As blackpowderscout said, no, I didn't pipe bomb it.
Using a light load of powder has been a common and much used method of discharging jags and stuck patched balls and bullets which were loaded without a powder charge for years.
If faced with this situation, it is best to ram the object down to the bottom of the barrel before trickling in the gunpowder to lessen the gap between the object and the powder.
98 percent of the time the small 3-5 grain powder charge won't damage the barrel even if the stuck object is a good distance above the breech and refuses to move down the bore.
(This situation seldom happens though because with a bit of water down the bore, most "stuck" objects can be rammed down the bore without difficulty.)
As for the "welding" method of removing the stuck jag.
Jags are smaller than the bore of the gun or they won't even start in the bore, therefore, they won't get stuck all by themselves.
The stuck ones always have a cleaning patch on them.
Because the brass jag is surrounded by a cloth patch it is not making electrical contact with the bore of the gun.
Arc welders by design operate on very low voltage using the high amperage to produce the heat.
The low voltage will make it difficult if not impossible for enough current to pass thru the cloth jag to energize the brass jag with the needed amperage to produce the arc/heat to "spot weld" the long brass "electrode" to it.
I strongly doubt that even adding water to the patch will reduce its resistance to the electric current.
If we assume somehow the undersize jag got stuck without a patch on it and it was actually making some contact with the bore so the welder would be able to provide the arc to "spot weld" the rod to the jag, IMO there would also be an arc between the jag and the bore.
This arc would damage the bore, leaving a scar that would end up tearing any cloth ball patch when the gun was firing a patched ball load.
Torn ball patches produce very poor accuracy in a muzzleloading rifle. :hmm: