OzFclass

HI All,

Is spin drift a just the name for the lateral deviating component of Magnus effect or are they 2 seperate effects of a spinning projectile?

Denis
This young bloke has a lot of good material which if you check out can answer a lot of questions. Check out: https://www.youtube.com/watch?v=OCA1njSxgv4.
My view is the Magnus force is the chicken and it came before the egg. Also check out: http://www.nennstiel-ruprecht.de/bullfly/
David.

Thanks for your reply David. I'll have read through the link and a watch of the you tube videos.

Cheers again,
Denis.

Denis, do a search on Youtube.com for "Rex Reviews Sniper 101". There over a hundred videos by this young man. I think people should find an appropriate topic, say, Coriolis or Temperature and get as much backgrounding as you can. These videos are excellent. Then maybe we can talk briefly about the appropriate use of ballistic software and a couple of extra tips. Do look specifically at the role temperature plays.

I am really only interested in long range shooting, and as Alan’s data suggests we have a problem from 700 onwards with scores falling away.

What I use ballistic software for is to find a true zero given the atmospheric density, longitude and latitude of a venue.
This compliments the true alignment of my scope with the vertical curvature of my barrel setup so my windage zero is correct at every range and I am not guessing the zero was off due to mirage, spin drift, Magnus force or Coriolis effect. I set my elevation by conducting testing at 140 yards in a north or south direction so that Coriolus effect doesn’t distort the data and it will be accurate at a different venue using a ballistic program. At 140 yards the effect of coning largely dissipates and the effect of wind and mirage is minimised. This means I am in a better position to learn from environmental changes at different venues.

Points to understand are that:
1) Coriolis is not an atmospheric condition but an earth movement condition when the earth travels from West to East while the bullet is in flight. It really only counts at long range. Your elevation may be in the order of 1 minute if you shoot West at one range then East at another at 1000 yards. Your ballistic program is accurate in this regard.

2) Humidity has a very small part to play in atmospheric density, even if it changes a lot and may only be in the order of .3 of a minute at 700 yards. However, humidity plays a huge role in creating an apparent aiming mark shift as the wind moves from boil to drift, to run, in both elevation and windage. This was quantified by a bloke by the name of Anderson when I was a lad and I see where it has been republished by another crowd putting their name to it.

3) Temperature is far the biggest influence in our game.
(a) Muzzle velocity rises significantly with temperature and can be a problem with slow burning powders and not so much with 2208. The best shooters vary charge based on muzzle velocity because it maintains the selected barrel tune based on load development. They establish charts of temperature variance in 10 degree F variance against charge to maintain the desired velocity. Although some of the best ballistic programs can plug these factors in, there is no substitute for your own data created by testing. As soon as muzzle velocity changes, the vibrations change, so does bore size and bore angle. This is why it is so important to shoot rhythmically so there is no variance in bake temperature. A compensation tune can mitigate against small changes in velocity. Some use a tuner to mimic charge changes by moving nodes forward or back.

(b) Higher temperatures can also suck more water vapour into the air mass which we notice on those hot sultry days to greatly affect the apparent aiming mark.

4) The barometric pressure whether based general air pressures with altitude or varied by weather patterns is the other factor in air density. The ballistic programs can estimate the effect really well.

5) Do realize the air density affects all external ballistic solutions. The speed of sound changes with temperature, so if you are on the edge of the transonic zone at long range where bullet velocity drops below the speed of sound you lose dynamic stability. If the Gyroscopic Stability is too much for a given atmosphere then tractability problems start to occur past the overturning moment and bullets land like a jumbo jet and are more exposed to movement as they slow at long range. Wind allowances, Magnus Force effect and spin drift values all change with an atmospheric density change. So while you may have adjusted for your tune with a powder or tuner change have you adjusted for the condition values? Are you over or under reading? A good ballistic program allows you to explore these concepts and being forewarned is forearmed. They enable you to pick a window in which to work.
David.

The Shooter
Accuracy
Velocity
Bullet Drag
Gravity
Wind
Stability (barrel twist)
Air Pressure
Temperature
Departure angle
Spin Drift
Humidity
Coriolis.

I think these are the things that effect a bullet in a rough order of importance for an F-Class shooter. Can't do much to change gravity and others have only minimal effect but are predictable if you want to take them into account for whatever reason.

Could be a few other things. Anyone think of any?

Norm wrote:Can't do much to change gravity

For those that travel a bit, gravity varies by location by about +/-0.35% over the nominal average of 9.81 m/s^2, but I'm not sure how much this might affect your zero going to from one range to another.

I'd slash Wind with Range Profile.

Uphill/downhill effect links in with gravity and David's note about bore angle and shooting position (mound) relative to the target to some extent? The remainder of which is taken up by recoil and pivot points subject to the shooting position required to point at the target? I'd definitely add recoil somewhere there in the list below wind and above departure angle.

