Tuner question (not for real......) :) maybe?

[Joe Woosman]

I know my opinion carries little weight here but I have a problem with the theory of positive compensation and the magic in a good tune being the barrels ability to compensate for velocity variations. I understand mathematically it can be shown to make sense. However, with the new super precision loading scales and the ability of people to load to single digit extreme spreads, why has this not proven to be a solution to short range tuning? Theoretically, if you have no velocity variation, positive compensation can do nothing for you.

Now let’s suppose there are variations in the bullets ignition to muzzle exit time but low velocity spread. I mean, how can there not be with the variables in ignition and friction? We just can’t measure it like we can velocity. Theoretically in this case, bullets exiting at a higher rising muzzle velocity, (or angle), would produce greater dispersion with exit timing variations versus a muzzle that was slowed nearer a “stop”.

Outside of computer simulation, has there been any measured confirmation that positive compensation is indeed happening??

 

[mks]

I know my opinion carries little weight here but I have a problem with the theory of positive compensation and the magic in a good tune being the barrels ability to compensate for velocity variations. I understand mathematically it can be shown to make sense. However, with the new super precision loading scales and the ability of people to load to single digit extreme spreads, why has this not proven to be a solution to short range tuning? Theoretically, if you have no velocity variation, positive compensation can do nothing for you.

Now let’s suppose there are variations in the bullets ignition to muzzle exit time but low velocity spread. I mean, how can there not be with the variables in ignition and friction? We just can’t measure it like we can velocity. Theoretically in this case, bullets exiting at a higher rising muzzle velocity, (or angle), would produce greater dispersion with exit timing variations versus a muzzle that was slowed nearer a “stop”.

Outside of computer simulation, has there been any measured confirmation that positive compensation is indeed happening??

Joe,
You are correct about zero velocity spread not needing compensation. The smallest group I ever shot was with a rifle that had essentially no compensation, because its CG was level with the bore. The other argument against worrying about compensation is that it doesn't make much difference at short range even if you have a sizable ES. I have done the calculations. It's not easy, because most external ballistics programs don't allow you to input muzzle angle. But if I recall correctly, the difference is in the 10's of thousandths range. The counter argument, though, is that every little bit counts. I know I have lost X's, points and matches where 0.010" would have made a difference.

Friction slows down muzzle velocity and exit time, so it should be subject to compensation the same as charge weight. Ignition and the pressure curve, and more, may have random effects that cause dispersion that can't be compensated.

Kolbe posted some 22RF groups before and after tuning that will make your jaw drop. For my CF tests, the compensation is not very evident by eye. You have to measure carefully.

Cheers,
Keith

 

[mwezell]

Mike,
Your drawing helps illustrate what we want the muzzle to do. If it is following a circle, then we get horizontal dispersion along with positive or negative vertical compensation. So to reduce the horizontal dispersion, we would pick the oval. Now which part of the oval? At the top or bottom of the oval where the muzzle has little vertical motion, slow and fast bullets come out of the muzzle at nearly the same vertical angle. So the slow ones hit low on the target and the fast ones high. This produces vertical dispersion, which is not good. At the same time, the muzzle has its fastest horizontal velocity here, so we also get the greatest horizontal dispersion. On the other hand, on the side of the oval where the muzzle is rising, slow bullets that exit the muzzle later are launched at a higher angle, so they have a chance of striking the target at the same elevation as the fast bullets. We tend to get the smallest vertical dispersion here. Right in the middle (half way between the top and the bottom), the horizontal velocity goes to zero, so this point minimizes horizontal dispersion also.

What we can't tell from just the shape of the curve it what velocity the muzzle has. There is one perfect muzzle angular velocity that will provide perfect compensation (zero vertical dispersion) at a given yardage, load and atmospheric conditions. Ideally we want the muzzle angle to be rising straight up at that perfect rate, with no motion horizontally for zero horizontal dispersion as well.

Delta = F/k, so tuner mass doesn't directly affect amplitude delta. It does have a secondary effect through increasing the sag in the muzzle due to gravity, which increases force F (or more accurately, the moment arm of the recoil moment). A more direct way to increase amplitude is to decrease stiffness k, or lower the CG of the rifle to get more recoil moment.

Cheers,
Keith

Lots of good points Keith. Good post.

