The 6mm BR Norma is versatile enough to work with a wide range of barrels and chambers. Here’s a run down of the choices.
Let’s look at barrel and chamber features this month. US SAAMI chamber / barrel specifications remain at 1-14in with a short throat, while CIP has 1-8in and employs much more ‘freebore’ in the Norma version.
Actually, you might specify a barrel with a rifling twist anywhere within a range of 7 to 14in, although both extremes are rare.
So how do we decide? As always, the process starts with the use the rifle will be put to and the bullets to be loaded. In practice, 8in and 12in are the two most widely used rates.
That’s eight for shooters who use long match bullets in the 95-107gn range and/or heavier expanding deer bullets, and 12 for ‘varmint hunters’ wanting to load 55-75gn pills.
The 1-8in rate actually performs well with light bullets down to 70gn, and therefore offers great flexibility. However, one thing that is really illuminating it whether it will work with really light, 55 to 60gn, ‘varmint’ bullets.
There are two potential problems in using these bullets in a ‘Norma set-up’. Even barely seating the bullet in the case neck will still see it make a big jump before it reaches the long-throated chamber’s rifling leade to the likely detriment of group size.
Worse, there is a possibility of explosive projectile break-up in flight thanks to a combination of the thin jackets used on these fragile numbers and the high rotational rate produced by 3,500fps MV and the twist rate.
The 1-in-12in twist rate is usually specified by short-range target shooters and ‘varminters’, being ideal for bullets up to the 75gn mark and also able to stabilise some 80-88gn flat-base match / varmint bullets that can give superb performance out to 600 yards.
Some opt for a compromise 1-10in twist rate barrel, which allows the use of 90gn match bullets alongside 70-87gn boat-tail ‘varmint’ bullets. All in all, the 1-in-8in twist / Norma set-up appears to offer much more flexibility without any apparent loss of performance.
If you go down the 1-10in route and want to take smaller deer with the rifle, Berger now offers an 87gn ‘Hunting VLD’ designed for this twist rate. How about the extremes?
Ultra fast twists, 1-7in or 7.5in, are only needed for the longest and heaviest match bullets, the 115gn Berger VLD and Tubb DTAC, although the latter usually works in 1-8in. 1-14in might be used to squeeze the last drop of precision out of a short-range bench rest rig firing 62-68gn flat-base bullets.
While still on barrels, the 6BR’s small case capacity and charges mean there is little internal ballistics benefit in going beyond 28in length, and you won’t lose much velocity by choosing 25 or 26in.
Turn or no turn?
Another key decision in a custom rifle build is whether to specify a ‘tight neck’ chamber that requires case-necks to be turned. While this is standard practice for the PPCs even on many fox rifles, there is near consensus that a full-house small diameter chamber neck section and heavily turned brass is not needed with 6BR.
Though a ‘tight’ 6PPC chamber might be specified at 0.262in and sees case-neck thickness reduced from 0.012-0.013in down to around the 0.0085in mark, many top 6BR competition shooters have gone for 0.270 to .271in neck-diameter chambers. This sees case necks given a light turn and a small cut into the shoulder to avoid a ‘doughnut’ forming inside the neck/shoulder junction.
Others don’t even reckon this is needed. With Lapua brass usually exceptionally consistent, it has become common to specify a relatively small neck clearance ‘no-turn’ chamber that gives around 0.002-0.004in total clearance (half that at any particular point around the neck) over rounds loaded in unmodified cases. If turning is done, it’s a partial ‘clean-up’ that only removes small amounts of metal off the thicker spots.
Such no-turn set-ups see the chamber cut with neck diameters ranging from 0.272in to 0.274in depending on taste. The cartridge drawing shows a maximum neck O/D of 0.270in on a loaded round, so the small end of the above range leaves virtually no leeway, only 0.002in overall clearance, and people relied on production batches having necks a half thou’ or so thinner than maximum CIP, but would have to turn necks if they got a thick batch.
We measured 25 pre Blue-Box new cases from lot number P00426203, and found that the modal thickness was 0.0130in with only tiny variations around that (maximums of 0.0002in below and 0.0004in above measuring three points around each neck).
Bear in mind that there are diameter variations between makes of 6mm match bullets. A survey done a few years ago produced measurements of 0.2427in to 0.2433in depending on make and model, Lapua the thinnest and new-model Berger VLDs the fattest, enough difference to affect clearances in an absolutely minimal clearance chamber.
Taking the largest of the factory HPBT bullets allied to the thickest neck found in my sample, I would have got a maximum loaded round neck diameter of 0.2695in, giving an over-close fit in my view in a 0.271in chamber and a just acceptable 0.0025in total clearance with the popular 0.272in chamber-neck. I would prefer another thou’ for peace of mind, so would specify 0.273in for this brass.
This relies on Lapua brass being consistent between lots, as well as being only slightly thinner than maximum spec. There has been an upset here with recently produced lots: the ‘blue box brass’ (from Lapua’s new blue plastic case boxes), which have turned out to be thinner.
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 neck thickness down to around 0.012in.
It’s only a coincidence that this came at the same time as the move from gold cardboard cartons to the blue plastic box, so don’t use the packaging as a 100 per cent reliable indicator.
Many users of no-turn chambered custom 6BR rifles now get an unwanted 0.002in extra neck-to-chamber clearance when they buy new cases. Anybody rebarrelling or having such a rifle built is therefore wise to buy cases in advance and measure them with a good quality tubing or neck thickness micrometer before talking to a gunsmith about chambers instead of making assumptions.