I built a 26wssm ~5yrs ago, that turned out really well. Haven't shot it as much as I'd like, with other projects going on though..
I went with a 35deg shoulder, 10thou total body taper, and necks turned to 13thou thickness(.290 loaded nks). My chamber set clearances extremely tight everywhere, and I had measured & culled a lifetime of brass before filling out the reamer form or deciding go/no-go on the project. Fired brass turned out perfectly as expected, and the cartridge surpassed my expectations in every regard.
Things learned with it(mostly beforehand luckily):
1. Win reloading wssm brass is the most consistent brass I've ever measured
2. It's the thickest brass I've ever seen or heard of
3. There is nothing 'wrong' with tight clearances provided the case is well designed, you've provided enough barrel steel around chamber, and you're rational with pressures
4. wssm brass is a serious neck turning challenge, due to it’s thickness causing excess springback to standard turning mandrels. This is exactly why the necks MUST be turned, for low ES.
5. With 139lapuas seated for bases at the nk-shldr junction, this is a fairly long cartridge. Yeah, short for ‘magnum diameter’, but not short. I went with a BAT RS, which is longer than short actions, and larger dia, and I’m glad I did. With this, I have sufficient port for loaded round ejection without pulling the bolt, and enough meat around the chamber to run this cartridge to potential without FL sizing nightmare to live with.
6. For improving this case body with single fireformings, you must lead dip anneal beforehand.
The learning here was worth a lot to me.
I think Winchester should have went only with 25, 26, and 28wssm, and that they should have provided WSSM brass for these(instead of WSM brass formed down). I also, concede that a 260imp could match the 26wssm, with way less trouble. But IMO, the 26wssm represents perfect capacity for ~140gr bullets, and my load results did support unique case design efficiency as calibrated in QuickLoad, and claimed in the patent.