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Old May 22, 2019, 03:32 PM   #26
robrob
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Or post 20.
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Old May 22, 2019, 06:03 PM   #27
zeke
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"This means the gyroscopic stability factor increases as the bullet goes downrange. That's what was vexing Stoner. The M16 rounds tumbled just fine when they hit a target up close, but far enough away they were they settled out and would just make pencil holes. He was trying to get them to tumble further out."

Find this very interesting, although for a slightly different subject.
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Old May 23, 2019, 09:38 AM   #28
Jim Watson
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Eugene Stoner would have disagreed with Berger's number. He wanted the military to go from a 12" twist in the M16 to a 14" twist to get the stability factor down nearer to 1.
Does this mean that the history I read which said that the M16 started out with the 14 twist varmint barrel and was increased to 12 to stabilize the FMJ boattail in cold dense air was wrong? (As Jeff Cooper said of another gun, rumor has it that it gets cold in Russia.)

I have variously read that the wounding mechanism was "tumbling" or a one-time yaw turnover or the bullet breaking in two at the cannelure. And that the standardization of a 14 inch barrel has done more to reduce velocity dependent effects than changes in rifling pitch.
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Old May 25, 2019, 01:55 PM   #29
edward hogan
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The 185gr Berger will serve with distinction; and is mag-length capable (from what I understand).

I have 168gr SMK and Nosler BTHP; not like I'm trying to get the last 50yd capablity. And they work...
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Old May 25, 2019, 09:14 PM   #30
Bart B.
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Bullet spin rate in rpm doesn't slow down as much, percent wise, as velocity as it goes down range.

Read Unclenick's comments and use Kolbe's formula in

https://thefiringline.com/forums/arc...?t-556530.html
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Last edited by Bart B.; May 26, 2019 at 07:53 AM.
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Old June 6, 2019, 07:45 AM   #31
Jimro
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You really need to think about what you are trying to do with a rifle and load.

For target work at known distance, the 220 SMK from a 308 is a fine option. The additional drop at 1k yards is irrelevant since the known distance ensures that you won't see vertical dispersion problems caused by distance estimation errors, and the 1.4 MOA advantage in wind drift for standard 10mph 90 degree cross wind will help you stay in the ten ring.

For tactical work, it's the opposite. The 175gr SMK has 3.6 minutes less drop at 1k (that's about 36 inches), which on a torso sized target is pretty significant. That makes it easier for a sniper or dedicated marksman to use holdover/under stadia lines on a scope reticle (smaller holds are generally better, keeping the target closer to the optical center of the scope).

The 220 SMK load is dropping 79 inches between 800 and 900, and 98 inches from 900 to 1k. The 175 is dropping 74 and 90 across the same distance. I know that 5 inches and 8 inches doesn't seem too significant, but the average human torso length is between 17 and 21 inches, that additional 5 to 8 inches of less vertical drop becomes helpful in making distance errors less of a factor for success of a shot where you might only get one chance. It's about a 25% advantage when you run the numbers.

So this is why the 175 SMK has survived as the DOD's projectile for sniper ammunition, despite other options doing better on the competition line. It performs equally well through bolt and gas guns, has a tangent ogive making it very jump tolerant.

If you run the numbers for across the course high power (200, 300, 600 yards) it's clear that the 175 SMK is probably the better choice for most shooters (faster action time and less recoil, minimal difference in ballistics).

So I hope this provides some context about how a dedicated target rifle shooting known distance for score across a string of shots is just different from a tactical rifle shooting unknown distance only going to take one or two shots.
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