Range Report Detecting yaw when loading at limits of stability

[dbooksta]

I've been working up a number of loads that push the limits of stability in my barrels -- combinations of long bullets and/or low velocities with twist rates that give Miller stability values approaching 1.

However Miller's formula isn't necessarily valid for some of the extreme bullet types and low velocities I'm testing, so I'd like to empirically verify when I've hit the limit of stability.

I don't have access to high-speed cameras, but I have read that in the old days ballisticians relied on "yaw cards" to detect stability. As far as I can tell these are just paper targets placed at various distances to see whether the bullet is going straight.

Now, I can certainly tell when a bullet is blatantly keyholing. But I don't think I can reliably detect more subtle yaw -- say 10 degrees off axis -- from the mark through a piece of paper. But 10 degrees of yaw would certainly be enough that I would want to keep these bullets out of my suppressors!

So are there any tricks to getting more precise readings on bullet yaw near the muzzle?

 

[Lt. Arclight]

Re: Detecting yaw when loading at limits of stability

That is probably a question you might ask Bryan Litz. When you say you want to empirically prove the limit of stability? Is this at MAX velocities or are you working on subsonic loads?

Short of a high speed picture- a yaw card won't work anywhere near the muzzle since it will be destroyed by muzzle blast. Short of shooting a bullet smaller than the bore-how could a bullet yaw 10 degrees after leaving the crown?

 

[dbooksta]

Re: Detecting yaw when loading at limits of stability

I'm most concerned with subsonic loads since those are ones I would love to put through suppressors. Of course all the silencer manufacturers warn against putting subsonic bullets through their cans, specifically because stability is so tricky with low-speed rifle bullets. I assume they issue those warnings because it <span style="font-style: italic">is</span> possible for a bullet to yaw significantly just after leaving the crown.

But now that I put it that way I wonder if the problem with subsonic baffle strikes doesn't instead have something to do with the bullet not outrunning the turbulence a suppressor induces in the propellant plume?

Based on what I've found so far it sounds like the standard distance to start yaw cards is 7 yards from the muzzle.

 

[Guest]

Re: Detecting yaw when loading at limits of stability

Stability in a subsonic load is always checked w/o the can attached first. When working up a new subsonic load, I'll first load above the speed of sound and check group size at 100yds. Then start droping the charge until it's below sonic on a 35* day, all the while checking the groups. If it won't shoot better than 1.5 min at 100, that load will not go threw my cans. My .308win has a 12 twist, button barrel.
I've 5 different rifle loads that are subsonic, with the 175smk being the best I, or many others have ever shot in a .308win.
One LEO I shoot with has shot that load many times into 1/4 moa an better groups many, many times.

 

[dbooksta]

Re: Detecting yaw when loading at limits of stability

So is it generally agreed that a (subsonic) load is silencer-safe if it groups about 1.5MOA at 100yds?

 

[Guest]

Re: Detecting yaw when loading at limits of stability

Thats just my own rules for my cans. If I had a can that required a major wind or elevation change from my, std uncanned, 100yd zero I'd not go above 1.5 for any reason. This is a problem I just went threw again, with my last 30cal can. It would move 3 IPHY up and 4.5 IPHY right at 100yds until I corrected that, I would not shoot any subs that printed above 1.0 IPHY. Now that it's corrected(only 2.5 IPHY up now, full power loads, no windage required) I can go back to 1.5 as my std max number. Should be able to get it to less than 1 IPHY of up with more indexing, I hope. This 22" barrel may not do that number, but we'll see. Short heavy barrels work the best.

 

[Greg Langelius *]

Re: Detecting yaw when loading at limits of stability

I'd make my judgement by looking at the other end of the trajectory.

If the bullet's rotational axis is tracking in line with the trajectory (achieving a smooth noseover/tipover); drag will be at its least, drop will be relatively consistent, and at its minimum value. As yaw occurs, more drag is induced, drop increases, and often, so does vertical dispersion.

So if your POI is significantly lower than your ballistic data would predict, and you're certain your numbers going into the equation are good, suspect yaw.

BTW, this is not just something that occurs with minimal stability, yaw can also occur when static (gyroscopic) stability is well in excess of what's required.

