After reading all of this, I feel that I should chime in.
First off, no rifle has EVER left here without being fired. EVER. Our chambers are minimum spec and will exhibit pressure signs before a rifle with a looser chamber will. Lots of people out there want to turn the 280 AI into a 7mm Rem Mag or push 140 grain bullets to 3000+ FPS in the 6.5x284, neither is a good idea. We also recommend full length resizing as neck sizing can lead to sticky extraction & hard chambering. Sizing dies that use collets are asking for grief too.
Second, even if the barrel threads were slightly undersize, this would have absolutely nothing to do with case head separation. I would go into the differences between class 1, 2, & 3 threads & how they relate to the gun industry, but I haven't the time or inclination.
The 6.5x284 has been standardized by CIP but is NOT recognized by SAAMI in the US, as such it is still a "wildcat" cartridge with no real dimensional standards to adhere to. As such, one company's head space gauge can differ from another company's. Also, in the absence of a gauge, if someone attempts to check head space by using an unfired case, this will give a false reading. In the case of the 6.5x284, the new unfired cases are about four thousandths of an inch short at the shoulder. This is so that the brass can be used in any rifle chambered in that caliber and it will fire form to whichever version the chamber is. Loaded rounds in unfired brass may not extract in an unfired state due to the shoulder of the case not contacting the shoulder of the chamber and thus not providing the necessary resistance for the extractor to snap over the rim of the case. This never happens after the cases have been fired. In any event, in that caliber, the headspace cannot accurately be determined by using an unfired case.
Since the OP never contacted us about the rifle, I can only speculate as to what the original problem was. I can say with absolute certainty that it was checked with headspace gauges at least twice while here at the factory. I can also say with certainty that it was fired several times with live rounds while here. I can also say with certainty that the 6.5x284 is a tricky cartridge to load & use due to the above mentioned reasons. There is factory ammunition available for it, but this can be problematic as well. In the US, no two company's chambers are exactly alike (unless they happen to get their reamers from the same source). Which chamber was the factory ammo made for? I don't know, the consumer doesn't know, & the ammo factory may not either! The same can be said for resizing dies! All of this adds up to a potential headache to be sure. The round also has short brass life. After a few loadings of full power loads, it becomes tired. This is merely a trade off for the performance of the round. If loads are hot, the brass has an even shorter life span. That being said, it's a popular round with a huge and loyal following. Most of them are advanced shooters and hand loaders who do not mind these "quirks". However, for a few folks, it can be a headache.
My suspicion (and that's all it is) is that the OP had a reloading issue caused by either tired brass, dies that didn't match the chamber, or excessive loads. He went to his dealer and for whatever reason had a bad experience there. I know the folks up there & while it's possible, it's unlikely. He then went to his personal gunsmith who very likely got an inaccurate headspace reading from gauges that differ from the ones we used or from an unfired case. He then proceeded to sell the OP a new barrel. While there is nothing wrong with this (it sounds like an honest mistake that would be easy to do), on a brand new rifle under warranty, I would have recommended he call the factory first. We have pretty damn good customer service, which the gunsmith well knows.
Why the thread dimensions were brought up is a mystery to me. They have absolutely nothing to do with the OP's issue, unless they were mentioned as ancillary evidence to show poor quality. They are in fact in spec for the class of thread that our prints call for, please see the beginning of this post.
I also feel that I should respond to the comments about the receiver being fit for only "light duty". The receiver is machined from 4140 Chrome Moly steel & is heat treated by Smith & Wesson for increased strength. It has been destruction tested in a lab & withstood HORRIFIC pressures. I have NEVER encountered a Model 52 action with locking lug set back. The Model 52 has been used to house 338 Lapua, Remington Ultra Magnums, 26/28 Nosler, 375 H&H, and 416 Remington just to name a few. It is hardly a weak action & is every bit as strong as any action that is machined from round stock & is likely stronger than some others.
I have to say that this entire thread presents the appearance of a "Cooper bashing festival", with a hint of "P River bashing" on the side. This is unfortunate and I have taken pains in the writing of this post to remain cool, collected, & professional.
At the end of the day, if the OP had an issue (real or imagined), he could have called us and it would have been resolved to his satisfaction. No questions asked.
I hope that any issues (real or imagined) with our rifles encountered in the future do not have to go the way this case did.