I am building my first "Semi-Custom" Rifle. It is a Savage 110 I want to chamber in 6mm-284. I want to push a 70 gn Nosler Ballistic Tip 3700-3800 FPS. I am going to use a Shilen Drop-in Barrel. Now what twist rate would be optimum for this weight bullet at that velocity. Also Shilen lists a .270 Neck and a .277 neck. I am assuming that the .270 neck is just a tight neck variation, Is this True. I Don't have any experience neck-turning and would rather not have to monkey w/ it if I don't have to. I am an experienced handloader but this is my first foray off the beaten path. Also I am going to buy a Redding Bushing neck die set for this project. What Bushing would you recommend for necking down the 6.5-284 brass. The same bushing that I would to just size them or go w/ an in-between first. Any help from you fine gentleman will be greated appreciated. Thanx

If you are going to shoot nothing but 70 grains or less, a 14 twist is plenty. But, you know that you'll eventually want to try heavier bullets so a 12 twist would be a good compromise. You'd need a 10 twist for bullets in the 90 to 100 grain range and an 8T for the biggest 6mm bullets.

A 270 neck will require turning while a 277 should not.

Ray

I think if I were you I would seriously look at heavier bullets with better BC's. You can shoot 87 grain V-Maxes with a .4 BC at 3600 fps with a longer barrel. 95 SMK's that I use a .5 BC for at 3450-3500 easy.

I think you will find that the 87 grain V-Max will stomp the 70.

Years ago did a 6x284 with 1/14 twist barrel with Berger 68gr bullet got 4100fps backed it down to 3800fps plus. I then build a 6Rem with 1/14 twist barrel that gave me 3700/3800fps using alot less powder.

If your a volume shooter 6x284 will burn a barrel out pretty quick.

I got a form/trim die from RCBS in 6x284 use that to form my cases. Mine was a tight neck .262 rifle.

I would get my brass first then measure neck thickness to see what bushing size you need. My Hornady 6.5x284 brass varies between .014 to .015 and Nolser varies between .015 and .0155. I didn't measure the Laupa brass I've got a tight neck 6.5x284.

quote:

Originally posted by Cheechako:
If you are going to shoot nothing but 70 grains or less, a 14 twist is plenty. But, you know that you'll eventually want to try heavier bullets so a 12 twist would be a good compromise. You'd need a 10 twist for bullets in the 90 to 100 grain range and an 8T for the biggest 6mm bullets.

A 270 neck will require turning while a 277 should not.

Ray

My 6mm/284 has a 1/10" twist. It gets over 3500 fps from 95-grain bullets, and should do 3700 easily with bullets as light as 70 grains. Ir will shoot half-moa with 105 grain Speer spitzers. 1/14" should do for 70-grain bullets. But I prefer a faster twist for heavier bullets.

I believe I got this off one of the forums here at Accurate Reloading. A.R. has become my evening reading as I drink my way through the Best Wiskey thread on Walters own General Discussions forum .


Greenhill Formula for Rifling Twists from Engel Ballistic Research

The Time-honored Greenhill formula was originally used for determining twist rates in the new rifled artillery in the 19th Century.. It is important to note that it is projectile *length*, not *weight* which is the determining factor when considering the best twist rate for your intended use. Weight can vary for a specific bullet length from use of different material and also is determined to some degree by projectile shape. Often long-range match shooters utilizing low drag or VLD projectiles will voice a preference for "lazy" twists; i.e. just enough to stabilize the bullet. The requirements of a Shooter who is utilizing both high velocity and Subsonic ammunition are quite different.

When utilizing subsonic ammunition there is another factor to take into consideration. The projectile not only has only (about) 1/3 of the forward velocity of standard ammunition in Rifle calibers, it also has 1/3 of the *rotational* velocity...think "RPM's". This makes for a less gyroscopically-stable projectile, so a faster rate of twist is indicated than by simply applying Greenhill alone. This wisdom was imparted to me in a conversation with the great barrel-maker Boots Obermeyer. It makes perfect sense, and doubly so coming from him, so we'll take it as Gospel.

Greenhill Formula For Rifling Twists

T*B=150*Sqrt (Density of Lead/Density of Bullet)

T=Twist

B= Bullet length

Both units have to be in 'Calibers', density portion is optional

**********

To determine the *approximate* twist rate for a given projectile (this varies a bit by shape):

Using the a 190 gr Sierra MatchKing bullet #2210M (.,30 cal) as an example. If we measure the bullet's length it is 1.375" long .

B=Bullet length in *Calibers* (Length/Diameter), hence 1.375/0.308= 4.46 *calibers* long

From formula we have T=150/4.46=33.6. This figure is now in *calibers*, so we convert the twist from calibers to inches. So T=33.6 x 0.308=10.34".

This is why most commercial rifles in .300 Win. Magnum come with a 1:10" twist barrel.

***********

One can go further and get involved in formulas for density, but for most conventional lead/gilding metal projectiles, the above will suffice. Note that this is for a normal High Velocity loading. Subsonics generally require a somewhat faster twist rate. If one plans to use both High Velocity and Subsonic loads in the same platform, a good compromise which works acceptably for both must be found. If the platform is a "dedicated" gun in which subsonic ammunition alone will be used, then a faster rate can be utilized in order to be able to accurately shoot longer (and thus heavier) projectiles subsonically and with good accuracy.

Example:

Twist rates for 7.62mm/.308:

High Velocity only: 1:11-1/8" to 1:12"
Both: 1:10"
Subsonic ONLY: 1:8"

Bear in mind that "dedicated" back at the shop might not be "dedicated" in the field. Most knowledgeable end users prefer to have the option of utilizing both loads so as to be able to address changing tactical requirements on the spot

quote:

I Don't have any experience neck-turning and would rather not have to monkey with it if I don't have to.

I chose not to go with a tight neck when I had by bench queen built. I wish I had. I can shoot just a red hair under 1/2-minute with its SAAMI chamber. Because I didn't go with the tighter neck, I will now always have to wonder if I could not have gotten down to 1/4-minute. I suggest you go with the tighter neck. You might bugger a few cases (just learn with cheap brass) in learning now to turn them but in the long run, you'll be glad you chose it if only to remove one point in the list of what causes failures in accuracy...

For 70 --I'd prefer a 10 or 11 ( Berger rec=13)
for 95 --9 ish
for 105--8 ish
for 115--7 ish

My 1-10" Shilen will stabilize the 105A-Max reliably,via 6mmAI.

The 6-284 would do likewise and more,due to velocity increase eeking more RPM.

That being said,9" is where it's at.

Email Bryan Litz AND buy his book.
Nuf said.


“Applied Ballistics For Long Range Shooters”

http://www.appliedballisticsll...index_files/Book.htm


Glenn

Put a 1:7 twist on a 6.5x 284 and watch pressures go through the roof!

If I were having another rifle put together I would opt for enough twist to stabilize the next heavier bullet than I expect to use. IOW, I would err on the side of too much twist than not enough. If I were building any 6mm I would go with a 1/10" twist unless planning to load bullets heavier than 100 grain or mono-metal bullets and in that case would step up to 1/9".