Instability issue in transitions to subsonic velocity range

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ckpj99

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In my ongoing quest to get a solid loading for 45-70, I think I might be running into a new issue.

My manuals estimate that my loads are running around 1300 fps and I'm running a 500 grain bullet.

I'm wondering if running that speed is going to make the rounds unstable after a really short distance due to the fact they are transitioning from supersonic to subsonic.

Is there an online calculator somewhere that will figure estimate the distance my load will drop below 1150 fps?

Also, I'd be interested to hear opinions on dropping my loading down to subsonic for maximum accuracy.

And finally, are there any long range 45-70 shooters out there? Are you using really hot load in Sharps guns to overcome the transition to subsonic or are you using subsonic rounds?
 
Ok, folks. Forgive my hasty post. I did a little Googling and found a great ballistics calculator from Hornady.

Just as I suspected, my loads are likely transitioning from supersonic to subsonic right at 100 yards, which might explain some of the issues I'm having.

MODS: Feel free to edit the name of the thread

My other questions still stand. I'm looking for the best accuracy possible out to 300 yards or so. No 45-70 shoots flat, so that's not much of an issue.

These are target loads, not hunting rounds. So should I increase my charge to overcome the sonic transition or decrease it so it never happens?
 
Check out the free JBM ballistics software online, too. Some bullet shapes are prone to become unstable in the transonic range, but not all. It happens to the 168 grain .308 match bullets with 13° boattails under some conditions, but not to the 175 grain bullets with 9° boattails. So it isn't simple. What does happen to all bullets is drag jumps up in that velocity range. That can, but doesn't always induce instability. Most magnum pistol bullets, for example, start supersonic and go subsonic the way to the target, but most do not destabilize in the process.

So, if you are having a stability problem, perhaps it is something else. Try out the stability calculator on that same site as well (bottom of the page with calculators).
 
Thanks for the link, unclenick. According to the stability calculator, it appears that for the bullet I'm running, the slower I run it the more stable it will be. So that makes me think I should try some tests going under 1125.
 
I don't know much about the caliber but believe It's not a long distance caliber . Can you describe the problem you're having ?

Key holing past 100yds ?

Very good accuracy ( under 2 moa ) out to a specific distance then groups just open up out of control ??

What makes you think it's a stability issue rather then the gun or shooter ?
 
Heavy bullets in 45-70 have been used for long range shooting for a really long time. Obviously, it's an old caliber and the levels of accuracy aren't as high as modern calibers, but I think 2 MOA consistency out to several hundred yards shouldn't be much of an issue.

Plenty of folks shoot 45-70 out to 1000 yards with success.

The problem I've having is that I'm getting really solid groups at 50 yards, maybe 1.5 - 2". However, when I take the gun out to 100 yards I start getting lots of flyers that open the group way up. Out of five shots, I might get three within 3 - 4", then two that fly off and open the group up to 8".
 
Obviously, it's an old caliber and the levels of accuracy aren't as high as modern calibers
Click to expand...

Uh...wait a minute. I think you've made a bizarre leap of logic here. I don't think there's anything inherently inaccurate about any cartridge, but cartridge accuracy only makes sense in the context of the entire weapon 'system'. I'm not getting all defensive over .45-70, I'm just suggesting that we can't really say the fellow's accuracy problems are inherently due to the fact the cartridge is 'old'.

If you're thinking that lobbing a 500 gr flat nose projectile at 1,300 fps out of a 20 turn per inch barrel might not be conducive to accuracy, sure...I'm on board with that. That this might be a reasonable expectation of 19th century loads for a .45-70, sure. Just let's not condemn the poor little .45-70 for asking it to do so much. Maybe that's just your point--I'm new to .45-70, but it seems like it might just be able to get 350 gr slug hopping downrange at 2,100 fps (or more) in a very smart little formation.

In any case, I'm just thinking maybe it's the bullet. Any unusual about it? I'm not a rifle guy, but I am educated in aerodynamics a bit, and I have to think some bullets are less affected by transonic transition than others. I don't know. The spin rate I would not expect to be changing at all--am I right? In other words, the spin stability is not affected at all, to speak of, during flight. If it's stable leaving the muzzle, it's stable at 500 yds as far as spin.

What's the bullet. I'm stupid, but curious.
 
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I have done a bit of BPCR shooting out to the mid ranges of 200-600 yards - although with .38-55 and .40-65, the .45-70 and larger just kick too much for my enjoyment.

Muzzle velocities are in the 1100-1400 fps range with very good consistency, single digit extreme spread is possible. The blunt bullets handle the transonic transition better than high velocity spitzers. There was some work with subsonic loads a few years ago but it didn't help. Anything gained on the lack of transition, if any, was more than lost in added wind deflection at the low velocity.
We certainly can't keep them supersonic to any great distance with black.

You don't say what your rifle, load, and bullet actually are.
There are a lot of reasons for poor accuracy in the details.
 
I'm getting a 14+ foot drop out to 400yrds which is not all that far but 14 feet is a hell of a hold over . 300yds is almost 7 feet of hold over and was my point as far as not the best long range choice . Will it work , Sure it will but is it the right/best tool for the job ? I always thought of the 45-70 as a 200yds gun . Sight it in at say 150yds and your GTG .

I don't know the math but that is a very big heavy projectile . I don't see it destabilizing that much . It should be fine to 75yds but some how in that last 25yds it starts flying 4" off . That would be a crazy trajectory if it were to keep going . Again I don't know much about the caliber and am just thinking out loud here .


You don't say what your rifle, load, and bullet actually are.
There are a lot of reasons for poor accuracy in the details.
Click to expand...

I'd like to know those things as well as the sights you are using . :)
 
As others have said you may find that a different bullet will make a difference. In these lower velocity cartridges the older style, more rounded nose, longer shank, plain base (as opposed to bevel based) type bullets will tend to perform a little better. Wind can be a factor at 200 yards and beyond, but unexplained flyers at 100 yards makes me wonder if there are voids in your bullets, or uneven or damaged bullet bases?

I always inspect bullet bases very closely, and any nicked or damaged in the casting process are culled. Weighing your bullets and culling those which are a grain or more out of the norm can eliminate those with voids which can impair their performance. Commercially cast bullets are generally made with a bottom pour pot, which, with home cast bullets, bottom pour technique with the heavies tends generate more voids than those cast properly with a ladle.

In general, the bullet seating process with cast bullets has to be much gentler than jacketed bullets, making sure to ease the bullet into the case without damaging it. Too I find that a properly dimensioned expander plug that expands the case to a nominal .002" over bullet diameter is helpful, so that cast bullets don't get squished from their sized/cast diameters. The RCBS or Lyman type expanders are pretty good, even then I typically buy a custom expander from BACO to open the cases which I have sized to .002" under bullet diameter. The Lee die set will flare the case mouth, but with the comparatively thick 45-70 brass, the flare alone might not be enough and your bullets might be swaging down to below groove diameter in the seating process. Also I find that 500 grain bullets tend to perform really well in this cartridge, with a lower Trapdoor velocities they tend to do darn good.

If the trans sonic transition is indeed the source of your problem, a heaver bullet might help. I like to think of the 405s as a "carbine" bullet, as the starting velocities are typically lower out of the shorter carbine tubes than they are out of the longer, rifle length barrels. --Even though our black powder era ancestors had technological limitations compared to modern, high velocity, smokeless cartridges we enjoy today, they were every bit as smart, maybe smarter, than modern folk. They had a recipe which worked well in the 45-70 Government round, and I find that if I can duplicate their successful recipes, I too can enjoy success. :)

HTH.
 
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