I myself have an RRA 9mm upper built on a Century lower with RRA parts kit and stock. The parts-kit hammer had to be fitted to the lower, but the replacement 9mm hammer dropped right in. I also had to file down the rear take-down lug on the upper to get it to fit the lower properly. I'm sure it was the Century lower that was out of spec, but it was a lot easier to modify the upper lug than to try to mill out metal from the lower.
For the mag-block: it would seem that generally the ASA drop-in block is one of the best. I, however, went the do-it-yourself route and bought a JRW two-piece block (no longer available) and then modified it to take sten mags. The big advantage here being that I bought 20 sten mags for about the price of one Colt mag. Granted, I did have to invest some time to modify the block and notch all the magazines, but what's a little sweat compared to a few hundred bucks in savings.
I've only put 100 rounds through the rifle so far, but it has performed perfectly. While notching the sten mags, I would periodically stop and cycle five live rounds through the mag and they all fed and ejected just fine.
If you haven't already, check out the [url=http://www.ar15.com/forums/forum.html?b=3&f=15]Rimfire and Pistol Calibers[/url] forum for some great info.
Here's a pick of my finished project:
[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=15227[/img]
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