I hate to see what's happened to Winchester's quality control - my first highpower rifle was a M70 in 243 back in '68. It would easily put 5 shots into well under an inch with Speer 80gr.PSP's, or Sierra 75HP's or 85BTHP's with WWII surplus H4831 that I bought for 80 cents/lb. The last new M70 I bought was a Heavy Varmint in 223 a few years ago. First barrel had metal displaced out over the bore at the muzzle crown - looked like someone had intentionally taken a 1/8" punch and given it a good rap, with half of the punch hanging out over the bore. Took it to my gunsmith for a fresh 11 degree crown, then when I fired it the first time, discovered the chamber was grossly oversize - cases came out swelled right in front of where the bolt counterbore ended. Sent it back to the factory for a new barrel. This one came with a clean crown, but the chamber was egg-shaped - you could see fired cases 'jump' when you rolled them across a smooth counter top. My gunsmith thought I should send it back again, but I thought "Why bother?", and had him set the barrel back about .060" & recut the chamber with a Wylde reamer. After all that, I found that the rough bore just didn't like moly'd bullets worth a damn, although it shot knotholes with bare ones. I sold it to help finance a Palma rifle project. From now on, when I want a good barrel with a good chamber, I'll pay my gunsmith to thread & chamber them. Don't know if Remington is any better, but I'm not willing to risk any more of my hard-earned $$$ to find out. I do still have a M70 Super Express in .375 H&H that's a beautiful rifle, and is very accurate. I bought it new in '87.
On the ammo compatibility issue - if the ammo is produced to SAAMI specs, it shouldn't make any difference who produced it, and ditto for chambers cut to SAAMI specs. The whole industry depends on these standards to avoid the problem you're having with WSM ammo not chambering. Winchester was buying barrels from Wilson when I got the HV, but I have no idea where they're getting them today.