It's not the bullet weight that determines the twist rate needed - it's the length, specifically the length as measured in calibres. If you have two bullets of equal length in a calibre but one is long-nose, boattailed and relatively light; the other blunt-nosed, flat base and 25% heavier, the heavy bullet actually needs a lower spin rate to be fully stabilised. Hence the 60gn and 63gn 0.224 Sierra SMP bullets and similar such as the 60gn Hornady that were designed for the .222 Rem and its 1-14" twist rate.
The 75gn A-Max averages 1.11" OAL. Run it through the Miller twist rule spreadsheet at various twists and here's what you get under 'standard ballistic conditions' (ASL @29.92 inches mercury air pressure; 59-deg F air temperature):
Let's assume that we get 2,800 fps MV
1-12" .................. 0.55 Sg
1-11" .................. 0.65 Sg
1-10" .................. 0.79 Sg
1-9" ................... 0.98 Sg
1-8.5" ................ 1.09 Sg
1-8" ................... 1.23 Sg
Because of its plastic tip extending the front end of the bullet out, the formula understates the Sg value for this type, but it won't be that much above 1.0. What's Sg? - It's the Coefficient of Stability. Anything less than 1.0 is unstable; at 1.0 it's theoretically stable, but barely so and a change in conditions (colder air than 59-deg, high pressure etc) will drop it below 1 back into instability. In practice, note the advice to use a combination that produces 1.4 or higher values (the 8 inch twist will definitely be there with this bullet). The latest thinking says 1.5 is needed as experiments have shown that bullets with lower Sg values will appear stable - group well, round holes - but see increased drag in flight, or to turn that around have their BCs degraded.
For many sporting applications, that's not a great issue but may be with this bullet as it is a long-range design and is favoured by foxers for longish shots in windy environments and is a popular mid range match bullet in 223 Rem for use up to 800 yards.
The bullet length and barrel twist rate are the key factors, MV has a relatively small effect - although that may swing things either way in a marginal situation. Run the 75gn A-Max again at 3,000 fps in the 9-inch twist and the Sg only climbs from 0.98 to 1.00. On the other hand substitute the 75gn Hornady HPBT (length 0.981") at 2,800 fps and the Sg value rises to 1.40 in the 9-inch twist. The slightly shorter Nosler 77gn HPBT Match (0.973") gets 1.47 largely because of its increased weight.
The 75gn A-Max and 9-inch twist issue is raised regularly on forums - the answer is 'maybe yes' to basic stabilisation, 'maybe no' because it is a true marginal combination. That also assumes that when the manufacturer quotes 1-9" twist rate that's what you get, which isn't actually so more often than not. Factory rifle barrels may vary by up to a quarter inch either side of the nominal rate. If you specify a bespoke barrel from a custom barrelmaker such as Krieger, Bartlein, Broughton etc, you get exactly what you order on the other hand. In fact, Bartlein takes orders to two decimal places just in case you fancy say 1-7.77".
This is what the spreadsheet looks like
Don Miller's Twist Rule
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Caliber
| 0.224
| Inches
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Bullet Weight
| 75
| Grains
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Bullet Length
| 1.11
| Inches
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Barrel Twist
| 8
| Inches/turn
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muzzle velocity
| 2800
| fps
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Temperature
| 57
| degrees Fahrenheit (59 is standard)
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Pressure
| 29.92
| inches of mercury (29.92 is standard)
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Sg =
| 1.23
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Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may
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gain some accuracy by going to a slower twist barrel.
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