THE HANDLOADING BENCH 6mm NORMA BENCH REST (Part 6)
loads I’d often use larger steps initially, say 0.5gn, then drop to 0.4 and down to 0.3gn as I approached the likely maximum. Some promising combinations were run again with 0.2gn or even 0.1gn steps over a smaller range to find the barrel’s ‘sweet spot’.
Most test loads used five batches, each of five rounds and were bench tested at 100 yards. Some match combinations were retested at intervals using the single favoured charge weight
Remington VS in the Shehane Tracker stock with Burris 8-32X50 scope installed – verticals were much improved.
Primers and Powders So, to the loading bench. Benchrest or small rifle magnum primers have been used throughout, at first in a rather cavalier fashion - switching between them according to whichever fell to hand first or other such unscientific methodology. I wouldn’t do that now, as while you may get away with this practice, there’s an equally good chance you won’t and a switch will affect group size and MV spreads adversely. I even re-check performance when starting a new production lot of primers now, as I’ve found that sometimes this has an effect. Nowadays, I pretty well stick to the CCI-BR4 model.
If primer piercing is an issue with your action, you should be aware that the Federal 205M Gold Medal Match has a slightly thinner cup than other makers’ BR and Magnum versions and is therefore more susceptible to this problem. Remington 7½BR primers work well in 6BR, but are also ‘softer’ than the CCI or PMC/Wolf magnums. Standard Small Rifle primers use 20% thinner brass cups than BR/Magnum versions (0.020” v 0.025”) and should be avoided like the plague by users with unmodified factory rifle actions and their looser firing-pins. Even with rifles using a custom action, the more powerful BR/Magnum versions usually give better results, 30gn of powder apparently needing a little extra oomph in the ignition department.
All powder charges were individually weighed on a set of RCBS 10-10 scales. How big should load development steps be? A good rule of thumb is to increase charges by 1% of the likely maximum weight - in this case 0.3gn based on the cartridge using around 30gn of powder. In practice, as I got a feel for
firing three to five by five-round groups. Whilst some cartridges in some rifles accept a large powder charge range and give similar results on the paper throughout, or with maybe four out of five weights performing well, I’d find ‘sweet spots’ - where group sizes shrank dramatically around a particular charge weight.
This is unlikely to be an inherent feature of the 6BR’s internal ballistics behaviour, more the effect of sticking an 8½lb barrel onto the relatively flexible Remington receiver. It is noticeable that with ‘naked’ 105-108gn bullets, smallest groups were obtained at a relatively modest 2,720-2,750 fps MV irrespective of the powder used. There will probably be another accuracy node 100-150 fps higher, but pressures would be too high for my action. The upside of
The Remy OEM and Shehane forends compared.
running at these MVs is very much longer barrel life than would be obtained from a custom action rifle belting them out at 2,900 fps or more and, I found 2,750fps quite adequate for 600 yard shooting.
Incidentally, running these figures through QuickLOAD suggested that it takes powder charge increases that produce another 8,000-10,000 psi peak chamber pressure to achieve that extra 150 fps from a 28 inch barrel, from the low 50 thousands to reaching - or more likely exceeding - 60,000 psi. At first sight,
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