Testing distance

[Norm]

I was wondering what distance everyone is doing there F-Class load testing at?

I'm doing some development at the moment and after reading a few things about barrel harmonics, velocity spreads etc. it got me thinking if there is an ideal distance to test loads for a rifle that will be mainly used at the longs.

Also does the barrel profile come into consideration? One of my rifles has a 32" slim profile barrel while the other has a 1.250" straight taper 31" tube.

Calibres I am using are 7-08ai with the thin tube and .284win with the cylinder.

Cheers.

[Brad Y]

Norm

I do all mine at 100m (lucky to have a farm to go to)

[Seddo]

I do initial testing at 50m (mainly pressure and velocity) and then 300y.

[Barossa_222]

I got taught to check loads and ladder test at 500 yards. To me this makes sense, as anything shorter may not show much deviation. With velocity testing range doesn't really matter as the chrony is at the muzzle.

[Steve N]

I like 200. Shows a bit more than 100 and groups not as influenced by wind etc as you can get at longer ranges. Loads that show some promise then tested at longer range depending on conditions.

[AlanF]

I think to be thorough you need to test at several distances. At 100 is okay for getting a wind zero (which can be adjusted by calculation for long range) and if you use a chrony and get low velocity spread at the same time as good elevation then its a good indicator of medium range accuracy. However for longer ranges you need to at least check accuracy AT long range, preferably in very good conditions and using a chrony to understand more about any high or low shots. The reason to test at long range is that projectile (BC) consistency is not revealed at short and medium ranges. For initial trials of different projectiles, primers and seating depths, 2-300 is handy for groups if you can see the bullet holes. And to ladder test, which I believe is one of the quickest methods of finding an optimum powder charge, then 500 yards in good conditions is about ideal.

[Quick]

I do testing at 300yd/m to start with and then use club shoots to finish tuning and then small picnic shoots to verify.

[macguru]

100 then 300m , or just 300 if it looks good

[DaveMc]

I have seen consistent (high quality/reliable) results for testing at all ranges (25m through to 1000 yards). I think you just need to go with what you have access to and make it work. At short range it is about group size AND velocity spread with a good chrono. At long range it is about picking ideal conditions. I am happy to develop a load at any range but the "confirmation" is always done over a few 12 shot strings at various distances (short and long) at the club shoots.

[williada]

Read this first, then if you want to know more read the next post.

IMO, there are two major factors in selecting a specific distance for group testing; (a) gyroscopic stability and (b) minimum wind effect. Group analysis may be combined with modern computer programs by looking at the trajectories of individual shots (knowing their velocities) taking into account their decaying ballistic co-efficient, to be a spot on predictor of long range performance. You merely check your elevation performance at longer ranges to confirm your prediction.

Gyroscopic stability can be broken into to further parts which directly affect long range performance. Understand these and you will know what distance is appropriate to test at with regard to the second factor, wind. With the wind factor, the goal is to minimise the impact of deflection on the test groups. So (a) the groups are not compromised by deflection horizontally; and (b) vertically, where the vertical spin drift (or aerodynamic jump) is taken into account which is about 1 vertically, for 10 in a cross wind at 3 o’clock for .30 cal. Spindrift needs to be imputed up short if wind is a factor in testing but it dampens over long range. Look up Litz’s site as Barry has discovered and you can calculate its size with varying wind direction for your calibre at a given distance. Or use Peter’s software to practice countering an effect of spin drift.

Wind deflection and gyroscopic stability are separate considerations to specialist compensation testing to measure the vertical plane of bullet impact, which initially can be done at 25 yards. I use a 25 yard test for remedial armouring so the barrel curvature is aligned with a compensation profile I am testing. These tests use large pressure variations to indicate a general compensation profile of a barrel. Compensation testing is an advanced principle, so I will only talk about the primary group size testing below in more detail. The vertical curvature of the barrel alignment also removes the problem of scope misalignment with the barrel and therefore changing zero’s with distance. Sometimes barrel torque has to be taken into account by these tests and a minor offset in the vertical takes care of that so the scope will line up with impact and compensation vertical is assisted. Remember Aaron’s link on another post. Get the basics right first, then you can tweak it if you want to.