Greg

 

[MitchAlsup]

Re: Detecting yaw when loading at limits of stability

Note on the stability formula(s):

The stability of a bullet is dependent on the density of the air it flies through. So a bullet might be completely stable at 5,000 feet in 100dF temperatures even with a stability number around 0.9. Another bullet might be unstable in antartica at sea level at -50dF even with a stability number of 1.2.

 

[Guest]

Re: Detecting yaw when loading at limits of stability

I've never used any math to be can safe, never knew it could even be done that way. I only did cast bullets 99% of the time(to save money) growing up I never thought about bullet length at all,
as everything was T&E for me.

I learned alot about cans an bullet types, that did, an did not work. Most of the time I'd take the can off an machine a tube that was the same size and weight as the can and install that, then shoot cast bullets threw the tube to see what worked and what did not.
I found 2 minutes was the lower limit some of the time, but some combs would go to 4 minutes w/o a strike, that is why I stop at 1.5 myself.
I also found the target and it's range would require a different load combo most of the time. Some worked well close, but sucked at long and many that worked well long, sucked at short.

One important lesson I learned the hard way, was don't use subsonic loads with a Winchester Silver Tip as the bullet. I hunted/tracked that Deer for 9 hrs after the shot.
Went back to cast bullets after that lesson, to this day I still think about that Deer and being stupid.

 

[dbooksta]

Re: Detecting yaw when loading at limits of stability

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Gunfighter14e2</div><div class="ubbcode-body">I found 2 minutes was the lower limit some of the time, but some combs would go to 4 minutes w/o a strike, that is why I stop at 1.5 myself.
I also found the target and it's range would require a different load combo most of the time. Some worked well close, but sucked at long and many that worked well long, sucked at short.
</div></div>

That last part is what scares me -- and I wasn't sure it's possible for a bullet to be inaccurate at short range but "settle" consistently thereafter. That means that at least in principle everything could look good outside of the muzzle blast range but you could still have a baffle strike?!

Sounds like building a "sacrificial tube" to detect strikes might be the best way to go in terms of protecting silencers.

 

[badbot]

Re: Detecting yaw when loading at limits of stability

I have a 1 in 12 twist TC and shooting cast from it I think that you could set cards @ 2 10 25 50 using a laser to line them up.
then pick say a 220rngcand load it at 1100 1000 900 800fps.
checking for yaw and trajectory for each load at each station.
after finding something accurate - yaw stable work up your dope
then try say a 180gr.

looking for twist/speed/rpms combinations that work for each bullet type/weight after finding something promising translate this to your rifle.

just thinking a tc supressed might be better it breaks down travels well has scope mounts won't need a sbr stamp, low cost and on and on.

 

[Guest]

Re: Detecting yaw when loading at limits of stability

Remember, I was useing cast bullets, which I still use to this day.

Back then I only ran baffles with +0.010 - 0.015 over bullet size, which means I only had 0.005 - 0.0075 to play with. Alot of cans of today run 0.030+ which gives alot of room to play with, assuming they run true to bore.

I am one of those types guys that once, I find the limits on something, unless theres a major change in the makeup I stick with tried an true, same as in, if ain't broke don't fix it.

With todays machines, gunsmiths, and can mfg's, I doubt you'll have an issue as long as you test fire for groups w/o the can attached first. I've seen a number of subsonic loads that were 1.5 moa an better at 100yds that printed 3/4 moa or less at 200 + yds. I have always thought that issue was in the bullet base vs crown, myself. I believe when the bullet leaves the crown if everything is not perfect, there is a little yaw induced from the gas pushing the base off ctr, that is self correcting from spin down range.

I've took a number of bullets and marked the bases with a knife and watched the effects on target over the years and find the same thing. I had a barrel recrowned and shot the best combos I could and all was well, then took and marked the crown at 12:00 just to see what would happen. Flat base bullets shot almost the same, but there was a difference. Boattail bullets, there was a big difference at short range, but at 300+ they would run neck an neck group size with a good crown.

I'm no expert at all, so take this for what you paid for it, just what I've found over the years on my stuff.

 

[Guest]

Re: Detecting yaw when loading at limits of stability

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BadBot</div><div class="ubbcode-body">just thinking a tc supressed might be better it breaks down travels well has scope mounts won't need a sbr stamp, low cost and on and on. </div></div>

We are on the same page as the TC Encore pistol in .308 can be had in a 10, 12 an 15" 10 twist barrel. I've shot subsonic 175smk's in a 12" 10 twist .308 and they do very well.