Gents,
An old trick that we use in the military is to use a few patches to fill/seal the chamber. Then we pour a good copper solvent such as Hoppes #9, or that Sweet's into the Bbl until it's about filled. We'd let the rifle stand for a while as the chemical worked. Then you'd pour off the fluid, saving it in another bottle/container, and then give the bore a stiff brushing. Patches run through would give you an indication of how effective your soaking was.
I understand that Midway sells a chamber 'plug' that makes it easier to do this. On a completely dry bore, a patch soaked in your favorite solvent will let you know if you need to do the treatment again. This is usually a MUST do with Surplus military weapons.
Incidentally, if a commercial rifle is fouling so easily (20 rnds), I'd be concerned that the bore was rough, and that it might need to be lapped. Keep in mind that if you do a really good job in lapping the barrel, you might just make it a tad over size, and this would play hob with your accuracy. From there you have a few options, but those are better discussed at another site - like "the Smithy", or handloading.