Taking the largest of the factory HPBT bullets, the revised Berger 105gn VLD at 0.24330 inches and the thickest neck found on the aforementioned brass, I would have got a maximum loaded round neck diameter of 0.2695 inches giving an over-close fit, (in my view, anyway) in a 0.271 inch chamber and a tight but liveable with 0.0025 inch total clearance, just over a thou’ at any individual point, with the popular 0.272 inch chamber-neck but many would want another thou’ total clearance for peace of mind. Using the 105gn Lapua Scenar, the thinnest of the factory bullet models that Jason measured, would give an additional half-thou clearance in any particular chamber form.
The choice of no-turn chamber dimensions relied on Lapua brass not only being very consistent between lots but also being just a little thinner than maximum spec. There has been an upset here with recently produced lots, the so-called ‘blue box’ brass (from Lapua’s new blue plastic case boxes), that have turned out to have noticeably thinner neck-walls than older lots.
Lapua explains that earlier
production lots risked exceeding the CIP maximum loaded round O/D if cases came out at the high end of its manufacturing tolerances, so reduced the norm taking it down to around 0.012 inches.
The fact that this apparently coincided with the move from cardboard cartons to the blue plastic box is coincidental, so the packaging form cannot be taken as a 100% reliable indicator. What this means is that many users of no-turn chambered 6BR rifles get an unwanted additional 0.002 inches total neck to chamber clearance when they buy new cases, also that anybody rebarrelling or contemplating having such a rifle built is wise to buy their cases in advance and measure them, instead of using old and possibly inaccurate assumptions about dimensions. Remember too, that if you want ultimate performance, very light neck turning allied to a 0.269 inch dia. chamber is still recommended and that may need to be revised slightly downwards now with brass that starts at 0.012 inches as that thickness leaves no margin for a reduction from turning.
Lapua v The Rest
Table 1 ‐ 6BR v .223R and .308W External Ballistics Cartridge/Bullet
MV .223 Rem / 80gn Sierra MK .308 Win / 155.5gn Berger 6BR / 105gn Berger VLD 2,800 3,000 2,800 * Free recoil energy in a 13lb rifle
D600(1) / D1,000(1) = Wind drift to the nearest inch for 10 mph true (90‐degree) crosswind. D600(2) / D1,000(2) = Wind drift to the nearest inch for a simulated 5mph change in a quartering (4 o’clock) wind.
62 Target Shooter Recoil* 2.8ft/lb 12.4ft/lb 4.7ft/lb V600 1,658 1,898 1,865
Both Lapua and Norma produce loaded ammunition, but with price and availability issues as they are, nearly all recreational users handload, so this source of brass is much less important with the cartridge. (Remember though, that health & safety rules require government and corporate users to use CE/CIP etc tested and certified ammunition only, so there is a residual demand for factory cartridges even in some special-use applications).
Fat Bottom Brass I said last month that the Remington and Norma versions of the 6BR share a common case and the differences are in the chamber throating, or freebore length and barrel rifling twist rate. Whilst it is true there is virtually no difference between them, Lapua brass is generally made with a marginally larger lower body diameter - just above the case-web - than its Remy predecessor or Norma competitor and if your gunsmith cuts the chamber with a ‘Minimum SAAMI BR Remington’ chamber reamer, a percentage of cases will suffer from a slight interference fit in the rear end of the chamber. They’re only a tiny amount too fat - one or two tenths
D600 (1) 34” 27” 26” D600(2) 15” 12” 11” V1,000 1,078 1,304 1,347 D1,000(1) 118” 91” 84” D1,000(2) 51” 39” 36”
So far, you might presume that only Lapua makes 6BR brass, given my exclusive focus on the Finnish make and, to be honest it might as well as this is the only one that most precision shooters use. Its dimensions – ‘blue box’ issues aside – are well known, its consistency is usually very good, it is very strong and lasts a long time with reasonable pressure loads. Although expensive, it still offers very good value for money bearing in mind its production quality and likely lifespan.
You still see Remington 6mm BR cases listed and always in already primed form which is unusual nowadays and causes storage, retailing and transport problems but, I’ve not heard of any being available in the UK. Norma brass is imported by RUAG Ammotech UK Limited but many case types are virtually special order only. Brian Fox (Fox Firearms UK) takes orders for small lots of Norma cases and that’s how many F Class shooters get their SAUM or WSM brass, so I’m sure that BR cases could be sourced this way if desired.
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