Finally solved my 12 twist stabilization problem!

[venison_burger]

Hello all,

I am so relieved to have finally figured out my Sierra 39 gr BlitzKing bullets. One of the reasons i bought the .204 Ruger was to take advantage of the good BCs of the smaller bullets for use on predators and just target shooting. When I started key-holing 39 and 40 grain bullets I was devastated. I thought my Remington 700 just wasn't going to shoot the heavier bullets and settled on a good load for the Berger 35 grainers.

Through lots of reading on this site I found that the 1:12 barrels are just on the cusp of stabilizing the heavier bullets. I've also read that some rifles will shoot factory 40 grainers but not hand-loads. With this information I theorized that if one pushed a bullet fast enough, the 1:12 barrel would stabilize the bullet. I figured that Hornady's blistering-fast factory rounds must do the trick with stabilizing the 40 gr bullets.

After failing to stabilize my Sierra 39 gr SBKs using BLC2 and Benchmark, I bought some Re-10x to try. Loaded up a 10 shot Audette Ladder starting at 25 grains, going up past the max load of 25.1 grains all the way to 25.9 grains. After warming up the barrel with a few shots I started the Ladder. The first couple shots hit a couple inches off of center, but after that the shots started tightening up. I finally settled on the 25.9 grain load which shoots .4 to .5 moa groups consistently. To prove that complete stabilization had been achieved I hit 4 balloons at 500 yards in 4 shots in a friendly competition with my brother-in-law. He was quite impressed.

Has anyone else tried this method to solving the dreaded rate of twist problem with the 1:12s? I am so relieved to have a good bullet working in my gun. I hope this helps someone with the same problem!

-Ben

[Fred_C_Dobbs]

venison_burger, my BL-C(2) load, 30.8 grains, on average was 23 fps faster than my RL-10X load, 26.2 grains. That was with 32-gr Bergers. Unless your BL-C(2) loads were way, way light, I can't imagine the powder could have been responsible for any instability. Regardless, I'm glad it's shooting better.

The RL-10X did turn out to produce very slightly better accuracy, but both of them are extremely temperature sensitive. Next time I reach the bottom of the jug, I'm thinking I'm gonna have a whirl with 8208 XBR. It's being touted as extremely temperature insensitive, plus a lot of shooters in a great many different calibers -- including .204 Ruger -- are reporting excellent accuracy with it.

[Joe O]

No doubt,a 1:12 can be a challenge with bullets in the 39-40 gr range.I have three 204s in three diferent guns (manufactures).They all shoot the 39SBK in
.3 MOA or better.Havent used 10X and BLC(2) only with 32 gr bullets.IMR 8208XBR is the best I've found for my 204r loads.Pours well,and gives great accuracy,with low pressure signs.I use 27.3 w/39 SBK

[Tokimini]

I had similar problems with my 700 in 204 shooting 39gr Blitz Kings. I was using 8208 XBR which worked well with 40gr Vmaxs but I could not get good groups with the BKs. I just got some Reloder 10X and was going to work up my own ladder for the Sierras. You saved me some work. Thanks. You theory of bumping up the speed to stabilize the bullet is an interesting one, however with the 40gr Vmax using 8208 XBR I found that my most accurate load was with 26.2 gr of powder. More powder and the groups started to spread.

[Fred_C_Dobbs]

A little extra MV won't have that much effect.

The reason a bullet has to be spun to be stabilized is because, in flight, it carries its center of pressure ahead of its center of gravity. That's like shooting an arrow fletchings first. It's a "statically unstable" condition and aerodynamic forces want to turn it around and let the CG lead the way. But just like a nice tight spiraling football, the spinning gives it a gyroscopic stability that helps it maintain its "point-into-the-wind" orientation. So, yes, driving the bullet faster will increase its RPMs but it also will increase the force of this "overturning moment," which reduces the extra stability you get from the added spin.


To quote Bryan Litz,* "Velocity also increases the force applied to the nose of the bullet at the CP, and strengthens the overturning aerodynamic torque which actually makes the bullet less stable. As it turns out, the effect that increased velocity has on the rigidity of the spinning mass is slightly greater than the strengthening of the overturning aerodynamic torque, so the result is a slight increase from the higher velocity, but not nearly so much as commonly thought, far less than a 1:1 correlation."


Bryan then cites the numbers from a generic 155-gr .30 cal bullet, noting that increasing its MV by 10% (which also would increase its RPMs by 10%) only increases its stability number by 3%.

I looked at the numbers on my 32-gr Nosler in JBM's stability calculator. Changing the input MV from 3800 to 4200 fps -- a 10.5% increase -- produces a 3.4% increase in the stability number, from 1.018 to 1.053. That's not a lot of extra stability to show for an additional 400 fps. If 1.018 isn't adequately stable, 1.053 ain't gonna be a whole lot better.

venison_burger, at what range were you getting the keyholing? I ask because, if it was 100 yards, I'd be curious to see if they still were keyholing at 200. If so, maybe they just haven't "gone to sleep" at 100 yards.


*Applied Ballistics for Long Range Shooting, Ch. 10, Bullet Stability, p. 150

[venison_burger]

My 39s and 40s keyholed at 100 and 200 yards. The first time I shot my 40 gr V-Maxs at 200 yards I had trouble keeping the key-holing bullets on an 8x11" piece of paper. I noticed with all of these loads that although the higher end of my pressure series still key-holed, they were more accurate. When I got my Re-10x I didn't bother working up from a minimum powder charge but instead chose to test my theory and start near the maximum. This 10 shot ladder was shot at 200 yards. There were small signs of key holing with the first few shots.

I didn't try this experiment with either of the other two powders. My "solved problem" might just be due to the new powder.

Its nice to get input from others with more knowledge on the subject.

I looked at the numbers on my 32-gr Nosler in JBM's stability calculator. Changing the input MV from 3800 to 4200 fps -- a 10.5% increase -- produces a 3.4% increase in the stability number, from 1.018 to 1.053. That's not a lot of extra stability to show for an additional 400 fps. If 1.018 isn't adequately stable, 1.053 ain't gonna be a whole lot better

Reading on this forum I've learned that lots of 1:12 barrels will shoot the 39-40 grainers and lots of 1:12 barrels won't. Assuming I have one of these on-the-cusp-barrels, could the 3% increase in stability have a significant affect?