I would say a 1-10 would be plenty for this bullet and for the 22-250, 26-27 is about as long as is really needed for top performance. You can go longer and get a few more fps but I don't likethe extra length on a 22-250.
I feel 25-26" is about pefect for the standard 22-250.
Contact your barrel maker and they will give you the best twist recommendation.
As far as barrels go, I would recommend Lilja as my top choice and Dan will not give you poor information on twist recommendations.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.
Agree here also on Lilja. The way I would do it is to send the bullet that you are planning on using the most to the gunsmith of your choice. He then can determine exactly what will be the best twist and barrel length.
I just had a Shilen Select Match put on my Rem BDL action last year, just went with standard throat, twist and barrel length. Damn good shooter with them 55gr V-max's.
West Coast of Washington
Assuming that you are talking about a Berger flat base 64, I have used that one and the former 65 grain for quite a few years. Is 500 yards a realistic distance, or are you thinking of a boattail design and wanting to reach out a little further?
If the former.
I would figure the muzzle velocity with your bullet of choice, and try to get as close to 195,000 to 200,000 RPM as possible. Use whatever twist is indicated, to achieve that goal. Can't find my formula, right now, but it's simple.
I haven't used the 64 gr bullets but have used the 60 gr in a Shilen 1/12 twist 22-250AI and know berger list a 1/12 twist for the 64 gr bullets my barrel lenght is 22". I did kind of a light weight walk around varmit rifle for the high country and my wife likes that rifle. Since your rebarreling an existing rifle you might want to consider staying with the same contour and barrel lenght. My other Shilen barrel 22-250AI has a 1/14 twist 26" barrel lenght and it will not shoot the 60 gr bullets. Well good luck.
I just bought a twist rate program. it's not here yet. But the basic rule is for a given caliber the lighter the bullet the less barrel twist.
So as the other person mentioned sub 200K is a wide standard used.
When they first came out with Super explosive bullets for the small calibers they had to give a vel. limit. I had a 220 swift and bullets were not in one piece at 200 yard paper. They were hit with shrapnel. The thin copper skin and high velocity/spin rates made them come apart right out of the barrel when the core lead in theory get molten because of barrel friction.
I do agree with the faster powder types uses in small bore guns the 26 inch is about perfect.
I have used target guns in the 28 -30 inch range and speed increase is minimal do to the fact that the pressure wave is so down on the bell curve and the fact the bullet is already going so fast that it only spends sub 1/1000ths of a second in the last inches of the barrel [img]images/icons/shocked.gif[/img]
.270 Ackley improved 29 inch 1.250 dia. target barrel 7mm STW 28 inch 1.250 target barrel. I also love my .458 mag for varmints and the biggest game in the world.