22LR subsonic - getting apps to match actual drops

[kevwil]

I've been trying to get various ballistic solver apps (iphone apps and JBM website) to come up with a BC that matches what I get in the field. The goofy part is that it seems the BC needs to be banded and needs to be higher as distance increases (to a point, I assume). In other words, the basic BC solutions have been giving too much drop past about 100 yards. And this is subsonic ammo, there is no transition from supersonic to subsonic as it flies.

I am curious if anyone can comment on why 22LR subsonic seems to act this way (stability? yaw?), and how I would go about building a BC that solvers can use to accurately (-ish) model my dope. Thanks! Apologies if someone has covered this and I missed it.

 

[DocUSMCRetired]

It has in part to do with Form Factor. Neither G1 or G7 is the perfect form factor for 22LR. It would actually be RA4. You have the G1, G2, G3, G4.... G8.. Which are the most commonly known. But anything that flies has its own form factors. Gs for spheres, Gl for blunt nose lead bullets. You have long cones, long cylinders, spinner rockets, finned missiles, and more. From that you have revisions, like the G7 Revision 2. The problem is, that you cannot compare bullet performance, if you have G4 on one box and RA4 on another. So, G1 is used, because it allows you to compare apples to apples. It has its own problems, specifically that a BC is velocity dependent and in order to be a true comparison you need the averaged BC. But that is a long discussion.

The short answer, is that what you are seeing is the use of a G1 BC for a bullet doesn't perfectly fit the G1 Form Factor. One of the short comings of BC all together, and why we are moving away from BCs towards CDMs.

 

[kevwil]

Thanks Doc! That's very helpful, I would not have thought the form factor could throw it so far off, especially given how the G1 profile looks like a pointed version of the 22LR bullet.

It turns out JBM has RA4 in it, and I was able to get somewhat close to my dope using 1.2 RA4 BC and 1000 fps MV, although it's 0.2 mil off at 150 yards. Better than nothing!

This should help, at least until you publish CDMs for Lapua Center-X. ;-)

 

[Ned Ludd]

But the real question is, "Where are you getting the appropriate RA4 BC values for rimfire bullets to input in JBM?". Without experimentally defined RA4 BC values for the rimfire bullets you're using, it doesn't seem as though you'd be any better off than using the G1 BCs, which at least are readily available for many rimfire bullets.

 

[Ballisticboy]

Isn't RA4 supposed to be identical to G1 at speeds less than 1400 ft/sec so G1 and RA4 should be the same for subsonic bullets?
I have to say though that the versions of G1 and RA4 that I have seen are not the same at all.

 

[Ned Ludd]

I have been using the G1 BC values for .22 lr bullets listed in Bryan Litz' book, "Ballistic Performance of Rifle Bullets" and it has worked pretty well so far. However, I have not tried to obtain predictions for distances greater than 100 yd, so it could well fall apart at longer distances as kevwil suggested in the OP. The potential pitfalls that may be associated with rimfire match ammo and ballistic apps appear to include: obtaining solid BCs, that they're typically subsonic, velocity banding if shooting distances greater than 100 yd, etc. Bryan's book is the best resource I have found so far for rimfire bullet BCs. The other issues I haven't yet really had to deal with. I'd imagine there might be a way to simply tweak the inputs a bit to get better output for rimfire bullets. Even though the specific inputs won't be exactly the same as for centerfire cartridges, the general idea and the math for the output should be nearly identical.

 

[flatlander]

We've been shooting some 22RF Precision Rifle matches, with targets out to about 220yds. I'm shooting SK Rifle Match ammo out of a couple of 40X/XB repeaters with Benchmark & Lilja bbls. I used a G1 BC of .145 in the BA solver, and have been within .1 mil most of the time (depends on how conditions change from what I input). I haven't actually chronographed any of this lot of ammo, but input a value of 1080fps. If I ever get off my lazy butt & take the time to set the chrono up, I'll test with whatever value that results in and see if it makes any difference.