Synthetic material I've been told will melt in the bore and cause problems. I don't think the water cleaning method would remove the melted stuff from the sides of the bore and then would create a place for rust to form under the melted material so I've been told.
You are correct ... That's why you ALWAYS burn with a flame, a sample of your patching material before you buy it or shoot it.
As taught to me in 1972 - To see if thicker patching would be more accurate in your rifle.
1. Place your lubricated COTTON patching material across muzzle, place 'rolling paper(s)' or other 'shimming material' (panty hose, bed sheet or whatever) on TOP of the COTTON patching.
2. Start the ball FLUSH into the muzzle (my instructor used the pommel of his a Ka-Bar).
3. Trim the interior 'shimming material'. Rolling papers would be torn and panty hose if used, was twisted and cut at the top of the ball.
4. The short stud on the short starter was then used to push the ball to below flush and the COTTON patch material was trimmed at the muzzle - with the Ka-Bar.
5. Finish loading and fire as you normally would.
If done as explained, (which takes longer to explain than to actually do) only 'Pure Cotton' ever comes in contact with the bore.
To get back on topic, as all barrels are not created equal, you CAN double patch to make the ball fit the barrel.
Cheers,
Smoketown