The practical place to start testing is where the gyroscopic yawing and pitching factors have settled somewhat, and the wind deflection is minimised and that’s about 130 yards - minimum. I prefer 140 yards due to variables in atmospheric density which is another discussion. Some scientists have seen the effect of a coning motion go out to 200 yards. It can be visualised as a corkscrew motion of the projectile about the trajectory path in the shape of a cone. The cone has several parts to it. The first phase of the cone or spiraling trajectory viewed in two dimensions is of most interest. It is caused by fast precession which dampen to slow precessions (frequencies of yaw i.e. bullet tilt away from its axis). The base of the first part of the cone is therefore greater towards the muzzle and tapers inwardly along the trajectory like a funnel in the second phase when the bullet is said to go to sleep. A third phase due to bullet instability where the airflow overwhelms the gyroscopic stability can occur at long range. Any logical person would base groups where the bullet has gone to sleep. Its as simple as that.

[williada]

Read on only if you want to know some reasons for my suggestions above.

People throw the term yaw around, but in simple terms it means tilting something off its axis. There are terms like in bore yaw, precession, fast or slow, yaw of repose and angle of attack which are descriptions of tilt from the bullet axis with the motion of the projectile in its path to the target.

In-bore yaw is critical to long range accuracy, and is essentially minimised by good gunsmithing and reamer selection and blueprinted actions (tight tolerances, no slop). It is most critical to reduce it before it gets past the overturning moment i.e. when the bullet flies down from its highest point to the target in its angle of attack or tilt off the bore line minimised. However, in-bore yaw will show up if you know what to look for about 500 yards by looking at group spreads on the test target, and it happens 500 yards is a practical distance relatively close to the overturning moment in a lot of trajectories of different calibres with sights set for long range. You can be sneaky and aim higher than your normal 500 yard elevation to mimic the bore angle for a longer distance - then look at the spreads.

In-bore yaw will influence the yaw of repose, which in the slow precession or low amplitude of the tilt caused by gyroscopic effects, which in isolation tends to be stable from the overturning moment on the downhill run. The in-bore yaw will lead to velocity variation with more drag. Normally, the yaw of repose is not a problem if velocity spreads are tight and above the speed of sound and if in bore yaw is insignificant. The projectile RPM does not reduce much and so the bullet is still asleep. Velocity changes will enlarge the group to a point in both cases where velocity drops off too much at very long range enabling the air flow to take over and destabilise the bullet. In other words, couple in-bore yaw (because it provides a bigger surface area for the bullet to react to as it slows down) with the yaw of repose, the resultant increases in velocity SD's will give you problems at long range with group size. The tilt of the projectile due to in-bore yaw exacerbates the other yaw factors associated with gyroscopic stability. Go to the butts a look for an old pill. If the rifling marks are longer on one side of it, it’s a sure sign of in bore yaw. A projectile is so malleable it will travel down the barrel in an offset manner, hence getting chamber alignment or concentricity of the projectile is paramount. Minimum chambers or jamming assist this. Sloppy work does not.


Alan's data is very significant in the development of his comparative shooting class system. It demonstrates scores of scope shooters fall away after 600 for F/TR, F/Standard and after 700 for F Open. What this says to me is that nodal tuning used by most shooters holds groups together up to 700 yards by and large. Also and hopefully, I have demonstrated on other threads, that the nodal tune has an association with higher velocity spreads. So the yaw of repose associated with in-bore yaw are not as big a problem up to 6 or 7 hundred yards because the overturning moment may not exist or just be starting. Ask yourselves how close are these distances to the overturning moment? To maximise scores past this distance, gravity plays a major role. So you need a compensation tune combined with an optimum charge weight tune where possible and for the bullet to travel nose down in its angle of attack past the over turning moment in a tractable manner. For a compensation tune and or an OCW tune, you still need minimum extreme spreads for long range because you are not tuning at the node, otherwise the yaw of repose path will do you because it is velocity sensitive at long range and more so if your barrel is not compensating positively. If you have a neutral barrel in terms of compensation profile, low extreme spreads will keep you in if all else goes to plan.

You have to be aware of these factors when assessing groups and weeding out causes like fouling issues or barrel/receiver joint problems. You will know one set of symptoms will occur at short range and another at long range. Fouling issues etc will be evident across all distances.

