We see many post here on the Handloading forum in regard to various problems in rolling your own ammo. They are difficult to asses and fix, especially when I don't know what technique is being used, or the prior condition of components involved.
Handloading your own ammunition is no different from any other endeavor requiring skillful hands and a large dose of common sense. I would consider it very similar to running a chainsaw. In spite of reading the instructions and all the safety precautions several times, you still are not a lumber jack!
Everything that applies to loading the 243 Winchester applies to the 270 WSM and vice versa.
Every brass maker of every caliber has a set of tollerances, or error limits, they are allowed to make and still have a "standard" product. Also they are self policing. Meaning the manufacturors check their own work, and they can either take the attitude of "that's close enough" or "scrap that one, we've got to do it right". Obviously, the latter would be a more costly product to produce. (i.e. Lapua) Certainly most will be between these two extremes.
Every die maker has this same set of "blueprints" for any given caliber to which his die dimensions must be made. There is some room for error, but that too is specified by the standards for the particular caliber.
Every gun maker is held to these same standards when chambering a gun in a particular caliber. Here again everybody checks their own work.
The Sporting Arms and Ammunition Manufacturing Institute (SAAMI) is the "governing body" in this country for these "blueprints" which are used by Ammo, die and gun makers. They do not establish the dimensions, except the pressure limits, but do establish the tollerences for all makers.
Some makers are not members of SAAMI and don't feel the need to folloow all the SAAMI established standards and tollerences. Some makers are members of SAAMI but don't have all their "products" accepted by SAAMI.
As I said all are self policing but I seriously doubt that there is a failure, by any manufacturer, to comply with the very liberal tollerences established by the SAAMI council.
I don't know who is in and who is out, but an example of this in years past was the Weatherbys calibers. U.S. SAAMI member ammunition makers wouldn't but Norma did make ammo for Wby calibers to a pressure limit much higher than what SAAMI would accept and therefore Weatherby would not pay SAAMI dues but still was able sell his products quite successfully worldwide. And, there was no known catastrophic failure of any Weatherby gun/ammo. Weatherby was the "governing body" for all Weatherby guns and ammo.
I know many of the calibers I shoot and load for are not SAAMI accepted. I don't care. I am the "governing body" at my reloading bench. I'm qualified to do that. I have a couple of rifles in custody now without SAAMI acceptance in calibers perhaps never heard of before. The 416 Alaska Express and the 338 Brewer.
You may find an individual set of dies, or a batch of brass that is a little too something, or even a gun with a rather large, or small, chamber, but that is rare. It is even much more rare for a die maker to make a die that won't size a case down enough to fit a rifle chamber, assuming correct caliber designations. Usually they will size too much.
The WSM, or WSSM series of calibers have very straight walls. Compare to the 30-06 case which has a pronounced taper. With the 30-06 case we can keep screwing the sizing die down until it is sized enough to go into the chamber and it will still be a 1/4 or even a half turn off the shell holder. With the WSM cases, with so little taper, it will need to be screwed down to touch or almost touch the shell holder. So the die manufacturers instructions of 1/8th, 1/4th or even 1/2 turn off the shell holder is not much help.
Any case fired in a chamber will go back in that chamber. Yes it may be snug, but the headspace is set and it will fit.
Some sizing dies actually destroy this fit. They size the neck down too far, (over working the brass) then when extracting from the sizer we pull the expander plug back through the neck and resize it back up to correct bullet holding dimensions. This actually pulls the shoulder forward a bit (because the expander was too tigh for the over sized neck) destroying the headspace dimension, and now it won't go back in the chamber. This is not an uncommon problem but it is almost always the fault of the brass having a neck wall dimension that is too thick.
I have encountered just about every problem a handloader could have. I see them routinely now days since I do so much 'seat of the pants' loading. I think it's the problems that keep me interested in the loading bench. Routine endeavors rarely interest me for any length of time.
We can have a problem with a 243 which will be the exact same as the problem for the 300 Ultra, for the same or different reasons. Rarely do I ever get enough of the story to fix the problem the first time around.
I once knew a pair of brothers, one of which bought a 300 WSM custom gun when they caliber first came out. (Actually a RJ back then) and I taught him handloading and he did quite well with just neck sizing the cases. His brother bought a Win M70 in 300 WSM and he loaded ammo for the brothers M70. Then he came to me because his brass, even new once fired brass wouldn't fit in his expensive custom gun anymore. At this point I wasn't told of the brothers M70 of the same 300 WSM caliber. When I began to examine his brass, it was obvious some of the cases came from a different rifle. He denied scrounging brass from the range and said it was all from his original purchase of new brass. Which it was. He said we just pick up our brass, take it home and put it in the case polisher, just like you said. (So this is clearly my fault) "Who's we", I asked? "Ah...Ken.... my brother." "And, what does he shoot"? Ahhhh....a model 70...300 WSM....!!" "Any chance the brass gets mixed"? "So..... we should keep it separated"? That would be good!
If you mix brass from different guns, you will likely need to full length resize. It may or may not say that in the instructions.