I have no idea if it is true, but read years ago I think at Benchrest Centra. That any charge over 40 grains would work better with a LRP. And i hear some on here say they get better results with SRP magnum. I have no way of testing this but would love to know.
There was pages and pages of posts on this topic at the time of Lapua 'Palma' brass adoption on the Long Range Shooting Forums (the Palma teams' / discipline's main forum), and a lot of hard questions, opposition too.
The queries/opposition basically centred on two aspects:
minor one - doubts that small primer 308 was 'legal' under ICFRA regs. ('Unaltered 308 Win or 223') That was quickly put to bed in that SAAMI doesn't specify primer or flash-hole sizes for any cartridge, and the US team captains had cleared it with ICFRA anyway.
the main objection - it wouldn't work. This went back to the early days of the Remington BR cartridges when Big Green (or anybody else) didn't make 6, 7, 30 or any other calibre BR brass, rather a thin-walled, small primer / flash-hole 308 Win case for reforming. Sometimes referred to as UBBR - Unformed Basic BR. Naturally, more than a few people tried these cases as made, straight from the box in 308 Win form attracted by their greater powder capacity than standard 308 Win because of the thin case walls, and the promise of more consistent ignition. A lot of these people also ran into ignition problems, it apparently being marginal. Some experimenters drilled the flash-holes out to the standard 0.079" diameter or thereabouts and partly or completely solved the problem, but this rather defeated the object of the exercise and experiments on the BRs and PPCs had shown that ES/SD rose if the hole exceeded 0.070" diameter.
There are many on this forum who can talk from direct experience about this brass and experiments / problems with it in those days. (The cases are still remarkably common in the US even now, many years after their limited production ended.)
Anyway the SDP 308 Win concept was abandoned back in the 70s, 80s, whenever exactly this was, as one of these good ideas in theory that simply didn't work in practice. Then a highly successful US Palma shooter whose name I can never remember - but I know will be supplied here by at least one AS Forum member - retried the Rem UBBR cases some years later using original Rem examples, and got superb results. He convinced his fellow Palma team members to look at the concept again and in turn their captains persuaded Lapua to make a few thousand trial SP 308 cases. ......... and the 'Palma' case was born and put on general sale, albeit with some caveats such as not to be used in cold weather.
The fact that 90 odd percent plus of FTR shooters use this type of brass, loading primarily Hodgdon / ADI extruded grades in the US and on my side of the Atlantic Viht N150 in 44-47gn charges tells you that it works with 40gn or higher charges with at least some popular powders. It also works from my own experience with some ball-types, but not all (eg disastrous results with Hodgdon / St. Marks CFE-223). I have sat alongside former national champion and world class FTR competitors working loads up or testing new powder lots, and I have no doubts (my own personal experiences aside) about the tiny groups they shoot and the small MV variations they see.
Here's the final part of a trio of articles on small rifle primers with the test results on a 45.5gn Viht N150 powder charge from some 15 primers models. I don't see any sign of ignition problems! (However, I had poor results previously in cold conditions - mid 30s F - with its stablemate N140 which appears to prefer an LR primer whilst N150 had also performed superbly on the same day in the same temperatures - go figure!)
http://www.targetshooter.co.uk/?p=2662
The 'magnum' SP primer issue is much more about primer cup thickness and strength rather than primer brisance.
My personal opinion is the type
is 'marginal' for 308 Win size cases / charges and those of similarly dimensioned cartridges, so it's no panacea and some users end up unhappy with them. It's a case of suck it and see in the particular application and with different primers / powders. The easy, simple option is to buy large primer brass when there is a choice, and for field use ('hunting'), I'd advise large primer every time.
The other factor is that the SP versions of 0.473" dia head cases are
much stronger / tougher than their conventional large primer brethren. In the UK - can't comment on US practice - SP 6 and 6.5mm Creedmoor Lapua brass is used by many top competitors in Tactical type comps with some impressive MVs from heavy bullets, no doubt with some 'impressive' (read scary) chamber pressures to match. But boy, do these things perform often holding their own with out & out F-Class rifles at up to 800 yards.