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Recommended bullet weight for .308 w/ 16.5" barrel and 1:10 twist

This is a discussion on Recommended bullet weight for .308 w/ 16.5" barrel and 1:10 twist within the Reloading forums, part of the Firearm Forum category; I'm new to the .308 and would appreciate some education. Looking to maximize the accuracy of my 16.5" barrel with 1:10 twist. Apparently lighter vs. ...


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Old January 16th, 2012, 05:38 PM   #1
 
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Recommended bullet weight for .308 w/ 16.5" barrel and 1:10 twist

I'm new to the .308 and would appreciate some education. Looking to maximize the accuracy of my 16.5" barrel with 1:10 twist. Apparently lighter vs. heavier bullets make a difference?

Rifle is a scout set-up - I'm interested in the 100-300 yard range.

Thanks in advance!



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Old January 16th, 2012, 05:48 PM   #2
 
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I have 3 Remington 600s an a SOCOM16 so I hear what you are saying an you prettymuch have the same general range expectations that I do. I am somewhat partial to a 165 gn bullet and so are my guns. The 150s are fine in many respects but my rifles prefer a 165 gn when the distance opens up. I don't run 180s because I don't need a bullet that heavy
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Old January 17th, 2012, 06:31 AM   #3
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My Ruger M77 Tactical 308 has a 1-10" twist rate as well. I found that it seems to prefer heavier bullets with that faster spin rate. I've been using Hornady 168 grainers with better accuracy than 150 grain bullets.
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Old January 17th, 2012, 06:25 PM   #4
 
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Thanks Gents! As I said, I'm new to the .308, but it's sounding like heavier bullets are the way to go for my rig.

Thanks again!
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Old January 18th, 2012, 08:26 AM   #5
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Also, experiment, see what your particular rifle likes best.
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Old January 18th, 2012, 08:35 AM   #6
 
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ive been shooting battle pack 147gr DAG ammo thru my SOCOM lately, and she loves that stuff. i stay away from the heavier weight stuff (175 and 180 gr) simply to keep from punishing the op rod.
in my Savage Scout i do use the commercially loaded 180 gr ammo, it "bucks the wind" better and does allow for some longer shots.

Last edited by Lee D; January 18th, 2012 at 08:38 AM. Reason: punctuation
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Old January 18th, 2012, 10:43 AM   #7
 
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168 grain would be my suggestion in that setup. I am a big fan of the .308 Winchester.
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Old January 19th, 2012, 05:44 AM   #8
 
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First you have the right idea....with a 16.5 inch barrel the rifle is a 300yd gun.
As far as bullet weight goes each rifle is an individual and since 300yds is your max range boat tail bullets may be a moot point.

I own a Ruger Compact in 308 and it too has a 16.5 inch barrel. The main issue is velocity loss. Heavier bullets will drop more over 300 yds but hit with more energy. Lighter bullets drop less but have less energy. I would note, 150, 165, and 180 bullets should still give you 1000 foot pounds at that range, so this too may be moot.

I would try several different bullets in each weight class and see what it likes.

I shoot mostly 150 rem PSP core locks. Depending on the powder, they shoot 1.25 inchs to .6 inchs. My gun likes these at the 2500 fps range. Each powder varies as far as velocity loss so a chronograph is your friend.

I have heard good things about the 168gr bullets and if punching paper is the game they may work good for you. But if you want a hunting bullet they are not the best since they don't expand.

Finally, if you are not going to reload yourself, try the cheapest off the shelf federal 150gr bullets. I shot a few and they were just about 1 inch @ 100yds.
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Old January 19th, 2012, 09:48 AM   #9
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SmokingGun, There's a lot of confusion about bullet weight, barrel length, and twist rate. Let's start with the basic design .... the 308 Winchester was designed for a 24" barrel with a twist rate of 1:12 and a 150 gr bullet with a muzzle velocity of 2820 fps. With this combination, 150 gr bullets would maintain stability for at least 600 yards ... even more with match grade bullets with a higher ballistic coefficient.

Twist rate and muzzle velocity affect the bullet's spin rate after it leaves the barrel. When the spin rate best matches the bullet, it will maintain stability a long ways down range. If the spin rate is too slow, at some distance down range the bullet will become unstable, start to wobble, and then start tumbling. If the bullet is spun too fast (called over stabilization), the bullet will develop a spiral path much like a cork screw and will tend to spread the groups at closer distances. At a considerable distance down range, the spiral will dissipate and the bullet will fly in a straight path. Long distance shooters call this phenomenon "going to sleep". At any distance down range where the bullet is still stable and not spiraling, it will maintain good accuracy. So ... bottom line ... for best down range stability, you want the spin rate to match the bullet and the muzzle velocity, however at hunting distances of 300 yards or less, it really doesn't matter much because most bullets will remain stable well past that distance.

Internal ballistics have a direct impact on external ballistics. As you probably know, factory ammo is loaded very close to SAAMI max pressures. If you increase bullet weight you have to reduce the powder charge or chamber pressure will skyrocket. This is proven by looking in any reloading manual. When a heavier bullet is used with a lighter powder charge, velocity is reduced.

Bullet spin rate is a product of muzzle velocity and the barrel's twist rate. The higher the velocity and/or the faster the twist rate, the higher the bullet's spin rate will be. The formula for spin rate is: 12 divided by twist rate times velocity (in fps) times 60 (number of seconds in a minute) = RPM. As an example: the above 150 gr bullet with a MV of 2820 fps and a twist rate of 1:12 ... 12/12x2820x60=169,200 rpm. According to my references, 170,000 rpm is the optimum spin rate for a 150 gr .308 bullet ... very close to .308 Win specs.

