Anyone tried these?  I've heard rumours they may not extract because of chamber size issues and problems with the fluting.

Or better yet, anyone know where I can get a good G3 barrel?

Don't waste your time. Of the few people I've seen who bought one, nobody has been able to get it to work properly! The flutes weren't cut deeply enough. Your best bet would be to pick up a parts kit. The FMP (Portugese) barrels are fine because they are built on HK contract equipment. They go for less money than genuine HK barrels. I've had good luck getting HK parts from www.robertrtg.com