Marginal stability?

[tiny68]

Back in 2007 I purchased a 0.224" 1:9 twist PacNor blank and had it chambered in a 22-250 AI. It finished at 28". It was my first Ackley cartridge. I experimented with several powder bullet combos. I was trying to find the highest BC bullet I could shoot fast. 75 AMax shot very well with IMR4381 at 300 yards. I shot 300 yard load ladders with the 80AMax and 80SMK with RE-22, IMR 4381, IMR4350. Only IMR4350 performed poorly. I even talked with Ed Berger on the phone and he sent me a dozen 90 Berger VLDs to try (and told me they wouldn't stabilize - he was correct). I was obsessed with velocity back then and found I could run the 69 SMK at 3520 fps with IMR 4007SSC and shot many sub 1" groups at 300 yards. So that was my load for that barrel and my long range coyote gun for next 10 years.

Fast forward to today. IMR 4007SSC is no longer available. Going back and looking over my load data and notes for a new load. I did a bullet seating comparison with the 80 AMax with IMR 4831. I was averaging 3340 fps was getting 0.5" groups at 100 yards. I have a couple of boxes of new 22-250 Lapua brass waiting to be fired-formed. Running the numbers in the Berger stability calculator I am getting a SG of 1.26. That is the marginal stability range. Why did my bullets fly so well? I have confirmed the twist is really a 1:9.

Just trying to pick a new direction. No more IMR4007SSC and only a few hundred 69 SMK left. I have a bunch of the 75AMax, 80AMax, and the 80 SMK. Should I explore the 80s more or stick with 75 AMaxs?

Thanks for input, Tim

 

[jelenko]

As the Berger calculator says,
Your bullet is MARGINALLY STABLE.
Your bullet stability is marginal. You may shoot good groups under these conditions, but the BC of the bullet will not be optimized.

 

[tiny68]

Did the 80 AMax get renamed as the 80 ELD or is the ELD a different bullet?

 

[jelenko]

The ELD's are a different bullet. My understanding is they are essentially the same as the Amax's but with different plastic in the tips.

 

[Orr89rocz]

Someone told me the berger calc doesnt really apply for polymer tipped bullets

 

[Oso]

What are you using the rifle for and frequency of shooting? You are shooting a 22-250AI...it has a very limited barrel life from an accuracy perspective. General consensus if shot at typical velocities then after 1000 rounds through it the accuracy begins falling. Your barrel is now 13 years old...what is your round count through it? What you are wanting may not be feasible with your current setup.

If you want to chase heavies (match bullets) then maybe this is perfect time to rebarrel to a 1:7 twist. At the very least you probably need to bore scope the throat to determine if it is time to set back the barrel and rechamber. There are a lot of great shooting 52gr-69gr bullets that will perform better in your 1:9 twist.

 

[JASmith]

I recall reading that Berger recommends twists that give marginal stability (1.26 is about right) for best accuracy. The discussion might be still available in their online literature.

 

[jelenko]

Pretty sure that Berger says only that you can shoot good groups with marginal stability.
From Berger's website:
"Bullets flying with SG’s between 1.0 and 1.5 are marginally stabilized and will fly with some amount of pitching and yawing. This induces extra drag, and reduces the bullets effective BC. Bullets in this marginal stability condition can fly with good accuracy and precision, even though the BC is reduced. For short range applications, marginal stability isn’t really an issue. However, shooters who are interested in maximizing performance at long range will need to select a twist rate that will fully stabilize the bullet, and produce an SG of 1.5 or higher."

 

[jelenko]

Someone told me the berger calc doesnt really apply for polymer tipped bullets

Knowing Berger, I would guess their calculator does account for the plastic tip. The calculator doesn't ask for the length of the plastic tip, but, since it's Berger's calc, I'm thinking they account for the tip based on the bullet that's chosen.

FYI. JBM Ballistics calculator does require inputting the length of the plastic tip. The results from JBM have always been within 0.0X of the results from Berger's calculator. For example, one might be 1.24 and the other 1.26.

 

[MagnumManiac]

I have burned out 3 22-250 barrels and have 3 22-250AI barrels in 28”, 30” & 32” with 1:7” twist.
On the 3 22-250 barrels, they all got 2000+ rounds through them before accuracy dropped to unacceptable levels.
The 22-250AI barrels I use for mid range F-class running the 77gr CC bullets are at 440, 713 and 980 round counts and the throats look barely used. The AI design is easier on throats than the standard 22-250 case.
I use H4350 or Varget, depending on availability.
These 2 powders are quite easy on throats in comparison to other powders.
OP, with a 9” twist, I think you would be best suited running 75 or 69gr pills in your rifle.
The issue with BC is that it changes with velocity, if the bullet is too slow, it defeats the BC and falls short.

Cheers.

 

[tiny68]

What are you using the rifle for and frequency of shooting? You are shooting a 22-250AI...it has a very limited barrel life from an accuracy perspective. General consensus if shot at typical velocities then after 1000 rounds through it the accuracy begins falling. Your barrel is now 13 years old...what is your round count through it? What you are wanting may not be feasible with your current setup.

If you want to chase heavies (match bullets) then maybe this is perfect time to rebarrel to a 1:7 twist. At the very least you probably need to bore scope the throat to determine if it is time to set back the barrel and rechamber. There are a lot of great shooting 52gr-69gr bullets that will perform better in your 1:9 twist.

This is set up as one of my varmint rifles. Mainly coyotes. I shot some paper preseason just to make sure everything is still on and I know my drops. I have almost 500 rounds down the tube. It is a great year if I get a chance to fire I 25 coyotes. I have really been pleased with the 69 SMK and what I could do with IMR4007SSC. It will try some other powders to try and reproduce that success. I have plenty of Varget an H4350. I am very low on H380 and what I have is committed to my regular 22-250.

When I do a new barrel I try the 1:7 for the heavies.


Thanks, Tim

 

[Oso]

69gr SMK is such an easy round to load for. I run it in a 223AI at 3000fps without pushing it hard. There are some nice 80-85 gr bullets that your rifle would love when you rebarrel. In the meantime you have got to kill more coyotes or trees to justify the rebarrel.... 500 rounds to go!