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Old October 6, 2013, 07:32 PM   #1
brijohn
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155gr Amax twist rate

I am new to the forum, tons of info so hopefully this hasn't been covered. I will be loading for a Ruger M77 Mark II, 22 inch barrel, 1 in 12" twist chambered in .308 Win. I want to use 155gr Hornady Amax in front of Varget. The box says 1 in 12" twist or faster which mine is, will it stabilize or is a 1 in 12" borderline?
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Old October 6, 2013, 07:43 PM   #2
precision_shooter
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1:12 should be fine and may be optimal for that bullet. Im loading 155 amax with varget but I have 1:10 twist. They work fine as long as your muzzle velocity is sufficient.
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Old October 6, 2013, 08:06 PM   #3
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Only one way to find out if your rifle likes that bullet. Load some up and try. Most of my 30's like the Sierra 155's.
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Old October 7, 2013, 09:28 PM   #4
brijohn
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Thanks for the info, time to experiment.
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Old October 8, 2013, 08:05 AM   #5
Bart B.
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As the .308 Winchester is the offspring of the 7.62 NATO which was developed with a 22 inch 1:12 twist barrel for bullets about 145 to 172 grains in weight, if your rifle doesn't shoot 155's very accurate, it's not the twist that is the problem.
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Old October 9, 2013, 08:38 PM   #6
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The 155 AMAX really likes about 2800 fps so try to get there and see if it works in your rifle.
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Old October 10, 2013, 07:34 AM   #7
Bart B.
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With due respect to snolden's suggestion you try to get 2800 fps with that bullet, attempting to reach a given velocity in your barrel may not be a good idea. We don't know its internal dimensions and if they're on the big side of specs, you'll need to load powder charges way over maximum to do that.

Besides, bullets have no nervous system and therefore can not "like" any speed they fly at. They don't know how fast the leave the muzzle nor how fast they spin. There is an rpm spin rate each bullet shoots most accurately. It changes with twist and muzzle velocity. Any combination of twist and velocity that spins it best will produce the best accuracy.

Your 1:12 twist barrel will shoot that bullet somewhere between 2700 and 2900 fps with max loads and it'll do just fine.
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Old October 10, 2013, 08:40 AM   #8
old roper
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Most of the suggestion on velocity for 308 with the 155gr AMAX @ 2800fps come from this

http://www.hornady.com/store/308-Win-155-gr-Match/


as noted velocity is from 24" long barrel
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Old October 10, 2013, 10:03 AM   #9
Bart B.
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From the "Scrapbook" page in the above link:
Quote:
Nobody could have imagined the accuracy of a 155 gr projectile at 700 and 800 meters. Firing in 9 o'clock and 3 o'clock winds of 10-15 km/h, from 800 meters, we were able to score groups as small as 6" while firing from the prone.
6" at 800 metres (875 yards) is excellent for any factory ammo. Too bad we don't know what the larger groups were.

Other 155-gr. 30 caliber bullets have been doing that well and better since 1991; first Sierras then a few years later, Bergers.

I wonder what twist Hornady uses in their .308 Win. test barrels for the 155 AMAX.
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Old October 10, 2013, 01:36 PM   #10
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And don't forget the Lapua Scenar 155.


Brijohn, Here is a link to a stability calculator. You need bullet total length, plastic tip length, bullet weight, bullet diameter, bullet muzzle velocity, atmospheric conditions (though you can use standard defaults) and barrel twist. The calculation output is an estimate of the gyroscopic stability factor you will have. For hunting, Sierra recommends that gyroscopic stability factor fall between 1.3 and 3.0 for adequate accuracy, and for match shooting that you try to get it between 1.4 and 1.7. Not always possible, and many modern match bullets are made well enough to shoot well with the higher side hunting numbers, but those ranges are the ballpark to shoot for.
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Last edited by Unclenick; October 10, 2013 at 08:59 PM. Reason: typo fix
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Old October 10, 2013, 05:54 PM   #11
brijohn
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More excellent info. According to the calculator with the conditions today it figures out to 2.049, will load some up and give them a try.
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Old October 12, 2013, 07:45 PM   #12
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That JBM calculator is a great tool...
When I ordered the barrel for my 7-08 I was told that a 9.5 twist wasn't fast enough for the 162 Amax by many forum "pundits"- and the calculator said otherwise.

The calculator was correct.
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Old October 13, 2013, 08:30 AM   #13
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I was told a 1:13 twist .300 Win Mag barrel would not shoot 180-gr. boattail bullets smaller than 2 foot groups at 600 or 3 foot ones at 1000 yards when I had it built as a longer range hunting rifle. Sierra's 180's went into 1 MOA at both ranges when checking it out. JBM's calculations put those bullets at 1.485 stability factor; at the low end but good enough.
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