Most 80s are only maginally stabilised at best in a true 1-9-inch twist barrel. When you're as close to the line as they are, you risk getting into the situation that a bullet shoots well on one location in particular weather conditions, but won't perform worth a damn under different conditions.
The first thing to understand is that it's not weight per se that drives the rifling twist rate to bullet relationship, it's the bullet length in calibres. The longer the bullet in relation to its diameter, the greater its propensity to turn end over end in flight - throw a fat, short straight stick as per a spear, then do it again with one half the diameter and twice as long and you'll see the long version starts to tumble quicker. In fact for a given bullet length, adding weight makes it more stable not less, so an inch long bullet that weighs 90gn say needs a slower rotational speed for stability than one of the same length that weighs 75gn - that's why all-copper bullets need faster rifling twists than heavier conventional gilding metal jacket + lead core models. Talking weight is a convenient shorthand, because generally speaking a heavier bullet is also a longer bullet therefore needs a faster rotational speed, but when you compare bullets with radically different shapes - HPBT VLD say against a same-calibre, same weight, flat-base round-nose hunting type, the latter is much shorter and works happily in a MUCH slower twist barrel.
the 80gn 0.224-inch Sierra MK is 1.066" long, so its length in calibres is 4.75 which is long. The 30-cal 185gn Berger BT Juggernaut is 4.39 calibres which is why it manages great with a 1-12" twist at similar velocities to an 80 in .223 Rem, and the Juggernaut will even stabilise in a 1-13" twist barrel in most normal competition shooting conditions.
The late Don Miller developed an excellent shorthand calculator that works on a simple ExCel spreadsheet - Google "Don Miller's Twist Rule" or do a search on this website and you'll soon find somewhere to dowload it from. You DO need to know the length of the bullet you want to try out. You may also need to measure the ACTUAL twist rate of your barrel using a cleaning rod, jag and tight patch, also described in the Daily Bulletin on this site on more than one occasion. If you have a Krieger, Bartlein, Broughton etc and know exactly what twist rate the barrelmaker quotes on it, don't bother, but factory rifle barrels are often a bit out from the nominal rate and that 0.25 difference can have a major effect on an otherwise marginal relationship.
Here's what the Miller spreadsdheet says about the 80gn Sierra MK at 2,800 fps in a 1-9" twist rate under standard ballistic conditions:
Don Miller's Twist Rule
Caliber 0.224 Inches
Bullet Weight 80 Grains
Bullet Length 1.066 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 = 1.18
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 is the coefficient of stability. Theoretically, 1.0 and anything higher is stabilised, but partly through the calculations used involving some 'shortcuts', essential to keep this great little tool usable and simple, partly through getting things to work in real world as opposed to theoretical conditions, 1.4 is advised as the minimum Sg value to be aimed for.
At 1.18, assuming the barrel is exactly as it says on the box at 1-9", it really is marginal and a lot depends on where you shoot (altitude) and in what conditions (warm or cold air and atmospheric pressure on the day), and at what distance(s).
Let's run Miller again and assume you want to use this bullet on a seaside range at virtually 0ft altitude in winter with the air right on freezing and in anti-cyclonic high atmospeheric pressure conditions. We get:
Don Miller's Twist Rule
Caliber 0.224 Inches
Bullet Weight 80 Grains
Bullet Length 1.066 Inches
Barrel Twist 9 Inches/turn
muzzle velocity 2800 fps
Temperature 32 degrees Fahrenheit (59 is standard)
Pressure 30.92 inches of mercury (29.92 is standard)
Sg = 1.08
BUT ..... now it's high summer and you've gone to a match at Raton which is ~6,500 ft ASL. Each 1,000 ft rise in altitude decreases atmospheric pressure by ~1-inch of mercury in any given weather condition, so we'll say the air temperature is 80-deg F and the atmospheric pressure is reduced by 6" Hg to 23.92:
Don Miller's Twist Rule
Caliber 0.224 Inches
Bullet Weight 80 Grains
Bullet Length 1.066 Inches
Barrel Twist 9 Inches/turn
muzzle velocity 2800 fps
Temperature 80 degrees Fahrenheit (59 is standard)
Pressure 23.92 inches of mercury (29.92 is standard)
Sg = 1.53
At 1.53 Sg, the combination will now work just fine - no worry at all.
The temperature / pressure issue is of considerable concern to the military - that's one of the reasons why new rifles and cartridges are subjected to winter Arctic weather tests, mainly about weapons' functioning, but also to ensure the ammunition still performs including bullet stability. The original AR-15 prototypes retained the 1-14-inch twist rate used on the .222 Rem that its cartridge was based on. The rifle and ammunition failed the US Army Arctic tests as the bullets which had been fine in normal weather in the continental USA were found to be wildly unstable on 500 metres targets in cold air. Stoner changed the rifling specification to 1-12" as used with the early versions of the M16 and it became the SAAMI standard twist rate for the civilian .223 Rem subsequently.
At this point, there are two further things to take into account.
First, the 80gn SMK and Nosler CC are about the shortest of the 0.224" 80s - load 80gn VLDs and the A-Max and they're all longer, so the effective Sg drops below 1.18.
Secondly, while a marginally effective spin rate and hence Sg might produce round holes and acceptable grouping at 100 yards, the bullet's performance at longer ranges may be seriously compromised. The results of tests done on 0.224" bullets at various spin rates / Sg values will be published in the April issue of the online magazine TargetShooter in a paper by Elya R Courtney and Michael W Courtney of BTG Research, Colorado Springs.
http://www.targetshooter.co.uk/
due out in the next few days. It shows that when the Sg drops down to somewhere in the 1.20-1.25 range, the bullet sees a massive increase in air-drag, or to put it another way, its BC is seriously compromised. The tests were done at 100yd using two chronographs and measuring how much speed the bullet lost in flight.
So, your apparently well stabilised bullet may not perform anywhere near what you expect out at 500 or 600 yards, never mind 1,000.
You're now saying you shoot your 80s in a 9-twist barrel at 2,850 fps or 2,900 fps and the extra speed will raise the Sg and make the bullets stable. Sadly, although an MV change can move a relationship that is right on the margin one way or the other, you normally need very large MV changes indeed to affect the Sg materially. Returning to the original calculation that gave the 80gn 1.18 Sg under standard ballistic conditions, 2850 fps increases it to all of 1.18 - ie too small to make a difference to a calculation done to 2 decimal places, 2,900 fps gives you all of an additional 0.01 raising it to 1.19 and going up to 3,000 fps does the same, ie to 1.20.
