Small primer brass in larger cartridges (> capacity than that of 6.5X47mm Lapua) has two origins.
The first is that Remington produced very thin-wall SP / small flash-hole 308 Win brass in the dawn of its BR cartridges back in the 1970s/80s. That was because although SAAMI registered, Remington didn't intend making any factory brass or ammo initially. BR adopters bought this special 308 Win brass and used forming die sets / trimmers to make their own. It was variously called 308 Win 'Competition' or 'UBBR' (Unformed Basic Bench Rest). There is still a considerable number of these cases in circulation.
Many people tried loading these 308 cases as supplied and results were mixed, but overall rated unsatisfactory. This was due to perceived ignition (insufficiently vigorous) issues in all but the most favourable combinations and environments. Some people reamed flash-holes out which at least partly solved the problems. So, the idea of SP versions of larger cased numbers (ie larger than the BR and 220 Russian / PPCs etc) pretty well died out for a generation.
Then secondly, around 15 years ago, the US Palma teams started looking at reviving the SP 308 Win with a view to reducing MV ES/SD values. This was driven by a team member whose name I forget who had produced excellent results with Remington UBBR based handloads. The team captains approached Lapua which made up a number of SP cases (easily done as they used the same case-head format as existing Lapua 6BR and 6.5X47mm designs) for team (and company) evaluation. Around the same time, (2004/5) Lapua was developing its 6.5X47mm, and primer / F-H dimensions raised a fundamental design question. Both forms were developed and after extensive trials, the SP / SF-H form was chosen. That raised viable SP powder charges to ~40gn levels from the BR's 30gn.
Meanwhile a full season of secret tests by selected US Palma team members took place in real-range conditions. The US team was then in the run-up to an international 4-yearly World or Palma championship fixture and these tests validated the concept. With the standard US Palma recipe of Lapua case, 155gn Sierra Palma MK and 44 or 45 point something grain charge of H. VarGet, there had been no ignition issues and ES values were reduced by a third. The reduced primer energy saw slightly reduced average MVs and charges were increased to return them to the previous LP brass levels, around 1gn more needed. This was then announced in the US Teams' Forum
http://www.usrifleteams.com/lrforum/
and that the team then being selected and trained up for the following year's fixture would use SP ammunition. It had been cleared with ICFRA and Lapua had adopted the SP brass as a production item and was about to put it on general sale so all competing nations' teams could follow the US example if they so wished. This was a VERY controversial move among American sling shooters and there were many and varied objections to its adoption on the teams' forum. Some questioned its 'legality' but most questioned the decision on the basis that it simply wouldn't work reliably enough given the 1980s' experiences.
The Palma shooters didn't increase pressures / MVs, their interest being entirely in reducing ESs and hence 'elevations' on the paper. F-Class and the new F/TR sub-discipline competitors soon took notice of this development and started adopting the new brass, also to reduce long-range elevations. Users soon discovered that the cases were considerably stronger though and survived heavy loads for longer. LP Lapua 308 Win brass only lasted around 3 or 4 firings in hot loads before pockets became slack; Palma variants appeared to have an indefinite life. So, people reasoned they'd safely accept yet heavier loads / higher pressures and this has become a common feature in many F/TR loading practices.
So, even before Lapua made SP Creedmoor brass, shooters interested in pushing their own loads' capabilities in other cartridges started to reform 308 Palma brass into 260, 7mm-08, 243 or whatever. With new companies entering the case market typified by Peterson Cartridge Co., it was a natural evolution to offer two versions of many popular match or long-range gong-shooting etc numbers, so a number of such now exist. With the Creedmoor cases being early members of these companies' ranges, SP/SF-H variants were obvious introductions and have been very successful commercially.
So, the main reasons for adopting this form in 6CM will be the hopes of helping improve precision, but also (likely primarily for many) to 1) run faster and 2) obtain better case life with max pressure loads.
Note also, that the small dia. (1.5mm / 0.059") flash-hole usually comes with the small primer pocket, but not always. Starline has decided to retain the 2mm / 0.079" diameter with its SP brass variants. PPU started producing Grendel brass with the large pocket / large F-H, but users complained of very short case life despite the cartridge's low SAAMI MAP ceiling. (A large primer pocket removes a great deal of metal / strength in its 0.439" dia. case-head, far more proportionately than in the 308 etc' 0.473".) SO, the company changed to the SP format for this model, but also retained the larger 2mm F-H despite the primer size reduction.
The possible downsides of inadvertent or unwise / uninformed SP brass choice in Creedmoor / 308 size cartridges / powder charges where there are such choices include:
- May not give proper ignition with some hard to ignite powder grades. (eg CFE-223 IME in 308 Palma brass)
- May not work well - or at all - in low temperatures, so inadvisable for winter 'hunting' use in cold regions.
- Excessive primer cup cratering, even 'blanking', in actions with large firing pins and / or poor pin tip fit in the bolt-face.
- Risk of seriously over-pressure / dangerous loads if 'unofficial' very heavy SP loads are switched to LP brass without a large charge weight reduction and rework of loads.
