I have a couple of 6AR Turbo 40 degree uppers that shoot very well. I have one with an 16" barrel and one with a 28" barrel. The 28" is for highpower rifle competition shooting. Shot the 28" yesterday at 600 yards with a scope to check some loads out. All (105 and 107 bullets and Reloader 15 powder) grouped well, but one load with 107 SMK shot 4 shots into around 1 1/2" with the the fifth hole being about 6" to the left. Had some switching wind which may account for the one out of the group. I was not watching the wind just trying to shoot fast.
If the 6mm bullets will do what you need I think the 6AR family would be a good choice and can be had much quicker than one of the WSSM's.
I have a 25 WSSM on order from DTECH that has around a 7 month delivery, The 25 caliber has some heavier bullet options if you are interested in that. I plan to use it for pigs and deer. The pigs is why I am interested in something larger than 6mm.
Good luck. Ken