canting reticle to allow for spin drift

Get or give advice on equipment, reloading and other technical issues.

Moderator: Mod

Message
Author
pigdog
Posts: 75
Joined: Mon Jan 30, 2017 12:07 pm

canting reticle to allow for spin drift

#1 Postby pigdog » Wed Feb 14, 2018 3:52 pm

is anyone putting a slight bit of cant on there scope to allow for spin drift instead of dialing it out?? JBM shows 25cm of spin drift at 1000m for me. I was thinking I could cant slightly to left which would be right on at 100 then dial up around 10mil and you could have the group print slightly left at 100 when dialled up 10mil you could have it account for most of the spin drift??? IS anyone doing this? Only issue I could see is if dialling for wind it would be adding a vertical component as well as horizontal but if you just hold off with reticle it would work wouldn't it??

bruce moulds
Posts: 2370
Joined: Sun Jun 19, 2005 4:07 pm

Re: canting reticle to allow for spin drift

#2 Postby bruce moulds » Wed Feb 14, 2018 5:18 pm

pigdog,
what you suggest is done by long range blackpowder shooters.
the average 45 cal long range rifle did have in the day, and still does now, about 2 moa spindrift at 1000 yds.
comeups at 800/900 and 900/1000 are 20 to 25 moa, and 1 moa on the vernier tang sights is 0.010".
thus 20 moa is 0.2"
canting the vernier tang so that the bubble on a spirit level just touches the right hand line with the rifle verticle seems to cater for this rather well.
it is a bugger of a job setting it up in a permanent way.
other guys just put 2 moa on a windage foresight.
the problem is testing it at long range, as the wind is a far more overriding factor than spindrift.
in modern fclass rifles spindrift is more like 0.6 moa, which is even harder to check the correctness of than 2 moa.
possibly an easier solution is to zero for spindrift at 600 yards or thereabouts, and then forget about it.
keep safe,
bruce.
"SUCH IS LIFE" Edward Kelly 11 nov 1880
http://youtu.be/YRaRCCZjdTM

GSells
Posts: 335
Joined: Sun Aug 13, 2017 11:04 pm

Re: canting reticle to allow for spin drift

#3 Postby GSells » Wed Feb 14, 2018 8:57 pm

Hi Mate , yes u can cant ur scope to compesate , but only do half the job at a 1000 yds at the expense of elevation inaccuracies.
It sounds like you have similar ballistics to a 155 gr 308 . Which mine was shot in For 200m for hunting ( these were early days !). Then dial up .5 of a mil for 300 yds and .1 of a mil left for spin drift . Then .1 of a mil held for about 680 yds . Then I dialed in .2 of a mil for 700 yds out to a 1000 yds .

So especially for first rounds hits on game , better to dial or add in your wind corrections .
On my comp rifle I have a marker that I can move move left as we go further out , which as Bruce has said , is about .5 moa at a 1000 yds .
Hope this this helps.
Regards Graham.

DenisA
Posts: 1429
Joined: Wed Nov 09, 2011 7:00 pm
Location: Sunshine Coast, QLD

Re: canting reticle to allow for spin drift

#4 Postby DenisA » Wed Feb 14, 2018 9:44 pm

I asked the forum about dialling for spin drift a little while ago. The canted scope idea was suggested but fairly quickly shot down.
Your right in that dialling wind will cause elevation but its in your favour with a correctly canted scope. If you dial left for a left wind that should theoretically push the bullet down, you will be holding off to the right side of the crosshair which is the lower side bringing the shot up a little.

I thought it sounded like sorcery in the end and a little too "how ya goin".

Since then, I've been dialling in spin drift with elevation for each range and its been a significant success to my shooting. Wish I started doing it 6 years ago. Its amazing how all of a sudden, when the flags or mirage looks the same but in the opposite direction, the correct hold is mirrored :oops: (except for elevation). I ask many shooters about dialling in spin drift and its surprising how many haven't bothered incorporating it in to there setup. I would honestly say, the majority I've spoken to.

I dont dial wind after I dial in spin drift, I hold off. Unless its significant like 6 + moa.

My rifle is zero'd at 100y. At 25y, it prints to the left of the POA. Heres a pic of the back of my plot sheet clip board that I keep with me. The spin drifts actually require 1 more 1/8 click than what I have written down.

