checking twist rates...

[Brad Y]

Hi Guys

After looking at the sierra and berger websites, im a tad confused. Sierra says you will need a 6.5 twist to stabilise the 90gr SMK but berger say a 7 twist will stabilise thier 90gr VLD. Not having the projectiles here to measure, I would say the VLD would be longer in length but why would berger say a 7 twist will do it?

For what its worth, im looking to put my tax cheque into a 28" true flite barrel 1:7 twist to shoot the 90gr bullets in either plain 22BR or a 22BRX. I know the 7 twist will do 80gr SMK's and VLD's and Amax's too but its up in the air whether it will do the 90's. Any suggestions on this scenario?

[Gadget]

22BR with a 1:8 30" Hart, 80g vld, 30.5g vv150, CCI 450, set 5 th in to lands works realy nice. There running @ 3200 fps.

See links to 6BR web site.
http://www.6mmbr.com/gunweek043.html

[AlanF]

...Sierra says you will need a 6.5 twist to stabilise the 90gr SMK but berger say a 7 twist will stabilise thier 90gr VLD.... why would berger say a 7 twist will do it?

Brad,

My guess is that Sierra is simply being more conservative. Maybe in extremely unfavourable conditions you will need 6.5 twist. Favourable conditions are high velocity, high air temperature, and high elevation (above sea level). If you shoot often at close to sea level in cold conditions and you use a low velocity load, then you might be in trouble with a 7 twist. Personally I would go with the 7 twist with a 22 BR. If it was a .223 then I'd be more concerned.

Alan

[Matt P]

I think Spencer Dunstall tested some 90grn in a 223 with a 8 twist barrel with good results so 7 in a brx MIGHT be fine.

[Brad Y]

Yep ive had confirmation from a pretty reliable source (Robert Whitley in the USA) that it will work but im definitely going to use the BRX to make sure. I should be able to get the 90's to 3000fps. My range is about 5 mins as the crow flies to the beach so yes we are very close to sea level. Again i wont be shooting to 900m unless i start getting real confident....

[AlanF]

Brad,

In general US long range shooters are at higher elevations than us, and from memory, elevation variations have a bigger effect than the other two factors. So they often use slower twists than we do and get away with it (until they get their rifles out in winter near sea level ).

Alan

[Brad Y]

Oh well i guess I will find out soon enough if it will work or not. Roberts discussion with one of the berger guys indicated that their 7 twist recommendation for thier 90gr VLD actually had some margin for error built into it. I guess i will know when i get it done. There is a pretty decent range of bullets to pick for this barrel- 80gr SMK, 80gr Amax, 80gr berger VLD, 90gr SMK and 90gr berger VLD.

[KHGS]

7" twist X 30" long will work fine with 90 grn bullets in 5.56mm @ 2750 fps. .22BR is very close to 22/250 in performance & in my opinion would stabilize 90 grn bullets with 8" twist, 7" would be preferred though to be safe.
Keith H.

[Brad Y]

Thanks Keith

Im going to go a BRX 7 twist just to be safe and 3000fps will be achievable. I dare say when i get around to it, I will order a true flite through you, as the 20 cal you sent for me (to SW firearms in Busselton) is mirror smooth. Just a shame that the local target shooters are getting in before me with barrel work- one bloke is getting 7 barrels done over the next week or so and ive been waiting to get my 20 practical chambered for a while....

[RDavies]

I used to have a 7" twist 22BR and yes, it did stabilize 90gn bullets.It was reasonably accurate, but then the barrel was only about 20" long. My 8" twist 22/6mm IMP didnt stabilize them. (well it half did on a hot day, but accuracy was not good and the occaisional bullet would be way off target. Since I dont have a fast twist 22 barrel anymore I will be able to send you some left over 90gn JLK bullets when I find where I hid them.

[Brad Y]

cr500?