SPINDRIFT

bruce moulds · Post by **bruce moulds** » Mon Feb 07, 2011 7:03 am

at long ranges, have you ever seen the condition come onto a full boil, had to shoot, and go to a deadwind zero, fire, only to see the shot go into the 5 ring at 3 'clock?
chances are that you set your deadwind zero at 100, and spindrift has altered that for long ranges.
the problem is that at long ranges many other factors come into play, so getting a good deadwind zero there is difficult to say the least. for example, we can all think of a range where a boil means you need a bit of right on, which makes it even more confusing.
one possible solution is to set the deadwind zero at 100, but a bit to the left, so it is correct for 600, making it more of an average, a bit out at 300, and a bit out at 900, thus minimising the problem.
i think jbm might have a spindrift calculator.
bruce moulds.

RDavies · Post by **RDavies** » Mon Feb 07, 2011 7:27 am

Yep, definitley seen it. I usualy set my wind zero at around 600 and factor just under 1 moa at 1000yds.

bruce moulds · Post by **bruce moulds** » Mon Feb 07, 2011 7:37 am

that is about what i would have guesstimated, so it's good to hear some backup.
thanks.
bruce moulds

Woody_rod · Post by **Woody_rod** » Mon Feb 07, 2011 9:23 am

This is why 100 yard setting have no importance to the actual ranges we shoot. I usually also go for a 600 yard zero, or close to it.

If it is not in the middle, why not just adjust it. Such things are not worth worrying about.

Matt P · Post by **Matt P** » Mon Feb 07, 2011 10:40 am

Woody_rod wrote:This is why 100 yard setting have no importance to the actual ranges we shoot. I usually also go for a 600 yard zero, or close to it.

If it is not in the middle, why not just adjust it. Such things are not worth worrying about.

It is worth worrying about, when the wind crosses over centre, to make the right wind call you MUST know where zero is !!!!

Matt P

Woody_rod · Post by **Woody_rod** » Mon Feb 07, 2011 11:16 am

I thought we were talking about 1/4 MOA or something like that? Do you know anyone that can read wind that good?

For me, it is a feeling you get after shooting a fair bit with the same rifle. It seems to consistently shoot one side more than the other, but only a small amount. Is this the same with you guys? E.g., every time you shoot, regardless of the conditions, the group seems to be consistently one side or the other?

ratshot · Post by **ratshot** » Mon Feb 07, 2011 12:57 pm

Spin drift does make a big difference the faster the twist barrel the worse it seems . my FO rifle wind zero is set for 1000 yards because it only shoots 1000 to 1200 yd . as i was sighting in a 300wsm yesterday , and had a mate there who i had only told about drift the night before so it was a good chance to prove a point . so we bore sighted the 300 wsm at 1000 lined windage up to centre as there was only 1/2 point right wind on the flags it was pretty still , so let the first shot go , 3 ring at 3 oclock wacked 2 points wind on centre 6 , got mate to bore sight again barrel pointing at 3 ring left side of target, whats that about, Spin Drift. end of lesson.

bruce moulds · Post by **bruce moulds** » Mon Feb 07, 2011 5:15 pm

rats,
what is a point?
bruce moulds

BATattack · Post by **BATattack** » Mon Feb 07, 2011 5:57 pm

we are lucky in australia.

we live in the southern hemisphere and most of us use right had twist barrels the coriolis effect and spindrift all but cancel each other out.

i from memory the spin drift of a .308 is approx 1MOA at 1000yd if sighted in at 100yds and the coriolis effect is approx .4MOA in the opposite direction leaving the total effect of .6MOA.

in the USA spin drift and coriolis combine to create 1.4MOA of movement from a 100yd zero. makes me wonder why they dont use more left hand twis barrels in the US.

bruce moulds · Post by **bruce moulds** » Mon Feb 07, 2011 6:18 pm

bat,
does that depend on whether you are shooting in a northerly or southerly direction?
surely one way you would have to add the two together, and the other, you would have to subtract one from the other.
bruce moulds.

IanP · Post by **IanP** » Mon Feb 07, 2011 8:06 pm

Why not download my excel spreadsheet thats been up for over 12 months for gyroscopic stability and spin drift calculations.
http://www.safclass.com.au/downloads.html
What was it that John Lennon sang "nothing you can do that aint been done" aint that the truth of it!

IanP

BATattack · Post by **BATattack** » Mon Feb 07, 2011 8:49 pm

bruce if you are south of the equator the horizontal deflection due to coriolis is allways left of the short range zero.

AND the further you travel south the greater the effect becomes. just the same as water going down the drain

BUT the VERTICAL component of coriolis is effected by the shooting direction (north,south,east,west). if you zero your rifle shooting in a north or south direction and then proceed to shoot east your shot will land approx 3" high at 1000yds and 3" low shooting west.

so there ya go burce another excuse to keep up your sleeve!!