Did you leave a load of 777 in your gun for a couple days before cleaning or forget to unload it one time for several days, months. To me it looks like that is at the breach and it corroded in the barrel. If that is the case, you will never get that out. Also, that looks to be up about as high loose powder would go too when loaded??
If it is down at the beach, is it that big of a problem other than a sightly issue??
My old mz had same problem after i forgot to discharge it for several days. seamed to bond to the sides and i never got it out.
didnt affect the pattern becuase it was at the breach though.
mpo....