The bullet tip is where it all happens with regard to yaw. The outliers in our group may indicate (a) the presence of excessive yaw or (b) bore angle variability. Whether they are induced by in bore factors or acoustic shock wave factors from vibrations or gyroscopic stability external factors, we still have to check all areas if you want extreme accuracy.

It has always been my method to try and put the ducks in line. I suppose this is up to technological debate. Assuming you are smithed correctly, I consider gyroscopic properties to be a resultant factor not a direct one. In this sense the Magnus Force, which relates to the change in pressure on one side of a spinning bullet at its outer edge, (hence the relevance of radial diameter of a projectile mentioned in a previous thread) is most important. The resultant action whether it’s a pitch up or down of the projectile or a yaw left or right is found 90 degrees away from the point of pressure and in the direction of rotation. In terms of aerodynamic stability, the Magnus Moment and Centre of Pressure must have the shape of the bullet balanced with the projectile’s Centre of Gravity, otherwise it becomes aerodynamically unstable. In relation to yaw, which relates to gyroscopic stability too, Magnus Forces actually decreases yaw angle. Hence 7 mm should be ballistically superior to 6mm given the radial diameter and all other things being equal. Same applies to bullet length because we see the greater the bullet length, the greater the aerodynamic jump. So bullets also need to be selected for benign conditions as well as rough conditions. I have also mentioned in a previous thread talking about vertical shear winds (which are common at Moe Rifle Range), that a shorter, large radius bullet, would be better with this type of disturbance - maybe a tangent ogive pill in severe conditions.

As also mentioned before, the torque produced by the rotating bullet is directly linked to the radius of the bullet which produces the gyroscopic stability which also resists movement off its axis. We have derived stability factors to determine the ideal twist rate e.g.1.4. It assists the aerodynamic stability of the projectile and so we have different twist rates to match that individual bullet shapes. This is not the same as form factor which relates to BC. The BC we use is the proportion of an historical bullet form given the value of 1.0 so we can compare other bullets against it in terms of its ability to overcome air resistance which is a function of mass, diameter and drag co-efficient.

That being said, the effect of airflow on the ogive will tilt or yaw the projectile and we see this in bullet drift to the right in a right hand twist barrel and down post the overturning moment in the projectile trajectory. If a bullet is over stabilized there is a problem at long range in terms of tractability in that it won’t follow the trajectory tightly, and like in-bore yaw but on a greater tilt angle. It presents a greater surface area like a jumbo jet landing with nose up, which leads to greater bullet dispersion due to drag increasing and eventual instability. Beware that over stabilised bullets will lengthen the fast spin cone on one hand but according to Vaughn the slow precession will be minimised and vice versa. So what you see at your test distance might have another impact out further as was indicated when I tested sets of barrels for the NRAA in project Penumbra. I am very wary when you map your short range test groups out horizontally, of groups that open in the last section of the profile when charge rates are increased. That was seen in a horizontal plot of Tim's positive barrel profile on an earlier post.

The other effects that have an impact on testing distance are aerodynamic jump and nutation - the later being different. According to Brian Litz, "Aerodynamic jump is what causes groups to slant when shot in varying wind conditions. When the bullet exits the muzzle, into a cross wind, the bullet tries to yaw to align itself with the airflow. The bullet yaws to the side, gyroscopic action causes it to nose up or down by a small amount depending on the wind direction. This initial yaw has an effect on the trajectory, and is known as aerodynamic jump. The more severe the cross wind, the more pitch the bullet ends up with. Flying to the target at a pitch angle will result in an elevation error that's proportional to crosswind”.

So don’t do testing in a fishtail wind as this markedly affects elevation spreads. It’s a short range problem and dampens over the longer ranges. Hence 500 again is a logical place to test. Shooters will notice test groups arc, centre, form triangles, string left and right up and down and at angles. These patterns seem to contradict the aerodynamic jump and so are really false positives in group analysis up short. Often the results don’t pan out at longer distance for this reason amongst others. That’s why I confirm loads selected again at 300 and 500 for their authenticity, unless I can have a handle on wind speed and direction during testing to impute the aerodynamic jump or discount it. Still scratching your head Tony? Craig?

