barrel twist ?

47studebaker

fast twist for lighter or heaver bullets ??

Trying to get better groups with a 7mm Mag.

TWalker

As a general rule, heavier bullets, longer bullets, and lower velocities call for faster twists.

Hawk Carse

Unless you are trying something weird like the new 183 gr match bullet, the factory twist is adequate for any standard bullet. You just haven't hit on the right one yet. Or maybe you have read too many gunzine articles and internet brags and expect too much out of a mass produced rifle.

charliemeyer007

I have yet to see a 7mm that didn't shoot the 162 Hornady boattail well. How far out of the rifling and a concentric cartridge followed by powder and primer choices are my usual playground as rate of twist is fixed by the manufacture. I like glass bedded actions with mostly free floated barrels except for about 5# of damping force up pressure at the end of the stock. I don't keep rifles that can't get by with neck sizing only.

I like the quicker end of the range usually. RPM's don't shed at all compared to velocity.

nononsense

47studebaker,

Personally, I'd like to hear more about your question and what tests you've run already.

Longer bullets require a faster twist rate.

More than just a general rule, bullet stability is basically controlled by the twist rate. It is the length and diameter of the bullet which are required to determine that twist rate. The formula most often employed is the Greenhill formula.

Here is a work sheet for the formula:

http://kwk.us/twist.html

As simple as the formula looks, there is a lot of background math involved as well as form factors or the shape of the bullet.

There is a ton of reading you can do with regard to twist rate and the Greenhill formula. But if you need an in depth study of bullets in flight, read this:

http://www.nennstiel-ruprecht.de/bullfly/

Best.

47studebaker

Years ago I bought a 7mm Mag for a possible trip to western US. So far I have not had the chance/money to go but kept the rifle in case I drew a PA Elk tag or got a chance at a bear. One of the guys I used to hunt with gave me a couple of hundred rounds 7mm Mag he had been using before he quit hunting. They were marked as 152Gr. I couldn't get a decent group so I put the gun away.
About 4 years ago I bought a TC Contender with a 7mm TCU rifle barrel and began to reload for it. After trying 152 and 139 bullets I settled on 120's as the most accurate in the TCU rifle barrel.
I loaded up some of the 139's in 7mm Mag but haven't shot them yet. Today I bought a box of 162 and loaded them up. I'll be trying them this coming week as PA Bear opens on the 21st.
I haven't yet figured out the rate of twist on the Colt Sauer (made in the 70's) I tried finding the rate of twist using a patch and then a bore brush but the cleaning rod would not rotate.

nononsense

47studebaker,

Unfortunately your Colt/Sauer isn't on my twist rate list. However other sources suggest that it should be either 1:9" or 1:10" based on their models. Either one should stabilize the 162's at 7mm Rem Mag velocities.

To measure the twist rate with a patch and cleaning rod, the rod must have a ball bearing handle. Cheap rods do not always have this due to the cost. The patch over the brush needs to be somewhat tight in order to fit down into the grooves to get the rod to rotate.

You might consider that there could be another problem with your rifle other than twist rate given that it's a 70's rifle probably with a wood stock...

I would check the forearm and barrel channel for clearance around the barrel. I would also check the recoil lug area for debris and maybe a pad used under the the end of the forearm for upward pressure on thinner barrels. Then I would have the barreled action bedded.

Best.

I was reminded this morning by a fellow member that these rifles probably had an adjustable barrel tension system. You might consider removing the barreled action from the stock to see what you find.

While you're at it, remember to check the recoil lug and the pocket it resides in for debris. Then check the barrel channel for contact other than the system mentioned.

Best.

47studebaker

Thanks to all for the ideas.

47studebaker

quote:Originally posted by 47studebaker
Thanks to all for the ideas.


After some range work this week I'll post some results.