String or cable to the end of the rifler and winch it through instead of push it through?

I now want to do inward-groove rifling on my next gun, but I have to build smoothbore first!

A foam wad is ok when it's thick(~2"), fails/leaks when it's too thin. That may be your extra pressure bang.
DO NOT do front wadding. That will really screw up your accuracy, nevermind the extra drag.
Hot glue doesn't hold well, but that's good if you use absolute minimum: the smushing of the wad ...

Ah. You want something inconspicuous and low profile while remaining potent. How do you plan on adding the air to the chamber?

Going to either gut/modify an auto compressor or use a hand pump(which I haven't tested at high(>80psi) pressure.) Not inconspicuous, and puts a lie to my 'lugging ...

On the energy and momentum conservation: you're not just pushing the bullet forward and ignoring the gases pushing forward!

The push on the bullet is split equally between the barrel->gun->shooter, and 1/3 sounds fair for how much goes to the bullet, so 1/3 would also go to the gun.

Some of the ...

I say your tanks are all too big. That maintains power for the shot, but with a smaller tank, say metal, you can fill up a lot faster and go higher pressure.

Every time I think about it, I want to get out to my mom's and (re)build it (drill press and an acre with a hill for backdrop.)

(Edit ...

Wouldn't you want the quiet one on the _sniper_ and the loud one on the _cannon_ ?

Let's say paintball, since that's about the diameter he built(ignore the home defense crap and felon comments) he wants good range, so a longer barrel, and to be unnoticed, so a quiet pump.

I had pretty good ...

But if you can achieve the same pressure on the airsoft (not likely, but let's say) the larger surface will get a significantly bigger push (19lb at 800psi vs 35lb on a 6mm) and significantly higher muzzle vel. All else being equal, the airsoft doesn't have as tight a fit to the barrels, which may ...

If you can make a steel pneumatic, throw in a coupler and a spark plug and voila, you have a hybrid.
The point is a *cheap*, very quiet, highly portable(needing to lug or purchase fuel defeats that) launcher.

It's an "without being a gun" ambition, not a "look at my really big gun" one.
I saw ...

(BTW the above assessment is correct about what I had)

Arranged with the seal cap facing forward with the barrel and a short length opposite barrel on a 'h' joint makes an ok simple shoulder rest.

That placement also makes it ideal to avoid injury when the thing blows apart on you!

The end ...

BTIS SPA?
Ok, fine, spray and pray with removable barrels.
(Updated in response to comments)
Brief history of my spud guns:
1st Gen: Ungainly 2ft 4" chamber with badly sealed spark wires. Parted out. Fired intermittently, Hair spray fuel.
2nd Gen: Interchangable barrel and straight or U adapter ...