I seem to recall the old "ammonia dope" to remove copper fouling was pretty good, but you had to cork the barrel (as you mentioned) at both ends to make sure air could not get at it or else it would corrode the barrel itself where the air contacted it.
It's in Hatcher's Notebook in the ammo development and gun corrosion chapter, but I don't feel like looking it up right now.
Yes, ammonia will attack/remove copper and is in many modern fouling dopes, but maybe the concentrations are less or the reaction with iron and air is suppressed.... maybe by the goopy or oily additives. Yes, most antifreeze is propylene glycol plus other stuff to inhibit corrosion on the steel and aluminum engine parts.
Might work with chrome barrels, though.
I'd just stick with commercial jacket fouling removers and not experiment too much (or at all) with household ammonia, which they usually say is 28%*.
At least not on a shootin' gun. Don't forget manufacturers and the (then) Ordnance Department had lots of barrels to play with... along with the ammo to do the fouling with.
A review of that chapter I mentioned will be instructive. Outdated, but instructive.
Terry, 230RN
* The percentages cited vary quite a bit.