Plecotus wrote:I presume that the 1x10" recommendation is to do with bullet stabilisation but I'm more than happy to be corrected on that. My real question is would it be worth trying this bullet with my barrel or am I wasting my time (and money!). I'm assuming and problems would not be safety-related but again, I could quite possibly be wrong.
Correct, it is about stability, not handloading / firearm safety per se (although a completely unstable bullet and resulting inaccurate shot could conceivably produce indirect safety issues).
Stability and the rotational speeds needed to achieve same are primarily driven by bullet length (in relation to calibre) not weight, although as bullets become heavier they usually become longer and a shorthand convention has arisen which says such and such a twist rate will stabilise bullets of such and such a weight. Always rather misleading, this approach is often very so because of the development of very long 'pointy' bullets including those with lightweight synthetic tips like the TMKs which therefore need more spin than a 'fat' blunt round-nose flat-base design. (A 308 200gn VLD type design needs some two inches 'faster' rifling pitch than a same weight / calibre RN flat-base equivalent for example.)
Then there is the issue of how much stability as there is stability and there is full stability for optimum performance and precision. Stability is measured by a coefficient denoted by the letters Sg and determined by very serious multiple calculations if done to the full extent as it is for instance by military researchers and developers of artillery projectiles. Fortunately, a ballistician called Don Miller developed a shortcut calculation which is used these days by all rifle bullet companies, ballistics apps and suchlike. If you Google
Don Miller's Twist Rule you'll find several sites where you can download a simple Microsoft Excel spreadsheet to let you calculate a bullet's Sg value. To do a simple calculation you need five things - calibre, weight, MV, rifling pitch rate in inches, and the tricky one for most people, the bullet's OAL in inches. (Where you shoot in what environment is also important to some, but let that stand aside for the minute.)
The 195gn TMK's OAL is 1.465" (Bryan Litz, Ballistic Performance of Rifle Bullets, 3rd Edition). Put this into the Miller spreadsheet with an assumption of 2,500 fps MV and we get:
Don Miller's Twist Rule
Caliber 0.308 Inches
Bullet Weight 195 Grains
Bullet Length 1.465 Inches
Barrel Twist 11 Inches/turn
muzzle velocity 2500 fps
Temperature 59 degrees Fahrenheit (59 is standard)
Pressure 29.92 inches of mercury (29.92 is standard)
Sg = 1.35
Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may
gain some accuracy by going to a slower twist barrel.
Sg value is therefore 1.35 in this combination in standard ballistic / meteorological conditions.
So what does the Sg tell us. In theory, anything with an Sg of 1.0 or higher is stabilised - ie won't tumble. In practice, you need a lot more leeway for all sorts of reasons. In practice, I've seen combinations with Miller calculated Sg values around 1.1-1.15 apparently shoot OK at 100 yards but be hopeless at say 800. Recent research has come up with two crucial findings:
A minimum Sg of just under 1.3 is really important. Bullets spun to produce Sg values below a value around 1.26-1.28 will usually produce round holes and apparently group well at short ranges, but their efficiency in flight is badly affected seeing a large increase in drag and turbulence in the airflow and hence a large decrease in its effective BC.
Traditionally 1.4 was the magic number as its says on the Miller Spreadsheet printout. Today, that has been revised to 1.5. With long pointy BT modern match bullets which the TMK is a good example thereof, anything under that degrades the BC value. Berger Bullets has a twist stability and BC calculator on its site set up for its bullets, but easily adapted to other makes if you know the bullet's OAL and its BC. You just select the nearest Berger equivalent and overwrite the data to suit.
http://www.bergerbullets.com/twist-rate-calculator/
Litz's range tests give the 195gn 308 TMK an average G7 BC of 0.298 / G1: 580 (a good result). Putting the various bits into the calculator we get:
SG = 1.35
Bullet BC (G7): 0.298
Adjusted BC for 1 in 11" Twist: 0.285
Your BC is being compromised by: 4%
Minimum Twist Recommended: 1 in 10.3"
So, it will work in this set-up but you lose a bit of its aerodynamic efficiency. The optimum twist rate is 1:10.3 or faster.
However, there are indications that better long-range grouping and consistency are being obtained at yet higher Sg values from still faster twists. Many top-level competitors shooting the Berger 200.20X match bullet in F-TR have gone now to 1:9.5 or even 1:9 twist barrels and Litz has been experimenting with 1:8 in 30-calibre with the 185gn Berger LR BT 'Juggernaut' and seeing signs of improved stability. (The 200.20X at 2,700 fps in standard conditions needs 1:10.2 twist to achieve an Sg of 1.5, so in practice 10-twist barrels are apparently optimal.)
The other factor in all this is the environment we're shooting through - the air. It varies in its density and hence resistance to things going through it depending on altitude (the higher you are the thinner the air and the lower the drag it imparts); temperature (cold = dense; hot = less so); atmospheric pressure.
So, if you're shooting the 195 TMK at Bisley (we'll say at sea level as it won't be much above that) on a really cold January morning (say freezing = 32F / 0C) and with a high pressure weather system over southern England (let's say 30.5 inches mercury) and run things through Miller or the Berger calculators we now get a calculated Sg of 1.25. That is low enough to seriously compromise performance at 800 yards and up. On a hot low pressure summer afternoon of 75F, the Sg rises to 1.41 and performance will improve a little over those in 'standard conditions'. This effect is one reason why marginally stabilised bullets apparently and mysteriously work OK on one occasion but not on another. Go to the US NRA Shooting Centre at Raton, New Mexico (7,500 ft ASL) on a hot day and everything changes with an Sg value of 1.88 calculated and a theoretical twist rate of 1:12.3 needed to achieve the desirable 1.5 Sg value. (This is one reason why some of the American You Tube videos of amazing shooting on hot, high altitude alfresco ranges on gongs and milk jugs etc work - the thin air produces much less drag on the bullet so it loses speed more slowly and stays supersonic out to much longer distances.)
Summing up, 1:11 will work at 308 velocities with this bullet, but is marginally sup-optimal in normal conditions and maybe more than marginally in poor winter conditions. If you were looking to compete seriously in high-level competition with it and equivalent heavy high-BC designs at long distances , you'd want at least 1:10 and maybe even a bit 'faster' rifling pitches than that.