With a 16.5" barrel, velocity with the same 150 gr bullet is reduced to 2584 fps. With the faster twist rate of 1:10, here's the results: 12/10x2584x60=203,040 rpm. As you can see, the faster twist rate more than compensates for the loss of velocity, making a 150 gr bullet too light and will "over stabilize".

Using QuickLOAD, I found the best bullet for your 16.5" barrel and twist rate. It is a 162 gr bullet (not common) so the closest common bullet weight would be 165 gr. With factory 165 gr ammo loaded to 2700 fps from a 24" barrel, your 16.5" barrel will develop about 2440 fps. 12/10x2440x60=175,680 rpm. The optimum spin rate for a .308 165 gr bullet is 182,000 rpm, which is very close to the calculation ... a 162 gr is almost perfect.
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Old January 19th, 2012, 02:52 PM   #10
 
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With that short barrel I'd worry about powder as much as the bullet, you want something fast so you get a complete burn in the barrel. My shortest .308 (actually my wifes) is 22 inches and really likes 150s, esp. Barnes TTSX, with 47g Varget, but it seems to shoot 3/4" MOA with just about anything. But she has the 1:12 twist barrel. With that shortie you've got I'd look at IMR 3031 or maybe something even faster.

Now I always thought the 1:10 twist was for heavier bullets, so I myself would look at 165s also. Sounds like a great hog gun!
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Old January 19th, 2012, 06:55 PM   #11
 
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Quote:
Originally Posted by Iowegan View Post
SmokingGun, There's a lot of confusion about bullet weight, barrel length, and twist rate. Let's start with the basic design .... the 308 Winchester was designed for a 24" barrel with a twist rate of 1:12 and a 150 gr bullet with a muzzle velocity of 2820 fps. With this combination, 150 gr bullets would maintain stability for at least 600 yards ... even more with match grade bullets with a higher ballistic coefficient.

Twist rate and muzzle velocity affect the bullet's spin rate after it leaves the barrel. When the spin rate best matches the bullet, it will maintain stability a long ways down range. If the spin rate is too slow, at some distance down range the bullet will become unstable, start to wobble, and then start tumbling. If the bullet is spun too fast (called over stabilization), the bullet will develop a spiral path much like a cork screw and will tend to spread the groups at closer distances. At a considerable distance down range, the spiral will dissipate and the bullet will fly in a straight path. Long distance shooters call this phenomenon "going to sleep". At any distance down range where the bullet is still stable and not spiraling, it will maintain good accuracy. So ... bottom line ... for best down range stability, you want the spin rate to match the bullet and the muzzle velocity, however at hunting distances of 300 yards or less, it really doesn't matter much because most bullets will remain stable well past that distance.

Internal ballistics have a direct impact on external ballistics. As you probably know, factory ammo is loaded very close to SAAMI max pressures. If you increase bullet weight you have to reduce the powder charge or chamber pressure will skyrocket. This is proven by looking in any reloading manual. When a heavier bullet is used with a lighter powder charge, velocity is reduced.

Bullet spin rate is a product of muzzle velocity and the barrel's twist rate. The higher the velocity and/or the faster the twist rate, the higher the bullet's spin rate will be. The formula for spin rate is: 12 divided by twist rate times velocity (in fps) times 60 (number of seconds in a minute) = RPM. As an example: the above 150 gr bullet with a MV of 2820 fps and a twist rate of 1:12 ... 12/12x2820x60=169,200 rpm. According to my references, 170,000 rpm is the optimum spin rate for a 150 gr .308 bullet ... very close to .308 Win specs.

With a 16.5" barrel, velocity with the same 150 gr bullet is reduced to 2584 fps. With the faster twist rate of 1:10, here's the results: 12/10x2584x60=203,040 rpm. As you can see, the faster twist rate more than compensates for the loss of velocity, making a 150 gr bullet too light and will "over stabilize".

Using QuickLOAD, I found the best bullet for your 16.5" barrel and twist rate. It is a 162 gr bullet (not common) so the closest common bullet weight would be 165 gr. With factory 165 gr ammo loaded to 2700 fps from a 24" barrel, your 16.5" barrel will develop about 2440 fps. 12/10x2440x60=175,680 rpm. The optimum spin rate for a .308 165 gr bullet is 182,000 rpm, which is very close to the calculation ... a 162 gr is almost perfect.
This, Sir is more of an education than I anticipated! This will be printed and kept with my hand loading reference material. Fantastic response, thank you so much for your time!

Great responses - all. Thank you very much!
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Old January 21st, 2012, 04:09 AM   #12
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I had a Socom 16. I loaded the Hornaday 155gr Amax with RamShot Tac powder. Loaded to the specs in the hornaday reloading manual.

With the very course open sights on the Socom 16, I was able to shoot 5 shot groups at 100 yards that were right at 1.25"

I love the 155gr Amax!

R65
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Old January 21st, 2012, 06:39 AM   #13
 
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Try Nosler 150 Gr Bt---Primer Fed 210 Match---IMR-4895---42 Grs---COL 2.810---5/8 Turn Lee Crimp Die...Quarter can cover 200 yards Three Shots...
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Old January 21st, 2012, 07:12 AM   #14
 
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So does that mean the closer to a 24" barrel you get, your weapon will shoot well with more diverse bullet sizes in factory loaded ammo?
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Old January 21st, 2012, 12:07 PM   #15
 
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Quote:
Originally Posted by Ruger48 View Post
Try Nosler 150 Gr Bt---Primer Fed 210 Match---IMR-4895---42 Grs---COL 2.810---5/8 Turn Lee Crimp Die...Quarter can cover 200 yards Three Shots...
Thank you for the input, but you've lost me here...."COL 2.810---5/8 Turn"
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