An Odd Characteristic of the SMLE

[Patrick Chadwick]

What about about spin drift.

That is the effect I already mentioned. The attitude of the bullet is the slight upwards tilt of the nose compared with the trajectory at that instant. As gravity starts to win and the bullet drops, so the attitude diverts more and more from the trajectory. The ability of the projectile to alter its attitude to follow the trajectory is termed tractability. But the bullet will always have some attitude angle, because it is always dropping under the influence of gravity. This means that the lower surface has a higher air pressure on it than the top surface. Putting it totally non-scientifically, the bullets "bites " into the air more on the lower surface, and thus tends to crawl in the direction of the spin.

A cricket ball has a very rough surface, and the seam makes a ring of extreme roughness that is used by a spin bowler to achieve a similar effect (generating different air pressure on different sides) and generate a curved ball. That's why they do all that curious polishing of one side on their trousers. Must be similar for a baseball pitcher. And if anyone ignorant of cricket or baseball finds that all a bit far-fetched, then watching over-the shoulder TV pictures of Shane Warne in action some years ago was a real eye-opener for me and anyone else with an interest in ballistics who was fortunate enough to see it. The bamboozling effect on the batsman was (wicket-)shattering.

If you can read German, then I recommend the volumes on ballistics by Beat P.Kneubuehl. There must be something similar in English. Not light reading, but it gets your ballistics knowhow off the anecdotal level pretty steeply.

Sorry if that was a bit involved, but you did ask!

[Patrick Chadwick]

And watching the fall and hits through binos, they all seem to go where the gun is aimed

But how are the sights adjusted? You may remember I once posted a photo of a machine gun sight that was identified as being from an M2, and that sight leaf had a very noticeable slant.

Here it is:

Attachment 36669-Attachment 36670-Attachment 36671

What does a Vickers backsight look like?

[Patrick Chadwick]

The rear locking of the Lee-Enfield action is well known to "compensate" at longer ranges but up and down NOT laterally.

I would be grateful for an explanation, as I still maintain that bullets have no memory. Without wishing to cause the slightest offense, "well-known" is not a scientific argument.

[Frederick303]

Not sure if this is relevant but I was discussing the use of optics on Machine guns with a German NCO from the 1980s Bundeswehr. He indicated the optics on the MG1/MG3 on the tripod was located to the left of the gun and that the gun was zeroed at close range with the point of impact being somewhat to the (right/left?) of the point of aim. The spin of the bullet would result in the actual group being on target at some long distance, but what that range was I do not recall. I seem to recall the maximum distance the MG was optically sighted for was 1600 meters or something like that. In any case if I understood him correctly the sighting of the mount/optics took into account some kind of lateral drift of the bullet at long range.

I also seem to recall that the left hand twist of the Lee Enfield and firing of the rifle in the southern hemisphere during the Boer War was the reason for the zero issue that is contained in both Reynold’s book and Skennerton’s book on the LE. I think the problem there was the rifles had some left drift at long range in the southern hemisphere which meant the Brits were putting their bullets off of the aimed target. This difference was related to the Corolis force if I recall correctly, which is in opposite directions in the northern and southern hemisphere.

http://en.wikipedia.org/wiki/Coriolis_force

(Jump to section on inertial circles halfway down page and read on from there)

Now perhaps the Aussie sniper who was firing his rifle in the northern hemisphere was referring to some slight lateral adjustment that had to be made for the difference in shooting a rifle zeroed in Australia (southern hemisphere) and a rifle shot in Korea ( northern hemisphere) at long range.

[Patrick Chadwick]

This difference was related to the Corolis force if I recall correctly, which is in opposite directions in the northern and southern hemisphere.

Careful, careful! This can rapidly slide back down to pub anecdotal level again, just as our brain cells were getting nicely warmed up!

If IF IF there is anything at all in the Coriolis idea, which seems to have been around as long as people have watched water going down the plughole, then any effect would depend on the direction in which you are shooting. And I have heard quite a lot of B-S over the years, but never that someone's rifle had a POI that altered if he changed the direction in which he was shooting.

Maybe I have just led a sheltered existence, or maybe all ranges have the same orientation. But combat is certainly not neatly arranged to be in the same compass direction as range practice, so even if the effect did exist, any built-in compensation would be 5/8 of f.a. use in practice, to use a British engineering term.

[Simon]

Er........... No Simon, the amount of offset (within the bounds of reason of course) between each manufacturer is pretty well academic to the sniper, as is the distance between trenches and anything else. The only thing he's concerned with is distance. There is no earthly logic in the zeroing at 300 yards theory........ it is doomed to failure at any other distance, near or far. Better to zero in exactly as taught, by the book as detailed by DRP in thread 27 and be sure that you'd hit the man at any (?) distance, albeit 1.5" to the right.

After reading 'the book.....' in thread 27, I cannot imagine for the life of me, some budding sniper telling the sniping school staff that he's had a brainwave and he's got a theory that it'd be better to zero his rifle at... and.......... Nope, it wouldn't work. Like I said. Better to hit 1.5" off or slap bang on if you aim off a tad than miss totally for want of remembering some mathematical formula. To be honest, some of them couldn't even remember the range scale clicks up the scale, knowing that it would be one of the end of course exam questions!

Peter,

Am I missing something here? How is zeroing at 300 doomed to failure at any other distance?

If the scope is off set to the left of bore by 1 1/2" then a point blank shot would strike the target at 1 1/2" to the right of the point of aim. It would then coincide at 300 before moving on to strike the target 1 1/2" to the left of point of aim at 600. The scopes of the day were only calibrated up to 600 anyway and click stops a luxury that wouldn't be seen on Brit scope until the advent of the No32. One things for certain once set you certainly wouldn't want to beggar about with the deflection setting on a SMLE Sniper with the possible exception of the Whitehead / A5 combo.

Until I can remember where I've seen it recorded I'll bow to Roger's black and white info in the PAM but I still think the 300 zero is feasible.

BTW??? The distance between trenches and anything else is academic. The only thing he's concerned with is distance.

My brain's hurting too now. time for nightcap I think

Cheers,

Simon.

[Patrick Chadwick]

It would then coincide at 300 before moving on to strike the target 1 1/2" to the right of point of aim at 600.

...And, since the left-hand rifling will produce a leftwards spin drift - why, it will indeed tend to bring the bullet back on course. Whether or not the spin drift effect is sufficient to curve the trajectory so much that it actually crosses over the direct line of sight again is a matter of conjecture - or rather, experiment. But since the spin drift depends on the ballistic characteristics of the bullet, the effect would also be dependent on which type of bullet is being used.

So the interesting effect cited at the start of this thread could really arise - with a rifle having left-hand rifling AND a scope offset to the left, AND zeroed truly (POI = POA, no offset aiming) at some intermediate range, AND using a specific bullet.

Does this all prove that left-hand rifling is superior to right-hand for offset scopes used over a wide range of distances? I suspect so, but don't want to stop anyone else chiming in on this most interesting thread. All soberly thought-through opinions are welcome. And a few measured facts would be nice, too!

[Rumpelhardt]

I once found a free ballistics calculator that took into account both spindrift and the Coriolis effect. I can't find it now and I don't remember much about it other than it made my head hurt.

[Patrick Chadwick]

I think the problem there was the rifles had some left drift at long range in the southern hemisphere which meant the Brits were putting their bullets off of the aimed target. This difference was related to the Corolis force if I recall correctly, which is in opposite directions in the northern and southern hemisphere.

That sounds to me like a very fancy excuse for poor shooting, after the Boers had demonstrated that their 7x57 Mausers were ballistically superior.

[Steve H. in N.Y.]

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