Saturday, December 6, 2008

Twist Rate.

  Most people.....OK, SOME people know that there is rifling in the barrel of their rifles and pistols which causes the bullet to spin, stabilizing it.  The twist rate varies from model to model to model and manufacturer to manufacturer.  ( rifling is twisting grooves cut into the inside of the barrel)  I recently had my AR rebarreled with a Douglas stainless 1/8 twist barrel.  It won't stabilize 80 grain Sierras.  Not enough twist.  (The twist rate 1/8 is one complete revolution of the rifling every 8 inches, hence: 1/8.)  My previous Douglas barrels have been OK, but this one isn't.
  All I have ever shot at 600 is 80 grain Sierra Matchkings.
  I've been struggling with this for almost a year, shooting good scores up close at 200 and 300 yards with 69 grain Sierra Matchkings then watching my score crash with some lousy groups at 600 yards with the 80s.
  I'm not happy when I shoot lousy scores.  The sensation was that I was shooting very well, but the 80s were never where you thought they should be.  I would break a shot I thought would be in the middle and it would be a 4:00 8 ring.  Then I would break the next shot down the middle and it would be a 9 at 2:00.  A couple of times I shot donut shaped groups AROUND the 10 ring.  Very frustrating.
  Today, I shot some Lapua 77 grain bullets that we got as samples on Commercial Row on Camp Perry at the National Matches.  I only had 20, and the match calls for 2 sighters and then 20 rounds for record, so I shot two Sierra Matchking 77 grain bullets for sighters.  The sighters were a 10 and an X.  First Lapua 77 just missed the nine ring for an 8 at 12:00.  I came down three clicks on the rear sight and started shooting up the center.  Result: 194X8 (out of a possible 200)  Everything on call.  Much fun when things work.
  Overall 485X16.  95X1 standing at 200, 97X3 sitting rapid at 200 yards, 99X4 at 300.  Beautiful day at Panola.

Update:  Yeah, if I had a 1/7 twist barrel there wouldn't be a problem.  Same thing with my 1/8 twist match rifle barrel- it's 24 inches long instead of 20 on the AR service rifle, so it seems to shoot 80s OK.  Those extra four inches or more twist rate make a big difference.


Anonymous said...

A novice comment .... but shouldn't the twist be faster than 1:8 to stabilize 80 grain bullets?

Anonymous said...

I just checked my twist rate chart. Forget the first comment.

AnarchAngel said...

A little... ok a LOT... more detail:

Robert Langham said...

Yep. Never a problem wth 1/7. For a while Douglas was advertising 1/7.7 I hear. Somehow that morphed into 1/8. This is about barrel three on this rifle, and along with my other two, probably about barrel six overall. No problems until now.
Lots of success with Douglas barrels over the past few years. Won the TSRA Highpower Championship once. Louisiana State Champ twice. Distinguished. HM. Very competitive. But this Douglas barrel stinks. It's wasted a competitive shooting year.

AnarchAngel said...


Mic the length on each of the bullets. Split the difference between the one that doesn't stabilize and the one that does, and that's probably the longest bullet you'll be able to shoot accurately from that barrel.

Remember, it's the length that's the determining factor not the weight, so two bullets of identical weights but different construction, or different profile will be of different length.

I was a 77gr SMK shooter for years, but about three years ago I switched to the 75gr hornady match. They've actually got a slightly better BC, and they'll stabilize in a 1:8 just fine (hornady says they'll stabilize in 1:9, but my math says different).

I find that the 77gr smk needs a bit more twist than other bullets of equivalent weight to stabilize. The 75gr Hornady a bit less. Funny enough those, the 69gr SMK doesn't need as much as other 69gr bullets.

Berger is the one that throws me off big time though; because the formula says one thing, and they say another (slightly slower), and they're general right about their own bullets.

Robert Langham said...

Had the jackets come off some 69 Noslers with 26 grains of Varget under them. Heard good things about the Hornady 75s. Haven't shot many. Shot almost exclusively Sierras.

AnarchAngel said...

That 26gr/69gr load mustv'e been cooking.

I get about 2850fps out of my 24" 1:7, with 24.5gr of Varget under the Hornady 75gr BTHP match (Winchester brass and CCI match small rifle primers).

Even better, it'll group into under 1moa at 600 if the wind co-operates.

I can push the velocity a bit higher without pressure problems in the winter, but the load loses consistency. Also, I live in AZ, and if I go over 24.5gr, it flattens primers when it gets over 110 degrees out.

I will say however, that I've never found a more consistent powder in this heat than Varget.

Anonymous said...


You need to switch to small rifle magnum primers (CCI 550). That is why you are getting inconsistent results, failure to light off all that powder (its a compressed load of varget, a stick powder). Your mileage may vary or not.

Robert Langham said...

The 77s gave me flat primers but the bullet was jammed in the lands on every one of them. Three popped out . (remind me to shake them out of the trigger.) 25 Varget. Using CCI 450 magnum primers. There's a better primer?
The Winchester SR primers that I had opportunity to check really chrono-ed well. Weighing every charge.