I have an A2 buffer tube I'm trying to replace. It is stuck tight. I don't have any specialized tools, just a wrench here and a punch there. I'm wondering how I should proceed?

It may very well be loctited. You may want a competent gunsmith to remove it. It may have to be heated up to help break it free from the loctite.

The rifle is a brand new Colt A4 if that helps.

You need to insulate the receiver in a vice, apply heat with a heat gun on low, and use the flats on the rear of the extension tube to torque off the tube with a wrench.

No open flame is appropriate, as it can damage the metal by heat treating it into a brittle state.  Just a hair dryer or heat gun will suffice.

+1 on heat...  you can use a propane torch just don't "Quench ' the receiver (drop it in cold water) "quenching" will change the hardness if you let it cool down slowly on it's own nothing changes.... aluminum expands greatly with little heat compared to ferous metals.. you just need to heat it just to the point of being to hot to handle..

i use the same method with wrist pins in pistons. when i'm rebuillding a top end on a engine.. and i've been building engines for 40 years and NEVER had a hardness change

hair dryer will work but you'll be there 2 hours... paint stripping heat gun would be better...

got a woodstove in your house running yet?  just put it on top of the wood stove for a few mins...

small amounts of heat also loosen all loctite's red, blue, green, ect

if heat fails..  wait a day... heat it again and this time use some "Kroil" or even AFT (automatic trans fluid) and let it sit.. not to hot so the oil smokes just enough to expand the aluminum...
hope this helps

Pete