Differences in velocity

[amce]

Hi all

Just placed an order for a Sako 85 Hunter blue/walnut in .308 Win. Tomorrow I'll be ordering a Sako 85 Varmint with a 600 mm barrel in .223 Rem. Just waiting on confirmation of what twists are in stock. Sako does the .223 in 3 twists: 1:14, 1:12 and 1:8. Know that tighter twist stabilises a heavier bullet but I was wondering about muzzle velocity. Can anyone tell me how mv varies with twist given same barrel length and same cartridge?

Cheers

 

[Jordan Smith]

Never have been able to see a meaningful difference that could be attributed to the twist rate with any level of certainty.

 

[paphil]

Go with the 1 in 8 twist. It will stabilize 75 and 80 grain bullets and give you an 800 to 1000 yard gun! Velocity is meaningless if you are dialing the distance. The heavier bullets will be slower out of the gate but have much higher BC and will have much higher down range velocicty and way less wind drift. You will love it! Run the numbers on a ballistic program.

 

[Nimrodmar10]

On the other hand, if you intend to shoot light thin-jacketed bullets such as the Hornady 50 gr. SX, or something of that nature, stay away from the 1-8 twist. They will shoot fine in a slow twist but will turn to smoke in a fast twist. I have to label my 223 bullets for either the bolt actions (slow twist) or AR Style (fast twist).

 

[Jordan Smith]

The more frangible bullets do okay in a fast-twist barrel, you just have to slow them down a bit. But why use an SPSX if you've got enough RPM to open the 50gr V-Max or BT up just fine?

I'd certainly go 8" twist.

 

[rscott5028]

MV is nice, but BC is forever

don't recall who said that

if you plan to shoot long range, go with the 8 twist

You can squeeze a little extra MV out of a slower twist if for no other reason than you can max out the light/thin jacket bullets without shredding them. That may be more of a factor for varminters or bench shooters using flat base bullets at shorter range.

-- richard

 

[Jordan Smith]

I'm getting 3350fps with the 52gr Speer HP from my 9" twist .223 with 28" barrel, with no issues. Fantastic accuracy, and no blow ups. I'm also getting 3050fps with the 75gr A-Max, also with great accuracy. I'm at ~4000feet ASL, so 9" twist works for me, but I'd go 8" if I were starting from scratch. If you want to shoot a truly great .224" long-range bullet, then go 7" twist and shoot the 90gr VLD.

 

[Bart B]

Well, a faster twist means the angle the rifling presents to the bullet is greater and that'll increase the resistance the bullet meets as it starts into the rifling. So muzzle velocity will change just like increased neck tension on the bullet does; it goes up. How much may be hard to measure 'cause in reality, it's probably not more than a few feet per second. A bullet entering a 1:9 twist barrel may leave 2 to 3 fps faster than a 1:10 twist one.

And I've never seen any load data suggesting the powder charge be decreased a bit for faster twist barrels.

 

[amce]

Thanks for all the input guys. Just waiting on the LGS to call with info on what the importer has in stock. If I have to place an order with the factory in Finland it'll take 6 months.

Cheers