Been thinking lately that we have the Where to Get Stuff thread for roll your own, that the holiday deal threads go well, but there's no equivalent the rest of the time for firearms parts and accessories. Not stuff YOU are selling, but stuff you see a deal on, or when you have a problem finding a good part.
It's not carbine specific, but since most serious-armed-guy customizing is to the AR platform, why not start here. DO delete, move, or just tell me to fuck off if this is wrong, bad, evil or there's a thread/section I don't understand how to use right elsewhere.
Also, because I found something I think is cool, do not want to keep to myself. Someone on another forum bought a couple cases of these lovely, green anodized receiver extensions when Colt decided to stop making some guns which used them. Selling them for $42 apiece.
Shown with a Giselle rail and CTR for color comparison.
I thought, so trustworthy a part, and so so pretty that I bought one just to keep on hand for when I have a project that deserves it. It's on the SH sale thread or just email him email@example.com.