I live in CA and got the plastic 10 rounder with my x39 kit. I have 9 25 rounders and 1 ten in 6.5(all C-products steel) in preparation for the 6.5 conversion. I took the plastic and steel tens,and two 25'rs out(across state lines and out of CA, of course) and, well...there's potential.
The plastic ten was kind of crap. The C-products mags took x39, but it didn't sit right in either size-the taper difference both front to rear and mag curve weren't right, and rounds were snug at the rear but loose at the front, with a tendency to nose down. I also realized that just blasting 20 rounds through each mag could get expensive, even with wolf ammo, and it was windy and raining, so I ran out of patience for trying to fight weather, conduct break-in and accuracy on the new conversion, and experiment with mags all at once. For what it's worth, the C-prod ten and shortloaded hi-caps seem to be a better option than the plastic x39 that came with the kit. I could stuff 22 or 23 into the big mags, but the more rounds the worse the fit problems were, so I stuck with 20 round boxes, and only ran each mag a couple of times.
Bottom line, I would need a LOT more experimenting before I would count on this set-up for anything important, but I would say it's worth looking into further, and I will repost here as I gain experience. The kit was being broken in, and the C-prod x39's are said to need break-in, so maybe the 6.5's do as well. I will definitely do some more experimenting-loading and unloading mags without shooting, seeing to breaking in kit to full reliability, etc. I wonder if changing out followers would help? It couldn't hurt, I guess. Ultimately , I expect to find that some mags will work for some guns reliably, but it won't be a plug-and-play, any-mag-in-any-gun swap. I had quite a few failures, but to be honest I've seen correct gun/mag combos that were as bad. I will try to see if I can get hold of x39 followers to try first, along with a little mag break-in. One caution, though-the nose-down condition ran the bullet nose into the front of the mag well hard enough to push the bullet pretty far back into the case, so I probably would have had a pretty bad overpressure if I hadn't caught that one and tried to fire it.
On a side note, I really want my XCR to be my do-all gun, and I don't think 5.56 is a do-all round. I'm pretty heavily invested in 6.5, but I'm starting to lose hope. Do any of you 6.8 guys think I could change followers in the 6.5 mags and get them to run in 6.8? They look a lot closer dimensionally than the 6.5/x39 mags. I suppose I'm being cheap if I'm considering a $550 conversion and $20-30 a box for ammo and stumbling on buying a few mags, but hey, I already have the mags and no gun for them, so if it works, why not?