The brass I was using was not over length but just for good measures I tried new brass and got the same results, after scrubbing the throat with what I had the pressure has went down but I am ordering some iosso bore cleaner to give it a good scrubbing, do high pressure loads cause more carbon build up? I was over max load for about 180 rounds...
Years ago I went to nylon brushes but was never really happy with results. Better chemicals now available do help but in the last few years I've gone back to phosphor bronze brushes for all scrubbing duties. Here in NZ phosphor brushes have never been cheaper and IMHO their use prevents problems like yours. Sure be careful with them around the muzzle crown and use a rod guide if you're worried about damaging the leade but for hunting rifles I don't bother.Oh and I'm currently nylon brush would I hurt anything using a bronze brush?