The small primer case using the basic 7.9 Mauser / 30-06 / 308 etc case-head design (and equally important, the smaller diameter flash-hole at 1.5mm v the standard 2mm) was originally designed for the 6mm BR Remington, the basis of one of the most effective precision designs introduced since WW2, and still the precision benchmark for many in its modern Norma form.Alpha1 wrote:Why small primers whats the benefit.
The primer / f-h form was subsequently adopted for the smaller 7.62X39mm case-head by Sako then Lapua for the 220 Russian match case, the basis of the PPCs (more recently Alexander Arms / Lapua's Grendel), and still regarded as the best precision design for short-range shooting.
Lapua made and tested both LR / 2mm f-h and SR / 1.5mm f-hole case-head forms of its prototype 6.5X47mm Lapua design, another outstanding precision design, and the same thing has happened more recently with its version of the Creedmoor case.
Around 12 years ago, the US Palma Rifle teams joint management approached Lapua (whose cases it had switched to from domestic Winchester) to make up 1,000 SR / 1.5mm f-h 308 Win cases for evaluation. The objective was to reduce MV extreme spread / SD values with the standard loads using H. VarGet or H4895 powders and 155gn Sierra MK bullets. This objective was achieved with an average ES/SD reduction of around 30% at the cost of needing a higher charge to achieve the average MVs used in the LRP 308 Lapua based Palma ammunition. Lapua undertook its own evaluations, obtained similar results, and put the case into production under the 'Palma' name, a rarely given permission from its US Palma Team owners.
More recently new entrants into the case manufacturing arena, notably Peterson have offered SR / small f-h versions of other cases - 243 Win, 260 Rem and a few others alongside the standard LR / 2mm variants.
You note a pattern here? All related to precision cartridges and/or improving precision and internal ballistic consistency.
Because it has smaller holes in the key case-head / web area, SR cases are considerably stronger than equivalent conventional cases and are much less liable to case-head expansion which which in turn expands the primer pocket ending the case's life when it produces a slack primer fit and poor seal. To give an example, LR 308 Win Lapua cases in early F/TR loads generally lasted around five firings before scrapping for expanded primer pockets. With otherwise identical SR brass, the case-head stands up to double-figure reloadings, often over 20, despite the use of loads giving higher MVs. Neck / shoulder annealing has become the norm, as brass hardening in this area has replaced primer pockets as the key case life determinant.
What's not to like? There are a few things:
1) Higher charges needed to obtain the same pressures / MVs
2) Weaker ignition and unreliability in very low temperatures and/or with some hard to ignite powders. (Hodgdon CFE223 gave me hang and misfires in 308 Win Palma brass.) 308 Win sized cases and its charges are at the upper limit of reliable ignition with this primer type and flash-hole.
3) Fat firing pin tips and/or those with a loose fit in the bolt-face aperture cause cratering and blanking with SR primers when they don't with LR models at the same pressures. The bolt may need to be bushed and the pin turned down in some bolt designs (or in AI rifles, a special bolt and striker obtained if a 6.5X47L barrel is fitted as these bolts mustn't be bushed.)
4) The additional case-head strength can be abused by handloaders. (The 6.5X47 in particular is regularly over-loaded to obtain large cartridge performance from this small capacity design.) Even if nothing breaks / fails disastrously, wear and tear is increased and stainless barrels are often worn out at low round counts.