 

[mwezell]

Joe,
You are correct about zero velocity spread not needing compensation. The smallest group I ever shot was with a rifle that had essentially no compensation, because its CG was level with the bore. The other argument against worrying about compensation is that it doesn't make much difference at short range even if you have a sizable ES. I have done the calculations. It's not easy, because most external ballistics programs don't allow you to input muzzle angle. But if I recall correctly, the difference is in the 10's of thousandths range. The counter argument, though, is that every little bit counts. I know I have lost X's, points and matches where 0.010" would have made a difference.

Friction slows down muzzle velocity and exit time, so it should be subject to compensation the same as charge weight. Ignition and the pressure curve, and more, may have random effects that cause dispersion that can't be compensated.

Kolbe posted some 22RF groups before and after tuning that will make your jaw drop. For my CF tests, the compensation is not very evident by eye. You have to measure carefully.

Cheers,
Keith

I agree, but if we tune as if we're tuning for positive compensation, we're also at a point where muzzle angle and or it's position at bullet exit has very near its least amount of movement. Win..win. IMO.

 

[Joe Woosman]

I have done the calculations. It's not easy, because most external ballistics programs don't allow you to input muzzle angle. But if I recall correctly, the difference is in the 10's of thousandths range.

Keith,

With the "10's of thousandths range", are we talking a few .010" or double digits up to .100"?

Can some of those using tuners describe what they see in group size when adjusting the tuner? I have never used one.

 

[jackie schmidt]

Keith,

With the "10's of thousandths range", are we talking a few .010" or double digits up to .100"?

Can some of those using tuners describe what they see in group size when adjusting the tuner? I have never used one.

Joe, I have fired thousands of rounds in practice, and in competition, using a tuner on a short range Benchrest Rifle. Many in practice over a chronograph.

First, velocity spread seems to have little to do with a Benchrest Rifles ability to shoot competitive groups, which for the sake of argument, we will say is sub.200 at 100 yards. With my Rail Gun, I have shot 10 shot groups in practice, over a chronograph, at the sub .150 level with a velocity spread of close to 25fps.

I always go to the line with what I hope is the tune that produces the smallest groups, regardless of any other parameters of ballistic performance.

As for what you can do to groups with a tuner. If you have a good barrel, and a good tune, say with a agging capability at about .150, I seriously doubt you could crank more than .250 to .280 worth of bad grouping into it with a tuner. And most of that will be in the verticle plain.

I have done it. I have taken my LV that was shooting quite well on a specific day, and turned the tuner until it got ragged. But, unless I did something like ignore the flags, or change the load, there is no way it was suddenly going to start shooting "fours". The groups will usually start going diagnol in the mid to high "two" range.

If you think about it, when speaking in terms of 100/200 yard Benchrest, that's a lot.

 

[mwezell]

Joe, I have fired thousands of rounds in practice, and in competition, using a tuner on a short range Benchrest Rifle. Many in practice over a chronograph.

First, velocity spread seems to have little to do with a Benchrest Rifles ability to shoot competitive groups, which for the sake of argument, we will say is sub.200 at 100 yards. With my Rail Gun, I have shot 10 shot groups in practice, over a chronograph, at the sub .150 level with a velocity spread of close to 25fps.

I always go to the line with what I hope is the tune that produces the smallest groups, regardless of any other parameters of ballistic performance.

As for what you can do to groups with a tuner. If you have a good barrel, and a good tune, say with a agging capability at about .150, I seriously doubt you could crank more than .250 to .280 worth of bad grouping into it with a tuner. And most of that will be in the verticle plain.

I have done it. I have taken my LV that was shooting quite well on a specific day, and turned the tuner until it got ragged. But, unless I did something like ignore the flags, or change the load, there is no way it was suddenly going to start shooting "fours". The groups will usually start going diagnol in the mid to high "two" range.

If you think about it, when speaking in terms of 100/200 yard Benchrest, that's a lot.

Actually, I see teens in good tune and high 3's to low 4's when completely out of tune. Realistically, I've never been that far out of tune due to conditions, though. It's just that the tuner will take it that far out. It could very well be that different tuner weight and/or barrel stiffness is why you don't see as much with yours. That's not a bad thing in itself, though..probably just a difference between tuner designs and the gun they're on. As long as it will bring the groups down, repeatably and consistently, I think that's what's most important.

 

[mks]

Keith,

With the "10's of thousandths range", are we talking a few .010" or double digits up to .100"?

