cord'n to some resources, the 356 is semi-rimmed.
i know that some people do not like a rimless cartridge. thats there option.....
i myself kinda like the performance from both rim and rimless. i can make'um shoot
no matter how bigga-butt they gots..............
Any case with a deep angled base/extractor cut and its rim diameter the same as its base diameter, is considered a rimless case. The 308 and 30-06 class of cases are rimless cases.
A case where its rim is slightly larger than its base diameters is considered SEMI rimless case. IE 32, 380 and 38 Super cases as well as the 444 Marlin are examples as SEMI rimmed.
A rimmed case has a larger substantially larger rim then the dia of the base of the case. IE a 257 Mag max 30-30 and its "class of calibers are all rimmed cases.
So it could be argued that the 307/356 are a semi rimmed case. BUT that rim is fairly substantial and specifically there to head space off of. SO based on this many (self included) consider this a rimmed case.
The deal with the 356 is multi faceted, first off, yes its a rimmed case. The problems with the 35 Rem and 35 Whelen are the same, poor and inconsistent head-spacing off the little shoulder of these cases. the shoulder of the 356 is no better, BUT it head spaces off its rim, eliminating this problem.
Secondly like most 35's its efficiency far out runs what it appears to be able to do on paper. Its fully capable of vel with in 10 + - % of its parent the 358. The 358 is better than 2600 from a 200g bullet.
Lastly, yea its cool, not to many people have one and it will nearly duplicate the 358. A damn good caliber to copy!
CW