Copper fouling

I got a Weatherby Vanguard 30'06 for my son about a year ago. I'd been
shooting Remington Core Lock 150 grain ammunition and found it increasingly
difficult to keep my groups together. Along with this, I noticed a brown
coloration inside the muzzle which wouldn't come off with the normal Hoppes
nitro cleaning routine after shooting.
With a little digging, I learn its copper. I found some copper solvent
which disolves the copper ok, although it seems to take a lot of patches to
get the barrel clean. Went back to the range today and it shot just fine but
after about 15 rounds I notice the streaky brown copper deposit inside the
muzzle again.
Is this caused by the ammunition I'm using, not enough time between shots or
what?
I also have a Remington 700 bdl and never noticed the problem with that
rifle...
Thanks for your help...
Mike




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