cut out and epoxy or plastic weld it to the sides of the lower. It may not be the prettiest mod, but it sure looks to be functional.

That's basically it. It looks like it just may be a go for those that don't want to do the full UMP conversion, but still want the option of a 25 round mag for their USC.
 
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