What tool/wrench to swap out AR-15 Receiver Extension

[Fusilier]

New to really taking apart my AR. What tool/wrench should I buy/borrow/make/get to swap out a receiver Extension? I'm probably going from a commercial tube to a mil-spec tube (or perhaps not?).

 

[tranman]

There's not really a point in switching...the only real difference is the sizing, you'd also have to replace your buttstock too.

 

[12GaugeShotty]

I think you're talking about the Buffer Tube? In which case you will need what is called a Castle Nut Wrench or an AR-15 Armorer's Tool which has a bunch of functions including the Castle Nut Wrench. You can buy some cheap ones on Ebay for like 10 bucks.