Nutation on the other hand, again caused by gyroscopic forces, is an initial axis wobble. Something you probably see on a javelin being thrown and not due to decay of velocity as mentioned above. It is damped by aerodynamic drag and disappears. Its path in the helix or corkscrew about the bullet trajectory caused by gravity has a series of flat spots when these loops pass through zero yaw according to Rinker. So very short range testing, say 20 yards is not advised for group size and long range extrapolation. It also means groups on the same elevation level, in plots with very short range testing can trick you, particularly if the group centres are compared.

Consideration of the fast coning distance is most important for me in testing. I think coning is determined by the action of gravity on the back of the bullet in the helical path while the bullet nose is trying to align itself with the air flow and is therefore it takes time for the gyroscopic stability together with drag to send the bullet to sleep. Similarly, but slightly different to aerodynamic jump in that pressure from bullet momentum in airflow terms has more force than the crosswind effect on airflow. Ballisticians recognise that there is a fast ballistic frequency that damps out to the slow precession which is persistent.

Coning should not be seen as effect of unbalanced bullets with loose cores or varying bullet jacket thickness that were commonly seen when we used the military stuff. The only solution to that is a slower twist rate and then the trade off is with optimal twist rates.

The last factors to consider are the jump angle caused by barrel lift and acoustic shock wave. As you change loads in testing, the jump angle and the torque twist of the barrel will alter bore angle; and so your groups will change shape and disperse more or less. Acoustic shock waves returning down the barrel as well as barrel torque can alter bore size as well as influence the angle of departure, and nodes. Having just mastered that, you may now be ready to refine it all with compensation tuning techniques and optimum charge weights. It’s taken me a lifetime and I am still learning.

So group size and dispersion have to be analysed with a lot of factors and trade offs in mind and therefore the distance for testing to sort the wheat from the chaff. I like 140 yards for basic load development, see if results hold at three hundred in compensation terms etc. and look for the flattest elevation compromise at 500 yards over 300 yards for long distance computer prediction. I may edit due to typos etc. David.

[DannyS]

Thanks David, great reading again.

Cheers
Danny

[Norm]

Thanks David.
Very interesting read that gets me thinking about some of the results that I have obtained with various calibres and loads at different distances. These results are what prompted me to ask this question in the first place.
While my close range targets suggest excellent accuracy, I am regularly plagued by scores that are lower in x counts than I think they should be. Even when the actual score is quite reasonable. Some fine tuning is obviously required.
Your comments also hint at the effects of twist rates and "in bore yaw" on the bullets actual real world B.C. While a stability factor of 1.4 is generally accepted as being enough to stabilize a bullet under average conditions, I have heard of testing that has indicated that a higher stability factor results in a higher BC over a long haul flight but this is getting a bit off the track.

[williada]

Thanks Danny and Norm. I think the secret lies in being able to read tea leaves. Not really, but you have to be brutally analytical in deciphering plots of groups in the horizontal and try and determine what factors are at play in their dispersion. I'm realizing that if a thorough grounding has to be achieved in ballistics, then practical one to one training has to be given before this can be mastered. So in that sense, it is like reading tea leaves to the untrained and those with the expertise should help others at the club level. You certainly have to have an aptitude for it. Otherwise take up shotgun shooting if you aspire to greater things.

Maybe Tony could comment how the length of time between shots exposes the fouling flaws in a barrel tuning with incremental load testing?

You are behind the 8 Ball at Moe with those uphill targets at trying to cope with windshear unless you can launch a hot air balloon with a flag attached to see what's happening.

Maybe I could shoot Pennant again, work with you and a small group and see if the team can giver Frazer's mob a tickle up Not that I think you need much help seeing you won scratch and handicap at Lang Lang last week. Cheers, David.

[AlanF]

...Maybe I could shoot Pennant again, work with you and a small group and see if the team can giver Frazer's mob a tickle up Not that I think you need much help seeing you won scratch and handicap at Lang Lang last week. Cheers, David.

Sounds like a challenge! You may have heard that Stratford-Maffra and Sale Clubs are no longer - we have effectively merged under the Rosedale name. BUT, for the forseeable future, we will continue to run two Pennant teams, Stratford-Maffra and Rosedale . We couldn't let go of the rivalry between the clubs .