Berger lists the 55gn HPBT as 0.915" OAL. Running it through the Miller Twist Rule Excel calculator, a 9-inch twist rate at 2,800 fps shows it as barely stabilised.
Don Miller's Twist Rule
Caliber 0.204 Inches
Bullet Weight 55 Grains
Bullet Length 0.915 Inches
Barrel Twist 9 Inches/turn
muzzle velocity 2800 fps
Temperature 59 degrees Fahrenheit (59 is standard)
Pressure 29.92 inches of mercury (29.92 is standard)
Sg = 1.05
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.
The recommended 8-inch twist rate gives an Sg (stability coefficient) value of 1.33 (which is also below the 1.4 recommended minimum result).
Anything above 1.0 is
theoretically stabilised, but a safety margin is required for several very good reasons, hence the recommendation of 1.4 as a minimum value.
However, four other factors need to be taken into account:
1) Ambient conditions and the firing locale's elevation above sea level. Low atmospheric pressure conditions and high air temperatures see a lower rate of spin required. The higher you are above sea level, the lower the air pressure you shoot in for any given conditions. The 1.05 value above is for 'standard ballistic conditions', ie 59-deg F and 29.92 inches mercury pressure. The Sg drops to below 1.0 at freezing point on a high pressure winter's day, but improves to nearly the recommended 1.4 if you were to use this combination at Raton (7,000 ft ASL) on a 90-deg summer day.
2) Accuracy of the quoted twist rate. This applies mostly to factory rifles with mass produced hammer forged barrels where the actual and nominal rifling twist rates may diverge either way. Hence marginal stabilisation situations sometimes work out fine or not at all in these rifles. Some custom barrel makers such as Bartlein have a reputation for producing twist rates that are
precisely what it says on the blank. Hart? No knowledge, but it may be worth doing a twist rate check with a cleaning rod and tight patch. If you're lucky, it may have been made with a slightly 'faster' twist than the nominal 1-9".
3) Stabilisation requirements are determined by the interrelationship of calibre, bullet length, bullet weight, bullet velocity. The main factor is the bullet's length in calibres rather the commonly quoted weight brackets. A long thin light bullet needs a lot more stabilisation (faster rate of spin imparted by the rifling) than a short fat heavy model. However, OTM or HP varmint type bullets often have a considerable airspace between the top of the core and the bullet jacket tip. As a result, the Miller and similar stabilisation programs may overestimate the degree of stabilisation and hence speed of twist rate needed. Not by huge amounts, but in a case like yours, maybe just enough to 'get away with it'.
Michael Courtney has done a lot of valuable work on this.
See:
http://forum.accurateshooter.com/index.php?topic=3834705.0
4) and finally .... another bit of great Michael J Courtney research that looked at the real world effects of marginal bullet stabilisation. He and his daughter used the .222 Rem loaded over a large range of MVs to change the Sg value and see if it affected the BC through measuring how much velocity the bullet lost using two chronographs, one each at muzzle and target. The results showed a substantial decrease in actual BC if the Sg value fell below 1.26 for the 52gn OTM bullet used in the tests - a very substantial BC reduction. This ties in with Sierra Bullets' comments that over decades of bullet testing to ascertain BCs at different velocities, that its technicians had also seen very marked spin-rate to effective BC links on occasions.
So, even if your bullets are apparently stabilised in this marginal twist rate, it's possible that the Berger quoted G7 BC of 0.195 won't actually be on offer in this set-up and that this will affect 1,000 yard ballistics adversely.
The upshot of all this is that the only way you'll know whether the combination actually works at long ranges and how well, is to go shoot it! The other thing that you may find is a lack of consistency, that is it works out fine one day but performs poorly on another; that it works on hot summer days, but not cold winter ones ......... and so on and suchlike! This is common in marginal stabilisation situations.
I hope it works out fine for you - good luck!
