Graham, I was hoping more people could contribute to practical experiences with 155’s and 185’s and answer Rod’s question too. In the absence of that, all I can offer is transferable knowledge which may apply to other classes which are faced with bullet choice and give reasons why things happen to the best of my ability for what its worth.
It seems the role of the Magnus force is not fully understood by the fact it is being seen only in the context of small vertical drift with head and tail winds, which also happens to be the context of where vertical shear causes problems when head and tail winds change vertical direction and density of the air flow over high mounds. My emphasis with Magnus force really related to 6mm v 7mm in terms of Magnus Moment on longitudinal trajectories, aerodynamic stability and radial forces. But I did not explain that and so give reason why one bullet or calibre is better than another when dealing with vertical shear.
The air flow being laminar stacks up in tighter layers above the mounds and unpacks in the gullies when dealing with head or tail winds. So in effect we have changing air densities as well. The wind speed is also a factor on the degree of deflection both vertically and horizontally. These effects are not to be confused with crosswinds which run faster in gullies in this case. The other factor coming into play is the trajectory of the projectile as to whether it pierces and flies above turbulence or does not pierce the stacked up laminar flow in the case of mound induced vertical shear. So Rod’s question is a good one about Belmont. I was used to shooting on flatter ranges in Williamston, Rosedale and the Wimmera. Now I face those upper targets of Moe.
Physics tells us the heavier mass is more resistant to disturbing forces. Also, the higher the rotational speed the greater the resistance to deflection. So rifling twist rate is important. But very important is the radius of the bullet as this relates to rotational speed and bullet mass to torque and also links to Magnus force application albeit small. This means that a wider fatter bullet has greater gyroscopic stability than a thinner longer bullet with the same spin because the angular momentum is less with the thinner pill. In a trade off, also consider whether deflection depends on bullet performance in terms of the ballistic co-efficient (BC) and downrange performance. If a lighter bullet in the same calibre has a higher momentum than a heavy bullet it will do better against the wind up short until a point in its trajectory where its momentum drops off because its velocity decays at a faster rate in a lighter bullet than the heavier bullet, which therefore has the long range advantage because its BC remains higher. Bullet flight time is critical to the amount of BC influence. Note that most BC’s are average figures and don’t paint the full picture of bullet velocity and BC decay. Ask yourself whether a VLD secant ogive which is generally longer, is as good as a Tangent ogive in these conditions if BC's are similar?
So if you can drive a heavier pill as fast with a similar twist as a light pill and with a radial advantage it is ideal. Would then, the 7mm be better overall than the 6mm? That’s why I think the trend to 7mm is more than a fashion, its being seen on the target in a practical sense.
The Magnus force also plays a huge role in the centre of pressure in relation to the centre of gravity of the projectile which determines the Magnus Moment hence projectile balance or aerodynamic stability and impacts on the overturning moment in the trajectory. This means the distance to the target is a big element in bullet selection and Magnus is involved as well as being a small part of radial advantage or torque. See
http://www.nennstiel-ruprecht.de/bullfl ... cesmoments So how do we relate this to bullet choice for dealing with high mounds? In long range, a higher trajectory of a heavy bullet may pierce that stacked laminar flow earlier and remain above it for most of its flight relative to a light bullet. In short range, the faster projectile is likely to offset mass arguments but relative BC comes into the equation. If it is a different calibre, the faster twist rates will resist disturbance. You then have to ask the question at what wind speed are there really problems with high mounds for different bullet weights of different calibres? In my experience, it is in high winds and perhaps others will share.
The Magnus force seen only in the spin drift vertical displacement, which has a small impact to be sure; and aerodynamic jump where the rotating pill aligns itself with the wind flow, (which I meant to say before this edit which is a connected concept and really what I should have emphasized here and you are right in that context Graham), is also responsible for vertical in small amounts as it relates to changes in wind direction but is also seen as lateral jump until it is dampened by gyroscopic stability at long range. Gyroscopic stability is really the overriding factor to be precise and there seems to be three steps forward and one back. Magnus is subtracted from these effects in this context as well.
In a right twist barrel, right wind raises the bullets nose and gives you a higher target impact while a left wind lowers it, with corresponding target impact. Knowledgeable shooters have been using it to advantage when wind speeds are high in terms of vertical drift/deflection in a fishtail where right and left winds raise you above and below centreline. These people are your competitors. You are only disadvantaged by not knowing it and it could be 1 minute in 10 minutes in a crosswind for 30 cal. where right wind raises and left wind lowers. Peter Smith's ballistic simulation allows you to practice this - certainly enough to tip you out of the super centre even if your barrel is tuned and not to be confused with falling off a node. So perhaps I would change my projectile or calibre for these special conditions.
Its very complex stuff and I have left out coning effects, in bore yaw and variable angles of attack post the overturning moment. But in relation to the Magnus force and the Magnus Moment which relates to centre of pressure and centre of gravity of the projectile for aerodynamic stability, the lift characteristics of gyroscopic stability are greater when the centre of pressure from drag is in front of the centre of gravity of the projectile. So the longer the projectile such as a VLD, the greater this vertical component will be. Perhaps another reason for shorter, or tangent ogive bullets to be given consideration in shear wind conditions over steep mounds. The language for all this stuff has evolved as more is learned. The Magnus force relates to the drift created by a spinning object and so does gyroscopic stability. Both can be increased by the radial increase of a projectile. David.
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