There's some debate about this. Traditionally, you went with the slowest twist that would stabilise the bullet - and that's still what benchrest shooters do.
The issue was slight imbalances in the bullet or its centre of gravity being slightly out through the jacket being marginally thicker on one side or the other, or other tiny faults. The faster it's spun, the greater the dispersion in such cases. It was real issue in the UK when military 7.62 was issued to Target Rifle shooters (same as your Fullbore Rifle). Radway Green 146gn 'Green Spot' had such poor bullets in some batches that fliers were inevitable, but a 1-14" twist reduced them noticeably.
Today, some people stick to the slowest twist format, but many such as Bryan Litz, Berger Bullets' ballistician say that modern match bullets are so concentric you can forget about an 'over-high' spin rate having an effect on group size, unless you go mad of course and overstress the bullet. There was also a belief at one time that if a bullet was over-stabilised at long ranges with a high trajectory, it wouldn't want to turn over fully at the MRT point and would approach the target slightly nose-up which increased air resistance. Mr Litz disagrees with this view too - and he knows a lot more about the subject than most, certainly me!
I've seen many people get very good scores with 155s out of 10" twist barrels - assuming everything is top-notch rifle and ammunition components of course. So the morale seems to be to err slightly on the fast side as that gives you more options. For instance, the 185gn .308" Berger BT L-R will stabilise in 1-13" just but if you want to use this bullet as an option alongside 155s, specify a 1-12" and you're very unlikely to lose anything with the lighter bullets.
My F/TR rifle currently has a 1-13.5" twist Bartlein and it stabilises 168s, the Sierra 175 MK and even the 190gn MK at short range in reasonable temperatures. It did very well with 185gn Berger BTs at 100yd, but was mediocre at long range later - barely held the black. The temperature was 10 degrees (F) lower on the long range shoot at the start of winter proper so I don't know if it was this factor or the increased range or both.
Anyway for 2010 I'm putting my money where my mouth is and will have a 30" 1-10" Broughton fitted that I picked up new but cheap and was originally going to try on a .300 Rem SAUM. I'll do comparative accuracy tests with bullet weights and see how the lighter models get on on it as well as heavies up to 210gn.
Powders? Viht is very widely available in the UK and cheaper than the American makes. We get Alliant, Hodgdon, plus some Accurate Arms propellants here. The only issue over Hodgdon and Alliant is that it takes 6-12 months for new ones to arrive. Re17 has just got here, and I doubt if anybody will be able to try IMR-8208 BRX during the 2010 Benchrest season. I'm very fond of many of the Hodgdon powders though, especially VarGet and H4895. Having tried VarGet in my F/TR rifle with 155s, I can see why it's so popular with you chaps. MVs weren't as high as we can get with N540 though - I would lose around 75 fps if I switched (from 3,085 fps down to just over 3,000).
Best wishes and a Good New Year,
Laurie
The issue was slight imbalances in the bullet or its centre of gravity being slightly out through the jacket being marginally thicker on one side or the other, or other tiny faults. The faster it's spun, the greater the dispersion in such cases. It was real issue in the UK when military 7.62 was issued to Target Rifle shooters (same as your Fullbore Rifle). Radway Green 146gn 'Green Spot' had such poor bullets in some batches that fliers were inevitable, but a 1-14" twist reduced them noticeably.
Today, some people stick to the slowest twist format, but many such as Bryan Litz, Berger Bullets' ballistician say that modern match bullets are so concentric you can forget about an 'over-high' spin rate having an effect on group size, unless you go mad of course and overstress the bullet. There was also a belief at one time that if a bullet was over-stabilised at long ranges with a high trajectory, it wouldn't want to turn over fully at the MRT point and would approach the target slightly nose-up which increased air resistance. Mr Litz disagrees with this view too - and he knows a lot more about the subject than most, certainly me!
I've seen many people get very good scores with 155s out of 10" twist barrels - assuming everything is top-notch rifle and ammunition components of course. So the morale seems to be to err slightly on the fast side as that gives you more options. For instance, the 185gn .308" Berger BT L-R will stabilise in 1-13" just but if you want to use this bullet as an option alongside 155s, specify a 1-12" and you're very unlikely to lose anything with the lighter bullets.
My F/TR rifle currently has a 1-13.5" twist Bartlein and it stabilises 168s, the Sierra 175 MK and even the 190gn MK at short range in reasonable temperatures. It did very well with 185gn Berger BTs at 100yd, but was mediocre at long range later - barely held the black. The temperature was 10 degrees (F) lower on the long range shoot at the start of winter proper so I don't know if it was this factor or the increased range or both.
Anyway for 2010 I'm putting my money where my mouth is and will have a 30" 1-10" Broughton fitted that I picked up new but cheap and was originally going to try on a .300 Rem SAUM. I'll do comparative accuracy tests with bullet weights and see how the lighter models get on on it as well as heavies up to 210gn.
Powders? Viht is very widely available in the UK and cheaper than the American makes. We get Alliant, Hodgdon, plus some Accurate Arms propellants here. The only issue over Hodgdon and Alliant is that it takes 6-12 months for new ones to arrive. Re17 has just got here, and I doubt if anybody will be able to try IMR-8208 BRX during the 2010 Benchrest season. I'm very fond of many of the Hodgdon powders though, especially VarGet and H4895. Having tried VarGet in my F/TR rifle with 155s, I can see why it's so popular with you chaps. MVs weren't as high as we can get with N540 though - I would lose around 75 fps if I switched (from 3,085 fps down to just over 3,000).
Best wishes and a Good New Year,
Laurie
