There certainly seems to be a huge variation in the quality of the Remington Barrels, which includes the chamber and throat. It sounds like a tap is used to cut the threads in the receiver, instead of cutting them with a single point tool on the lathe.
On mine, the 8mm fouls noticeably quicker than my 223. When cleaning the 223 which at 26" is the longest barrel I own, it has very low friction and needs very few patches before they come out clean and with no change in resistance. The 8mm typically needs a bore mop first, then some copper remover, then some more #9 and finally some Remoil followed by a dry patch. My one DPMS is similar to the 8mm barrel in that it needs more work to clean and then with the operating system, there are a lot more components that have to be stripped and cleaned and re-lubed.
I think the Remington 223 and the DPMS in 7.62x39 will be getting sold shortly. I already have a superior replacement for the Remington bolt gun and the DPMS in 7.62x39 will be getting replaced with an SKS. In reality the "functional equivalent" of the DPMS will be my Galil in 223 which I should shortly have sorted out. I have to say that with the poor quality receivers and shoddy assembly methods in use at Century Arms, you really have to love to Golani to put up with what it takes to make one of those misfits work. They make Remington look like saints.... On my rifle, they parkerized the inside of the bore, since they forgot to "plug" it when they parked the barrel / receiver. Then the receiver was out of spec, forcing the barrel out of position with a too shallow chamber, so they ground the back of the bolt lugs to make it fit, instead of fixing the chamber or receiver.
When I had the barrel replaced, it needed a replacement bolt, which came out of the surplus bin (at a smith that specializes in Galils). As we later found out, that bolt had been machined on the face that contacts the cartridge by 40-50 thou, altering the relationship of the round to the extractor... With the result that my rifle cannot eject its brass... So now, on the 3rd bolt, it looks like the extraction and ejection will work, but I may have to fix the chamber again.... As I say, they make Remington look like saints...