Six – the best?

Posted on October 16, 2019 by towens — No Comments ↓

Can the 6mm BR Norma cartridge be a success in target shooting? Let’s find out…

Target shooting isn’t all about the .308 Win – consider, for example, the potent 6mm BR Norma. ‘BR’ stands for Bench Rest, so one expects it to be more than ordinarily accurate.

Moreover, it’s a versatile design making a great long-range fox and vermin cartridge and working equally well on small deer, just achieving the English 1,700ft/lb ME legal requirement and comfortably roe deer legal in Scotland.

Oddly enough given the name, it’s rarely seen in benchrest matches – those shot at 100 and 200 yards anyway. It has seen use at 1,000 yards, even briefly holding the world small group record, but its real forte is in bench and F-Class competition at intermediate ranges, especially 300-600 yards.

It is big in continental Europe’s 300-metre three-position shooting, dominating ISSF competitions having displaced the once ubiquitous .308 Winchester.

Nevertheless, it doesn’t have things its own way with a growing band of small to mid-sized 6 and 6.5mm precision jobs providing alternative choices: the Tubb 6XC, 6mm Swiss Match, 6.5 x 47mm Lapua, 6.5mm Hornady Creedmoor, and the rediscovered .260 Remington.

The table overleaf shows the 6mm BR’s capacity in relation to other popular 6 and 6.5mm cartridges. It also faces competition from its own kin in the form of BR ‘wildcats’ and ‘improved’ versions, especially the 6 Dasher, a radically fireformed version of the basic case.

Remington v Norma

I could have called our subject ‘6mm BR Remington’ and that’s what you’ll find it listed under in American loading manuals, also in loading die listings. Is there any difference, you ask?

There is a substantial chronological gap: the American company regularised the dimensions of ‘wildcats’ based on Frank Barnes’ .308×1.5in in 1978, while Norma set new specifications and obtained CIP listing in 1995.

Case dimensions are identical for the pair except the early Remington form, which was shorter at 1.520in compared to 1.560in, the difference entirely in the neck length.

The real differences lie ahead of the case-mouth. Remington conceived its design as a short-range accuracy cartridge loaded with 60-75gn bullets, with cartridge overall length (COAL) set at 2.200in, bullets loaded to protrude around half an inch out of the case and barrel throating dimensions specified accordingly.

With short bullets the norm, a slow rifling twist of 1 in 14in is specified by the US SAAMI controlling standards body.

Norma followed the lead of some gunsmiths and competitors, who realised the case had enough powder capacity to give heavier, more ballistically efficient bullets, giving usable MVs for longer-range shooting. This required the barrel to have a suitable twist rate allied to ‘long throating’.

The ‘Norma version’ is designed around 105/107gn match bullets seated to a maximum COAL of 62mm (2.441in), and the standard rifling pitch is 1 in 8in. That COAL sees up to 0.88in of protruding bullet, two or three tenths up on the Remy version and needing much more ‘freebore’.

6 and 6.5mm cartridges off lots of perforance chooice, from the 6PPC to the 6.5-284 Norma

The reason for two ‘official’ names is that SAAMI and CIP standards include barrel throat and twist dimensions – change them and you have a new cartridge. Shooters, on the other hand, just say ‘6BR’ then discuss chamber and barrel specifications.

If you’re having a rifle built or rebarrelled for the cartridge, your gunsmith should quiz you closely on its intended usage and bullets, then recommend a suitable rifling twist rate and throat configuration.

6BR v 6PPC

The original Remington set-up made it very close to the 6mm PPC benchrest number, and a cursory glance might suggest they’re identical. They’re not, and are based on different cartridge ‘families’.

The BR started life as the 7.62x51mm NATO / .308 Winchester, or at any rate Frank Barnes’ much shortened .308×1.5in version, in which the 2.015in length 7.62mm / .308W case was shortened to precisely 1½in (38mm) and its taper reduced, originally intended for use in a new generation of lightweight military assault rifles.

This cartridge’s surprisingly high velocities with 150-180gn bullets, allied to great precision, impressed many designers, and Remington’s Mike Walker adopted it as the starting point for the BR.

Meanwhile, an unknown Russian took the 7.62x39mm M43 used in SKS and AK rifles, necked it down to .22 calibre, and made some other changes to provide Soviet Olympic shooters with a light-recoiling, super-accurate running deer cartridge.

The Finns adopted the design, calling it the .220 Russian and that, as most readers know, was the genesis of the .22 PPC and its 6mm sibling.

Savage Model 12 F Class rifles are available chambered in 6mm BR

The M43 / PPC family has a 0.445in case head and lower-body diameter, while the .308 / Barnes / BR family measures 0.473in, giving around 12-15 per cent more capacity for a similar length case. Nobody knows for certain why the PPC delivers such an unbeatable degree of short-range accuracy, but one possibility is that the powder capacity is exactly right for purpose.

The larger 6BR maybe has too much oomph for 100-200 yards, but gets better performance at longer ranges. The case head diameter difference affects the ease and cost of building a rifle, too. Any good .243, .308 or .22-250 can be simply rebarrelled to 6BR, while the 6PPC, with its near one-off case head diameter, requires an expensively modified bolt or custom action.

While Remington produced cartridge and chamber specifications in 1978, there was a 12-year gap before it manufactured cases or cartridges. The company saw it as a ‘factory wildcat’ in a trio of calibres: .22, 6mm and 7mm.

Alongside drawings to allow toolmakers to produce chamber reamers and dies, Remington produced a basic case: the .30 UBBR (unformed basic benchrest), a high-quality .308 Winchester case with unusually thin walls.

The ignition department was different from the standard 7.62mm NATO-based number, employing the small rifle primer alongside an undersized flash-hole (current Lapua 6mm BR Norma examples have a nominal 1.5mm or 0.059in compared to the standard 2mm / 0.080in), tests having shown these features facilitate a more consistent charge-burn.

Would-be BR shooters had to re-form UBBR cases in a tedious process requiring expensive custom ‘form and trim’ dies that probably saw a high case attrition rate.

Users also found that the brass had a tendency to undo some of the reforming work during each firing, giving increasingly difficult chambering and extraction.

With Sako and later Lapua .220 Russian brass available and requiring much less work to be turned into the PPC, benchrest shooters abandoned the BR, giving its competitor a 10-year start.