HANDLOADING BENCH PART FOUR
Countering that, we have a big plus in the design that is hardly ever recognised or commented upon – the exceptionally long neck given the cartridge’s overall size. At 0.312 inches (on a newly trimmed case that reduces its overall length to 1.550 inches) it is one of the longest you’ll find on a modern cartridge and means you have a lot of latitude in bullet length/ seating position to play with.
What do I mean by that? Take a chamber throated for heavy bullets that sees the base of a Berger 105gn BT down in the case-shoulder area. As noted, the case- neck is 0.312 inches long, the shoulder section another 0.163 inches deep, so the ‘case-body to shoulder’ junction is 0.485 inches below the case-mouth. The Berger 105gn bullet measures 0.691 inches from its base to the ogive section that will contact the rifling lands, so we want somewhere around 0.691 inches minus 0.485 inches = 0.206 inches of freebore ahead of the case to put the bullet base at or above the body/ shoulder junction. The bullet is 1.198 inches in overall length so that figure, minus 0.485 inches = 0.713 inches will be outside of the case to give a loaded round COAL of 1.550 inches (trimmed case length) + 0.713 inches = 2.263 inches.
That compares with the CIP COAL specification of 62mm/2.441 inches, so if we’re only loading this bullet we could make the freebore a bit longer and still be within spec. However, if we leave it at this, we can handle significantly lighter bullets. If you look at Table 1, you’ll see where I calculate the bullet base will be and how much of the bullet shank is gripped by the case-neck for this and another four bullets weighing 55gn to 90gn. These are rough and ready calculations based on bullet comparator measurements which aren’t 100% accurate in this role (their function is to compare rather than measure) and also assumes the bullet ogive will contact the lands at the COAL adopted.
What we see is that both 70gn bullets can be set to reach the lands with enough shank length still in the neck – a mere tenth of an inch suffices and both are well above this figure. The 55gn Nosler BST only has 0.055 inches being gripped - 0.070 inches if seated to give a 15 thou’ jump into the rifling, so would have to be seated rather deeper and thus have a larger than optimal jump - 45 thou. if a tenth of inch of the bullet shank is in the neck.
Tangent ogive bullets like this should perform OK in grouping terms with large jumps but it will affect velocities adversely, reducing the MV and increasing
68
Tony and Bruce Lenton use this 1-8” twist long- throated Winchester to good effect in BR competitions with stubby 68gn G&C handmade benchrest bullets, despite the theoretically ‘too-fast’ twist.
the spread. Interestingly, modelling the external ballistics performance of 6mm 55gn plastic-tip boat- tail varmint bullets at the 3,800 fps that 6BR can produce against an equivalent 50gn 0.224 diameter bullet in the 22-250 Rem. which can also attain this MV, shows there is no ballistic benefit obtained from using the larger calibre.
The Sierra Infinity programme gives the 6BR a 236yd PBR (Point Blank Range) if zeroed at 207yd and the 22-250 Rem., 239yd PBR if zeroed at 210yd, assuming a two-inch diameter ‘kill-zone’ in both cases. The 224 cal. bullet also moves a little less in the wind. To me, this shows there isn’t a ballistics case for the sporting shooter adopting the 6BR with a 1-14 twist that limits it to light varmint bullets. There are other arguments – like superior 6BR accuracy and better barrel life, the 6mm burning less powder than the 22 and forcing a smaller amount of gas down a bore with a 15% greater surface area.
How did my Remington with Clark chambering shape up to my hypothetical examples in Table 1? Norman Clark’s reamer saw the COAL for the 90gn Scenar at 2.318 inches and the 70gn Sierra Matchking at 2.191 inches - both 15 thou’ (0.015 inches) off the lands, so very close indeed. However, by the time I got around to trying 55-85gn varmint bullets in the rifle there was 2500 rounds worth of erosion to the throat, so actual jumps would have been considerably longer with the lightest bullets.
I’m not going to provide tedious lists of bullets but Table 2 shows the retained velocities and 10mph wind drift values for a selection of popular 70-108gn 6mm bullets at 2800 fps for 105s and equivalent MVs for other weights at 300, 600, and 1000 yards, this compared to .308Win. and 7mm external ballistics. Next month, I’ll supply rifling twist recommendations for a wide range of 6mm bullets at 6BR MVs, look at potentially suitable powders for the cartridge and move onto the first of my test loads.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98