After having picked up what is likely thousands of fired patches, I have never seen anything but fairly clean patches, some with still some lube on them
others with cuts, holes, charred curled strips of what used to be cloth, but no crud, either. There is just too much heat and flame behind that patch, to
have any 'crud' on them.
Perhaps what you are thinking is crystals of fouling or unburnt powder, might simply be chunks of compressed fouling, normal in the breech area of a gun
that is not wiped out for a given number of shots - as we do - never all day do we wipe - it is not necessary. When we clean, the barrel comes off and gets
dunked in water which dissolves the fouling - likely why we never see this 'gunk'.
When I clean with the barrel on the stock, I plug the vent and fill the barrel with cold water - let it sit for 10 minutes or 15, then drive a patch down the barrel
blasting the dissolved fouling out the vent or nipple seat- thus, no "gunk".
If you think that is unburnt powder, try lighting it. I cannot for the life of me, understand how there could be unburnt powder inside the bore after firing the gun,
especially with a load that is likely developing over 12,000PSI.