Can some of those using tuners describe what they see in group size when adjusting the tuner? I have never used one.

Joe,
Here is an example for a 0.308" bullet with BC of 0.366. For a spread of 3030 to 3040 fps with a fixed launch angle, the spread is 0.013" at 100 and 0.057" at 200. I have never gotten ES that low for 10 shots, let alone 50 shots for a full match. For a spread on the high side of 3020 to 3050 fps with the same launch angle, the spread is 0.040" at 100 and 0.172" at 200. One can judge according to their own ES, but the 200 yard dispersion is getting up there in the significant range.

Twist the tuner a little at a time, and the groups go from horizontal to round to vertical to round and back again. The round groups tend to be the smallest. It is repeatable, but not necessarily at the same settings as conditions change.

 

[mwezell]

Joe,
Here is an example for a 0.308" bullet with BC of 0.366. For a spread of 3030 to 3040 fps with a fixed launch angle, the spread is 0.013" at 100 and 0.057" at 200. I have never gotten ES that low for 10 shots, let alone 50 shots for a full match. For a spread on the high side of 3020 to 3050 fps with the same launch angle, the spread is 0.040" at 100 and 0.172" at 200. One can judge according to their own ES, but the 200 yard dispersion is getting up there in the significant range.

Twist the tuner a little at a time, and the groups go from horizontal to round to vertical to round and back again. The round groups tend to be the smallest. It is repeatable, but not necessarily at the same settings as conditions change.

That brings up another misconception..at least with some tuner/gun combinations. I often read people say that tuners only take out vertical. I find this untrue as when completely out of tune, big round 5 shot groups are common. Then the groups tend to stairstep diagonally. Next is where they go to just vertical before sucking down to just a bughole. This too may be somewhat dependent on tuner/gun idiosyncracies , but I see no reason why we should expect tuners to only remove/induce vertical, other than that I don't get that far out of tune from conditions alone...or never have. The only times I've seen the gun be that far out is when starting out or not knowing a baseline for tuner setting on a specific gun, due to taking it off or randomly moving it without knowing where it was set before.

The nice thing here is that it takes just a few shots to fix it..It's very typical for the range of tuner movement between in and all the way out of tune, with my tuner and a HV contour barrel of nominally standard lengths to be 4 marks. Thing is, that 4 marks in either direction, to be very close to your sweet spot, again. Now think about this for a second. If 4 marks on either side of your best setting takes you completely out of tune..AND there are 32 marks on the tuner..we have a one in four chance of being at or very near our sweet spot..basically anywhere along the tuners range of adjustment. Admittedly, this isn't 100% linear, but it's close.

Think about what I just said. With my tuner, it is typical to never be more than 4 marks from a sweet spot!! Let me repeat that..It is TYPICAL to never be more than 4 marks out at any point. Now, add to that, that realistically, I never use more than about two marks to maintain tune through any and all conditions I've ever encountered while shooting with a tuner.
I hope the picture I'm drawing here is becoming clearer now. That picture is just how EASY it is to use a tuner. These numbers are based what I've found to be true every time with my tuners, on many different guns with std hv contour barrels. The numbers are exact between different guns, but very, very close.

This is probably the most important point I'd like to make in this thread. That being just how easy using a tuner really is.

 

[mks]

I agree, but if we tune as if we're tuning for positive compensation, we're also at a point where muzzle angle and or it's position at bullet exit has very near its least amount of movement. Win..win. IMO.

Mike,
You seem to be stuck on Calfee's stopped muzzle mantra. If the muzzle is not moving, then you will have vertical dispersion due to velocity spread. To have perfect compensation and zero vertical dispersion due to velocity spread, the muzzle angle must be rising, and fast. Faster than typical short range BR rifles move, in my limited testing.

Let's think about it in terms of the two bullets, one fast and one slow reaching the muzzle in a bit over 1 millisecond after ignition. If the slow one takes 1% longer corresponding to, for instance, a drop of 30 fps compared to the 3000 fps MV for the fast bullet, then the delay in exit time is about 10 microseconds. During that time, the muzzle must rise to the new, higher launch angle necessary for the slower bullet to hit the target at the same point as the faster bullet. 10 microseconds is a very short time, so a lollygagging barrel is not going to get the job done.

Cheers,
Keith

 

[mks]

That brings up another misconception..at least with some tuner/gun combinations. I often read people say that tuners only take out vertical.

