This antechamber is common on foreign made guns. Many of those chambers are as small as 1/4", so they can be hard to keep clean for someone who doesn't know they are there.
The most common method of cleaning the antechamber is to use a patch wrapped around a .22 cal bore brush.
Depending on how deep and solid the fouling is, I suggest soaking the bore in a good cleaner, then get in there with a ball puller or .22 cal bore brush with the end cut and the center wire untwisted, somewhat, to make a small patch puller to break up any hard fowling, then scrub with a .25 cal bore brush.
Fouling often gets pushed into these small antechambers while swabbing between shots, with a too large diameter jag. I suggest removing .020-.030" from the diameter of the jag, so that a patch will slide past the fouling on the way down, but the patch will bunch up to pull any fouling out of the bore as the jag is withdrawn.
God bless
God bless