Completely normal, I personally clean mine by shooting it out........................you can google other methods. I do actually clean my rim-fire suppressor with an ultra-sonic and Simple Green Aircraft cleaner.
Don't do this if it's aluminum or if the finish can be damaged that way. What I do is use Kroil inside the metal pan that sits in the hot water US bath. The Kroil breaks up the gunk very well, any stubborn stuff can be picked out easy. It'll look new when done. The Kroil seems more gentle than does water and detergent. This is what I do for Liberty .22 cans.
Never really had to clean out the centerfire cans, but each mfg. has their own preferred method. AAC says you can use acetone and even gasoline (but gas must be cleaned out well) as well as just stick it in the dishwasher. It can be soaked overnight in some solvents too, then rinsed out. It's all in the card that comes with the cans and probably on their site too. I've seen some of the AAC cans have the finish stripped using US, but I don't know what they were using for media. Kroil won't do it, I know that. But I've never had to clean one and I have four of 'em. Truth be told I don't know how many each has shot but one of the SDN6's, my first one, has been used more than any other centerfire can I have and it hasn't been cleaned. More important is to clean the mounts off (brass brush, oily rag) and maybe the inside of the can where the mount fits. If it gets gunked up enough to hinder operation, then clean it. Otherwise leave it alone.
Salt and denatured alcohol, few tablespoons of rock salt and the can filled half or two thirds full the rest of the way with alcohol, plugged on both ends, shaken very well then rinsed with very hot water until positive no salt is left should get out any carbon or gunk. The rock salt won't dissolve in pure alcohol but will in hot water. This is the only way I can think of getting an abrasive action inside the can as opposed to a chemical one, and one that can be thoroughly removed and cleaned out. Possibly soaking overnight in Hoppes or Kroil or some carbon cleaner would be ideal if applicable or recommended, and may help the salt/alcohol mixture after well drained.
My advice to most of the questions on this forum is to just call the mfg. That's what I do, I come on here to get opinions about mfg.'s, not facts about them (unless the mfg. is like TBAC and is on this site and will answer all questions, many of which he's answered a million and half times, yet he's always cool about it.