This may be a long-used idea, but I just tried this recently,

 and thought I'd pass it along.

Cleaning the upper receiver of a AR style rifle, after you pull the bolt carrier group and charging handle.  I found a tube brush in a pack of them, having used smaller ones for other things, that's the right diameter for a snug fit in the upper, so I took it, sprayed a cleaner on it, put it in and rotated a couple of times, then sprayed some more and rotated again, then pulled it out and flushed out the interior, and the fouling ran out.  Saved a lot of reaching in with a cloth and scrubbing.

That cleaner is rated as safe for polymers and painted finishes, and seems to work pretty well.

As I said, I'd be amazed if this is anything new, but it was to me, and it works.

Also, I had a suitable piece of brass rod so I threaded one end to fit a chamber brush, bent the other end into a T handle, and used it the same way to clean the chamber/lug area, and it works great.  Do make sure you don't get the lug-cleaning portion into the chamber.  An old-style pistol cleaning rod with a ring end instead of a rotating one, would work just as well.