When I Googled "What does "6.5x55SE" designate on a cartridge box?" there were several pages of answers, mostly very short, with varying info, saying the same thing.
6.5×55mm Swedish cartridge
The designation 6.5x55SE on a cartridge box refers to the 6.5×55mm Swedish cartridge, which is a first-generation smokeless powder rimless bottlenecked rifle cartridge. The "SE" stands for the Swedish two-letter ISO country code. This cartridge is commonly used in Scandinavian countries and is known for its effectiveness and versatility in various shooting applications.
6.5×55mm Swedish, also known simply as 6.5×55mm, 6.5x55 SE, 6.5x55 Swede, or in its native military as 6,5 mm patron m/94 (6,5 mm ptr m/94), meaning "6.5 mm cartridge model 94", referring to 1894, is a first-generation smokeless powder rimless
This came pretty close.
The 6.5X55mm 'Mauser' / SE / SKAN situation is pretty complex historically, and probably only a handful of people in Scandinavia know all of the detail and nuances.
First, max pressure-wise, there are only two categories, and that only applies to Europe / CIP. 6.5X55mm SE is the
current standard CIP cartridge and rifle proofing one. That is, in the absence of any other factor this is
the European cartridge. Its MAP is 3,800 bar / 55,114 psi.
However, as that's excessive for some historic rifles, lower pressure standards are adopted by some European ammunition manufacturers for some of their products, geared to around 45,000 psi or maybe a bit less. In effect, although this is the original version so to speak, it's now no longer the standard, definition and standards-wise, rather a 'shadow version'. In fact, c.45,000 psi is too high for some historic rifle models I can think of and if I were loading for them, it'd be to considerably lower peak pressures. Pressures and hence loadings aside, there is no difference in cartridge or chamber dimensions.
In the US, SAAMI doesn't do dual-standards after bad experiences in the past with other cartridges where users failed to read warnings on ammunition cartons to only use their contents in modern weapons in good condition, or specifically not for named firearms. SAAMI rates the 6.5X55 at 46,000 CUP MAP, but if some of the US factory deer loads I've chronographed are anything to go by, I'd say the ammunition factories load them substantially below that level.
Then we get to the complicated bit - SKAN. First, it's an SE pressure-wise, so no difference at all in loads. It's all about standardising chamber and case dimensions. The big prone centrefire international rifle discipline across Scandinavia is an internal regional one based originally on 6.5X55 Service Rifle. When prone rifle shooting as overseen by ICFRA moved from Service Rifle to Target Rifle in the 1960s, most countries such as the UK simply dropped their old competitions and equipment and moved to 7.62 and the new single-shot rifle regs even if the first rifles were mostly rebarrelled Enfiled No.4s with cut-down stocks, or military Mauser 98 action-based. Scandinavia adopted those, but also stuck to their own version using originally service rifles with allowed improvements including match sights.
Then a problem arose. In these Scandi international comps, the host issues ammunition to all parties including the visitors. It turned out that when the 6.5X55 was originally and jointly developed by Norway and Sweden in the late 1880s/early 90s, that although a single set of 'toleranced' chamber and cartridge drawings were approved and issued to all parties, the various countries' armouries interpreted the tolerances differently. This led to situations where 'say' Norwegian manufactured ammo was 'tight' in a minimum headspace Danish or Swedish chamber, and in the reverse situation Norwegian competitors were issued ammunition with too much shoulder clearance in Swedish or Danish hosted events. Functional chambering problems with some batches of ammo at tolerance limits aside, it was claimed that some combinations gave the home team a competitive edge.
With the discipline remaining popular and desires to make it even more so, the participants got together sometime last century (1980s?) and decided to sort this issue out whilst also dealing with the matter of original rifle actions being worn out and nothing like state of the art in various things like lock-time. Rather than adopt the GB NRA TR situation of where every competitor decides on his or her choice from various competing designs that were being constantly improved (and tended to become more expensive in real terms at the same time) once adaptation of the former service models went out of fashion, the Scandis adopted a single design by Sauer, the STR, that everybody used. Switch barrel too, to allow quick change to 308 for those comps with non-Scandi countries and/or disciplines where only 223/308 can be shot, also to allow various standard barrel lengths to be available and swappable for different uses. At the same time, the chamber / cartridge discrepancies were sorted with a new and slightly different standard spec for the Scandinavian standard rifle, ie the 6.5X55mm SKAN. It is slightly different from SE. Most times there are no issues in ammo interchangeability with commercial sporting 6.5X55mm 'Swedish Mauser' ammo, but IIRC the SKAN chamber is marginally shorter from bolt face to shoulder datum line, so maximum tolerance cases from non-SKAN spec sources may not always chamber in the Sauer competition rifle.
The whole set up has worked very well AFAIK. The STR on its home turf is much cheaper than a state of the art TR rifle here and is much more flexible too allowing more than just slowfire single-shot matches in a single discipline. When a barrel is shot-out, replacement is a DIY task buying off the shelf chambered barrels at a fraction of the cost of machining, chambering, headspacing, and fitting stainless barrel blanks as is done elsewhere. Everybody uses the same model of rifle and shoots the ammo supplied in big matches, although I'm sure handloading is done by many in club matches where 'local' rules allow.