Air density takes into account atmospheric temperature, pressure and humidity. Altitude has some effect on air density, but more so as a relative base for the local air standard pressure. I think altitude and latitude combined though have a greater affect on local gravity, as there are still a number of locations worldwide that buck the standard atmospheric tables one might find in a flight training manual on any given day. Temperature is still my pick as the biggest influence, followed by pressure and humidity out of the three making up air density.

Good topic, very debatable.

Aaron,
Interesting about the gravity. Didn't think it changed much by location.
Temperature and pressure is one thing that mucks everyone about. For shooters at the one location temperature can vary so much throughout the day. Cool in the morning and bloody hot in the afternoon and that's just air temperature. Then what about you ammo sitting in the hot sun getting radiant heat into the powder to change its ignition temperature.
From a field shooters point of view. temperature and altitude have a unique relationship. In my area, I can shoot at one location and at sea level and have 29.8in/hg and 25 degC. Then an hour later I can change location and be up the high country with 26 in/hg and 10 degC.
The air pressure gets lower so flatter shooting, but as you get higher it can be lot colder so the air is more dense. This tends to have partial compensating effect.
Recoil is more a rifle effect thing than a exterior ballistic effect but still quite an important thing to take into account. A high shooting angle with a heavy recoiling rifle can shoot higher on the target if you don't drive it properly.

Another thing that I thought of is how the barrel throws its bullets. During ladder testing, my 7-08ai with 168gn bullets throws then lower on a test target at 200 yards as the velocity increases!

Hi David, I suspect that your third post was an answer to my other question that I posted yesterday and deleted. Sorry about that, I've reposted it worded differently.

I haven't been using a ballistic program in preparation for shooting at different locations. Although I don't travel often to shoot, that sounds like an interesting exercise that will demonstrate the effect and importance of different influences.

It sounds like I definitely need to be considering temperature more than I do in my preperations.

Thanks again.

Denis,
I have been doing a fair bit of Crono work, breaking in and finding a load for my new Bartlein 284 Win barrel that Frank Green kindly gave me. I am using 2113sc with 180 gr VLDs, as it has worked well in my 4 x 284 barrels prior to this new one
Initial work was at a pleasant 30-32 deg C over the past couple of weeks.
Testing loads at 54.8 gr and 56.0 gr this past weekend, which was a stinking hot 38 deg C on both Sat and Sunday, along with high humidity, my velocities were up an average of 30 fps. With the higher 56.0 gr load, it was definitely enough to put me off the node.
Temp changes and their effect on chamber pressure and muzzle velocity are one of the most useful features of the QuickLoad programme.
I shot the Goondiwindo OPM two years ago with a MagnetoSpeed on, testing loads and specifically primers, prior to heading down the the Sydney Queens. In my rifle, PMC primers gave me identical ES to the Fed 210 GM primers that I normally use, but right on 20 fps slower.......so an ideal option for me, for shooting after lunch in hot weather.
A couple of years ago, with my first 284 barrel, and using REL17, an increase in temp of just 6 deg gave me an extra 104 fps m/v. The rest of the tub is still locked in a cupboard !

Denis, I did assume you deleted the topic.

Tony, perhaps you could indicate for the benefit of other shooters what you found with regard to mound angle in the fly shoot. Aaron picked up on the point and yes I labour on it. David.

CONFESSION TIME : when David spoke repeatedly in recent posts about how critical it is to have rifles taken overseas (or any range away from home, by default) "tuned" for the mound angles, on the range where they will be shot, it occurred to me that perhaps that would seem unnecessary. I was thinking that perhaps going to the extent of checking and verifying mound angles was probably not really required. Elevation above sea level made perfect sense, but mound angles didnt. Until yesterday.....when on 6 FLY TARGETS IN A ROW..... I had perfect vertical for 3 sighters on a splash plate some 5-6 feet below the "score target", but on all 6 targets, over some 3 hours shooting, my first scoring shot was HIGH and LEFT, when moving up vertically some 5-6 feet.
So a miserable 0.4% change in angle (lifting up 5-6 feet) totally changed my zero.......not just once, but six targets in a row to hammer the point home that David has been repeating on this forum about mound angles being critically important for long range precision.

Now I get it ! And it has me thinking about the "tunes" involved when you see certain shooters repeatedly do well at a particular away range, but then repeatedly do "less impressively" At other OPMs. For example, Crows Nest-Gombungee has been really good to me. I won it two years ago, and got rolled by just one "x" by Col Sutton last year. The double 500 we finish on is a steepish uphill angle.
Yet every single time I have shot at Ipswich.....the home of the red hot Shooter McReid....I have had my a##e handed to me on a plate. The downhill, 800 m final range just kills my score every bloody time. Tune ? Or just the way the cookies crumbled, on the day ?

Ipswich is a hard 800 Tony because as I recall, your shooting blind, well over the top of the flags.............. but point taken.