Good point. The adjustments made during a match are fine tuning, which works on horizontal and vertical. Positive compensation isn't something you get by adjusting the tuner by a few marks. It takes large movement of the tuner and/or redistributing the weight in the rifle.

 

[jackie schmidt]

I addressed that in my post #52 where I discussed that dreaded "Horizonal tune".

By the way, what are the Threads per Inch of the moveable part of your tuner.? Mine are 36 TPI, which is a little over .007 inch advance per 1/4 turn.

 

[Jerry H]

Actually, a pressure trace chronograph will show variations in rise time, which in my experience, is significant to group dispersion. This is somewhat independent of muzzle velocity variation that indicates little of the grouping potential of a given load.

 

[mwezell]

Mike,
You seem to be stuck on Calfee's stopped muzzle mantra. If the muzzle is not moving, then you will have vertical dispersion due to velocity spread. To have perfect compensation and zero vertical dispersion due to velocity spread, the muzzle angle must be rising, and fast. Faster than typical short range BR rifles move, in my limited testing.

Not at all, Keith. I very much believe that you're correct in that the muzzle never stops, but I also believe that it covers a lot less area while it's at near the top and bottom..even if it doesn't slow at all. That's why I posted the pics. If the muzzle moves anywhere close to the elongated pic on the left, it certainly isn't covering much area if tuned for bullet exit just prior to max vertical displacement. It's just making a u-turn, so to speak.



Let's think about it in terms of the two bullets, one fast and one slow reaching the muzzle in a bit over 1 millisecond after ignition. If the slow one takes 1% longer corresponding to, for instance, a drop of 30 fps compared to the 3000 fps MV for the fast bullet, then the delay in exit time is about 10 nanoseconds. During that time, the muzzle must rise to the new, higher launch angle necessary for the slower bullet to hit the target at the same point as the faster bullet. 10 nanoseconds is a very short time, so a lollygagging barrel is not going to get the job done.

Cheers,
Keith

Is my math wrong? Would that not be 10,000 nanoseconds? Still an unimaginably short time.

 

[mwezell]

I addressed that in my post #52 where I discussed that dreaded "Horizonal tune".

By the way, what are the Threads per Inch of the moveable part of your tuner.? Mine are 36 TPI, which is a little over .007 inch advance per 1/4 turn.

Mine is .900 x 32 tpi, Jackie.

 

[mwezell]

Actually, a pressure trace chronograph will show variations in rise time, which in my experience, is significant to group dispersion. This is somewhat independent of muzzle velocity variation that indicates little of the grouping potential of a given load.

Good point. I need to send mine back for repairs and updating.

 

[mks]

Is my math wrong? Would that not be 10,000 nanoseconds? Still an unimaginably short time.

Yes, I am off by a prefix. It is 10 microseconds. An eternity compared to nanoseconds! Sorry about that.

 

[mks]

Here are some more exact numbers: For a 0.308" bullet with BC of 0.366 in a 22" barrel, muzzle velocity of 3029 fps and an air temperature of 80F, for perfect compensation at 100 yards, the required muzzle angular velocity is about 18 rpm, at 200 yards 38 rpm, and 300 yards 61 rpm. The speeds don't sound so impressive compared to, for instance, electric motors, but they are still faster than typical BR rifles move. Notice also the large difference among the yardages. To be perfect, the rifle needs not just a small tuner adjustment between yardages, but a large change in weight/stiffness distribution.

 

[Joe Woosman]

Keith,

Have you found a method of testing the amount of positive compensation a rifle setup provides?

 

[Fretka]

This equation shows that mass decreases frequency: omega = sqrt(k/m). Omega is the frequency. m is the mass. So frequency decreases in inverse proportion to the square root of mass. For a mass added to the end of beam (rifle barrel), here are the basics: http://iitg.vlab.co.in/?sub=62&brch=175&sim=1078&cnt=1 Yes, there is calculus in there.

So how much does a dime bend a barrel, and how did you measure it?

Comparing a barrel with the mass of a tuner to that without, clearly mass is changed but movement of existing mass would not change the fundamental frequency but would damp or amplify either the fundamental or harmonics (I think)! IOW tune it.

Holographic interferometry, 6 whole wavelengths of 632 nanometer light wavicles (!?) Fascinating to watch as I did use a method that allowed live measurements to be obtained.
Fretka