cut off half the receiver tube if necessary, and the bolt can always be cut down

He's got a set like this: http://www.yachtstyle.co.uk/images/GTC%20Hole%20Saw.jpg

Figure the smallest one has hes might do the trick, since the tube is apparently cast zinc, and he uses these for steel bodywork.

You're a gundoc, I seriously hope you're kidding.

I assume that's not the exact set since those are hole saws for ceramics, not metals.
Also, nobody makes a hole saw long enough for what you want to do, but you'd need a 1" holesaw.
If you did managed to find a hole saw close to the right size and long enough, it's going to make a big mess of things, especially if you have a bore just under the size of the holesaw, you'll need to start the hole by boring out the end of the buffer tube to the same size as the holesaw to begin with.

A spade or twist drill would be the best lower cost alternative, but a boring bar is still your best bet.

Honestly, if you're going to use the wrong tools, at least use the right wrong tools. You could set up a drill and a steel wool setup to slow file the internals. It would cost less but take longer to do.

What you NEED to do is use a boring bar, a boring head, and have access to a mill with a decent dial indicator and a good vice.

The link you posted looks like for wood working? For center drilling a pilot hole, then cutting a larger hole using the pilot as a guide?

EDIT: I would just buy a new buffer... you're looking at 10-15$ in tooling, and several hours of labor, and I personally value my time at LEAST $20 an hour... Your choice though. Some people love the labor.

just sell the tubes you got and make back some of your money and buy the correct one. instead of wasting other's time with silly ideas that will ruin the tubes and potentially injure your self.
unless of course you plan to video your attempt and post it on here for our entertainment.

Most likely for tiles, the grit looks very coarse though.
The drill is for hard materials, when it drills it turns to powder, thus the lack of any flutes

The hole saws for wood have large teeth and also wobble back and forth like crazy (on purpose) as a means to prevent you from clogging the teeth up immediately upon starting the cut. The wobble also makes them incredibly useless as boring tools, and the design of holesaws makes them impossible to start holes by hand without a pilot.

Those aren't the ones I'd be using, just grabbed a pic off of Google.

And yes I do love the labor.