Lilja 15 twist 22RF Anyone?
- Thread starter longbow
- Start date
Don't really know, so just speculation. While 15 twist might be better the last 1/3 of the bullets journey (after the bullet has slowed), in the first 1/3 of the trip the 15 twist might be bad in wind. So by 100 yards the bullet would already be headed off its mark and however good it might be in the latter part of the trip the bullets fate would already be sealed.
I'm no engineer, but I understand a bullet must maintain a certain RPM to maintain stability. A bullets length, weight etc effect how fast it must turn. Since most .22 bullets are very similar in those aspects, velocity and density of the air are a couple of things that can change. The colder it is the faster a bullet must turn to maintain stability. You can use a faster bullet (bialtholon) or a faster twist barrel. A slower bullet needs to turn about the same RPM as a fast one (talking .22). But to maintain that same RPM a slower bullet must come from a faster twist barrel to achieve that same RPM.
I think the thought here is that as the bullet slows down in the second half of a 200 yard trip to the target the 15 twist would maintain the correct RPM for stability.
But the fly in this ointment is that the faster a bullet spins the more effect crosswind has on it. If that wasn't the case we could just make all barrels 1 in 5 twist and forget it. You can over stabilize.
Manufacturers have used 16 twist for a gazillion years because it it a good compromise for all bullet speeds at typical .22 distances. Some in the benchrest crowd especially have went to a slower twist, 17 or even 19 to get better performance in wind at 50 yards. Mine shoot well down to 50 degrees or less with 17. But at times these slower twist can start to wobble at 50 yards. Oval holes in paper. Some are very sensitive to bullet velocity and manufacturer. I'm not sure 17 twist or slower would be a good choice at over 50 yards. But these benchrest guns are used for a specific distance (50 yards) and don't typically shoot outdoors in the winter.
Remington made(makes) a benchrest 40X with a 14 twist barrel. No one understands why they did that. The guns shoot really well indoors or dead calm, but any breeze at all and its unshootable.
I think the thought here is that as the bullet slows down in the second half of a 200 yard trip to the target the 15 twist would maintain the correct RPM for stability.
But the fly in this ointment is that the faster a bullet spins the more effect crosswind has on it. If that wasn't the case we could just make all barrels 1 in 5 twist and forget it. You can over stabilize.
Manufacturers have used 16 twist for a gazillion years because it it a good compromise for all bullet speeds at typical .22 distances. Some in the benchrest crowd especially have went to a slower twist, 17 or even 19 to get better performance in wind at 50 yards. Mine shoot well down to 50 degrees or less with 17. But at times these slower twist can start to wobble at 50 yards. Oval holes in paper. Some are very sensitive to bullet velocity and manufacturer. I'm not sure 17 twist or slower would be a good choice at over 50 yards. But these benchrest guns are used for a specific distance (50 yards) and don't typically shoot outdoors in the winter.
Remington made(makes) a benchrest 40X with a 14 twist barrel. No one understands why they did that. The guns shoot really well indoors or dead calm, but any breeze at all and its unshootable.
Really interesting ballistic fact: a projectile's forward velocity decreases much faster than its angular velocity. Implication being that any projectile will actually become MORE stable as it goes downrange. This is why long range (1000 yard) bullet stability can be tested by looking for oblong holes in targets at 100 yards. Its kind of cool.
Aside from slower twist having slightly less wind drift (i haven't seen it tested though, do you have any data?), the slower twist will have less bullet dispersion due to bullet imbalance. This is actually a bigger deal in rimfire than centerfire because our lead bullets exhibit far greater imperfections once they exit the muzzle. So, it seems to me rimfires would follow the same rules as centerfire: fast enough twist to stabilize and no faster.
-Mark
Aside from slower twist having slightly less wind drift (i haven't seen it tested though, do you have any data?), the slower twist will have less bullet dispersion due to bullet imbalance. This is actually a bigger deal in rimfire than centerfire because our lead bullets exhibit far greater imperfections once they exit the muzzle. So, it seems to me rimfires would follow the same rules as centerfire: fast enough twist to stabilize and no faster.
-Mark
In using the Don Miller formula for twist, a 1 in 16 comes out at about 1.33 with target ammo (40 grain, 0.470" bullet length, 1065 fps). That is pretty close to ideal. 1 in 15 will make it more stable than it really needs. Temperature down to 32F does not seem to affect stability very much. 1 in 17 is starting to compromise stability, but probably is still OK. Beyond that, is starting to be questionable.
An over stabilized bullet tends to wobble due to bullet unbalance, and reduce accuracy some, even at short distances.
An over stabilized bullet tends to wobble due to bullet unbalance, and reduce accuracy some, even at short distances.
This Miller formula sounds interesting but the numbers are a bit concerning for 17 twist, showing mostly in 1.2x range. I understand 1.4 is minimum with 2.0 to fast a twist. I would think the 17 would be a bit higher with with the performance it has shown. I just tried my 220 swift for grins and I got a number over 4! No wonder I can't hit anything(G). I'm going to have to play with this a bit more and recheck that I have the right formula and entered the right numbers.
Lower temperature - lower number - ok
Lower barometer - higher number -
Longer length - lower number - I need to recheck the length of my 220 and 22
Would be nice to be able to see what to expect in different conditions more precisely.
edit: Doing some searches I find the Miller formula is only good for supersonic boattails. I see there is a formula called McGyro on the JBM site I want to look at. I have used the trajectory formula at the JBM site before and it is good. I don't have time now.
Lower temperature - lower number - ok
Lower barometer - higher number -
Longer length - lower number - I need to recheck the length of my 220 and 22
Would be nice to be able to see what to expect in different conditions more precisely.
edit: Doing some searches I find the Miller formula is only good for supersonic boattails. I see there is a formula called McGyro on the JBM site I want to look at. I have used the trajectory formula at the JBM site before and it is good. I don't have time now.
longbow
Silver $$ Contributor
Going back and looking at Liljas 22 rf, I might of read it wrong the first time. I see he says 16 twist in two styles . The 15 twist must be more for pistols, short barrel.
Thanks for the response, I've had at least 10 of his barrels, everyone of the have been excellant. Pete
Thanks for the response, I've had at least 10 of his barrels, everyone of the have been excellant. Pete
Have been reading a bit about the Don Miller rule and found a few things which may be of interest. Here is his most recent article:
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf
Some things I learned were:
Velocity drops faster than spin velocity after the bullet leaves the muzzle. So stability increase as you go down range. If it is going to be unstable, it will happen at the muzzle.
If you read the article it appears his formula is very good for both flat base and boat tail bullet shapes. Also, seems good for different densities; lead vs copper.
Also he suggests using a different velocity correction factor for subsonic bullets of 1120 instead of 2800. I factored that into his formula and indicated stability for subsonic rimfire bullets goes up a fair bit. 17 is fine, even at 32 deg. F., with Sg of 1.51. Even at 18-19 it may be OK, if you like to live on the edge.
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf
Some things I learned were:
Velocity drops faster than spin velocity after the bullet leaves the muzzle. So stability increase as you go down range. If it is going to be unstable, it will happen at the muzzle.
If you read the article it appears his formula is very good for both flat base and boat tail bullet shapes. Also, seems good for different densities; lead vs copper.
Also he suggests using a different velocity correction factor for subsonic bullets of 1120 instead of 2800. I factored that into his formula and indicated stability for subsonic rimfire bullets goes up a fair bit. 17 is fine, even at 32 deg. F., with Sg of 1.51. Even at 18-19 it may be OK, if you like to live on the edge.
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