Twist Rate for .308

[commandernavi]

Hi all,

I purchased an SPS Tactical with a 20" 1 in 12 twist a few months ago and while I love the gun so far it seems like every internet forum is completely swayed to the AAC-SD with the 1 in 10 twist. When I purchased it, I was under the impression that the 1 in 12 was the better of the two options, but since then I have learned that the 1 in 10 allows you to fire bullets with a better ballistic coefficient and be subsequently a lot more accurate in long range applications. Since I am planning to move out past 600 eventually, will I miss out due to the twist rate?

If this is the case, why aren't all the 700s made in 1 in 10 twist? What benefits does the 1 in 12 have (if any)? I kinda feel I got the short end of the stick. Should I consider selling it and getting the AAC-SD? Pretty bummed at the moment .

Thanks all.

 

[roggom]

The 20" 1:12 will do fine with 178 and 175 bullets, plenty to reach out to 1000Y and take game (deer) at 700Y. Sure the 1:10 will better stabilize the 208's but do really need to use the 208's? The AAC has the faster bbl to stabilize very long and slow moving sub-sonic rounds, although it should do well with most long range target bullets.

I have the same rifle and here is what I have found:

155 Scenar, Flat but with a 20" bbl really can not wring that last 100fps needed for the higher BC
168 Amax, SMK, NOScc Very accurate sub 500, clover leaf at 100, just lack knockdown energy past 600 (for hunting)
178 Amax, SMK submoa across the board 100-750 (farthest I have taken it), plenty of power out to 700Y (for hunting), my go-to load for everything
208 Amax, so-so accuracy, may have done better in the 26" Varmint, my rifle didnt like them.

Of course every rifle is different.

 

[commandernavi]

Thanks for the response! What are the 208s used for? If the 178s work well out to 1000y what are the 208s better for?

Also, does the shorter barrel have a negative impact on the heavier pills' velocity that would make them impractical?

Do you think that loading the 155 a bit hotter would be a bad idea to get the extra fps?

 

[roggom]

The 155 have a bit more bearing surface and mimic the BC of the 178. I ran the at max Varget load, and they did do fine, but not as well as the 178 or 175. My 30-06 loves the 155's.

The 208 can be very effective if you can stabilize them, also they have a bc of .648. I ran the numbers and gave them a spin, but my rifle did not do so well with them. the 208's are another bullet, with a bit more velocity, could eek out some superior performance.

If you are not worried about downrange energy, you can make any of the bullets from 155-208 work. If you want to try a bit of bullets, check this guy out, he sells them by the bullet and ships them out real quick. Bullet Samples for Sale-Sportsman Rifle Bullets . He is not a sponsor, but sure beats buying 5 or 6 boxes of bullets.

 

[moose99psd]

diffrence between a 178 and a 175? a friend of mine has used 175s and is going to give me the recipe he used for his match ammo since he runs a 260 now, he tole me 175 smkbthp. so im curious as to the diffrence??

 

[roggom]

There are a couple of 178's the HPBT has a slight higher bc, and has a steeper ogive. It is also a bit more sensitive to seating position than the 178 Amax.

The 178 Amax and the 175 perform the same for me, and of course should be worked up separately as they are not interchangeable. Both do very well seated at mag length.


Also for OP, here is a quick vid, kinda explains the difference in BC and weight and choosing the best for your application. I hunt and shoot, so any load I make must be effective on game. The example is for 7mm, but the concept is the same for the 30 cal bullets as well.

Bullet Comparison - Heavy Bullets - YouTube

 

[commandernavi]

Nice video and thank you for the link about the sample bullets! Roggom, just to be clear, you have the SPS Tactical 1 in 12 and not the 1 in 10 AAC-SD, correct? I have a few boxes of 175 on the way that I am hoping will perform well. Is there a ballistics calculator like gseven that is free and that can give me the best load for my particular setup?

So far people have said that my rifle will like 168 and 178 is the max (I am going to try these out). Anything above that people have been telling me will not have enough stability to get good accuracy from.

Also, some shooters have said that the 210+ is worse than a good 175 overall. I don't really understand why they would say that. I am clearly very new to this and don't understand this well, so I am sorry for my ignorance and greatly appreciate the knowledge!

 

[roggom]

Yes, I have the SPS tac 1:12 20" bbl. I replaced the stock with a LTR HS precision stock, very comfortable to shoot.

The performance of the heavy bullets is very relative, bbl manufacture, twist and length. For the out-of-box remmy 20" 1:12 you will have superb wind deflection, great accuracy (hopefully), and sustained downrange energy with the 178 or 175. I have tested the 178 Amax/ Lapua / Varget combo and it has perfomed in 108 degree down to +4 degree. I run it around 2650 and use an FDAC for dope.

I also have a 175 SMK RP Varget that I run at 2600 and it is just fun and accurate. It is a good load, as the 175 pulled bullets will pop-up on the sale thread ever so often.


Here is a link to G7, the online program is free. If you have a Vortex scope, Vortex has their own version of the software as well.
gseven - Ballistic Program Vortex Optics - LRBC Tutorials

 

[2clicks]

Love my SPS 700 Tactical Although I've changed out the stock for a McMillan and put on a muzzle break It's a bug hole shooter at 100 and at 600 wears out the steel 10' plate! I detest the promotion of Varget But 44 grains behind the 168 max in winchester brass rocks well 42.5Grs of IMR4895 works wonders too (VVN140 is my all time favorite powder but it's pricey and I'm a cheap ass). Like the 175's and 178's I run those in my 26" .308 with 41.7 grains of IMR4064 Lake city brass winchester primers in all loads. I consider my 20" .308 a 600yrd and in rifle 175's do get better results at 600 though.

 

[commandernavi]

I thought G7 was only for 7 mm. I will try it, but I don't know the velocities out of my gun yet.

2clicks, I am assuming you have the 1 in 10 twist one with the threaded barrel, correct?

 

[Seekers]

Using the Green Hill formula they found that the ideal barrel twist for the 308 was just under 1:12. You'll notice the military uses 1:11.25 in some of their match/sniper rifles. But the 7.62 also used to be fired slower than it is now, as older boxes of Lake City Match Ammo shows the velocity at 2550 FPS. The additional velocity of the modern rounds makes up for any slight difference in barrel twist between 1:11.25 and 1:12 and the bullet is completely stabilized.

The 1:10 will stabilize heavier/longer bullets 180 and up.

 

[roggom]

I thought G7 was only for 7 mm. I will try it, but I don't know the velocities out of my gun yet.

No, that is the beauty of G7 software, it has BC's for most factory bullets in G1 and G7, and you do not "need" a chrony. You create a profile for the area you are shooting and then choose a bullet, approx velocity, zero point, and the smaller details (scope from bore, click value etc). Save and calculate, it will display a drop chart in MOA, or mil, inch. Go out and shoot at 100 and then as far as you can (75% of your max distance), and then make adjustments. If you are low or high, record that data on the sheet and to back and validate the info. The software will correct the velocity to get the correct trajectory. Of course you can use a chrony to get your velocity if you have one, but I have successfully worked up over 20 loads with the software and it works fine without.

There are tutorials on both the G7 and Vortex websites.

 

[commandernavi]

That's awesome. Thanks guys!