I highly recommend dialling spin drift instead of canting your scope.
You do not have the required permissions to view the files attached to this post.

pigdog
Posts: 75
Joined: Mon Jan 30, 2017 12:07 pm

Re: canting reticle to allow for spin drift

#5 Postby pigdog » Thu Feb 15, 2018 8:34 am

thanks guys I thought this might be a bit too "tricky" and if it worked everyone would be doing it I guess.

My next question is how accurate is the JBM spin drift calculator is that what you are using? Bruce mentioned .6moa which I'm not familiar with moa but that doesn't seem much?

Mt rifle 6.5x47L 1:8 right hand twist shooting 139 scenars at 2750 JBM tells me 25cm or .25mil at 1000m of spin drift. does this seem about right to you guys? I think these formulas they use came from litz?

DenisA
Posts: 1429
Joined: Wed Nov 09, 2011 7:00 pm
Location: Sunshine Coast, QLD

Re: canting reticle to allow for spin drift

#6 Postby DenisA » Thu Feb 15, 2018 8:59 am

I use an App called Ballistic to get me started. Its close enough and then experience fine tunes it.

0.875 moa at 1000y in the case of the above .308 (1 extra click over what's written) equals 8.75" x 2.54cm = 22.54 cm.

So yours sounds about right. Your close enough that you couldn't read that difference in the condition change anyway.

But then at 1000y you have the effect of Coriolis. Whether that's + / - all you need to know is that the spin drift calc. alone wont be exact. But the bullet never lies on the day, so prove it in sighters.

bruce moulds
Posts: 2370
Joined: Sun Jun 19, 2005 4:07 pm

Re: canting reticle to allow for spin drift

#7 Postby bruce moulds » Thu Feb 15, 2018 1:40 pm

pigdog,
do yourself a favour and swing over to moa.
while mils are becoming popular, they are designed for shooting people, and are far less useful for serious precision.
the people who market this stuff know more about selling than shooting.
another case of this is the plethora of fancy reticles being peddled as necessary for fclass, when in fact they are just a load of wank.
keep safe,
bruce.
"SUCH IS LIFE" Edward Kelly 11 nov 1880

http://youtu.be/YRaRCCZjdTM

pigdog
Posts: 75
Joined: Mon Jan 30, 2017 12:07 pm

Re: canting reticle to allow for spin drift

#8 Postby pigdog » Thu Feb 15, 2018 3:43 pm

bruce I don't shoot F class I'm just a field shooter mils is easy for my simple brain to work with everything is dividable and easy to work out in my head ie 10cm is 1mil at 100m and my scope is in 0.1 mil increments ie 1cm per click at 100m or 10cm per click at 1000m etc or 1mil is 100cm at 1000m. I only used metric other than the velocity which I work with fps.

GSells
Posts: 335
Joined: Sun Aug 13, 2017 11:04 pm

Re: canting reticle to allow for spin drift

#9 Postby GSells » Thu Feb 15, 2018 6:57 pm

I shot mils and still do in f class . Only switched for teams . Good to know both ! But Moa is better for f class! But don’t let mils stop you from competing.

Jbm is extremely accurate with accurate info fed into it . Even though I could manage without it going to a Comp , I just like to confirm my solution and try and get close to the middle as possible.

Bigtravoz
Posts: 109
Joined: Sat Apr 22, 2017 7:43 pm

Re: canting reticle to allow for spin drift

#10 Postby Bigtravoz » Sat Feb 17, 2018 9:59 pm

Each to their own, for me moa is easier, 1 inch per 100 is easy to work out. As for trying to cant a reticle there’s too many variables to “guesstimate” how much cant is correct even running a bubble level. I would rather work on getting my bubble in the middle and use mathematics or a calculator to tell me how many minutes I need to dial my windage screw. Spin drift is calculatable using velocity through given ranges based on the bc of your projectile and twist rate of your barrel, and is basically a semi constant. Somewhat along the lines of Coriolis effect. Reading wind is a far more difficult thing,&of far greater importance. Today at the range I came away with a win not because my $1000 omark was better than the $8000-10000 guns but I managed to read the wind that was up and down like a hookers undies, and adjusted as required.

My 2cents worth.

bruce moulds
Posts: 2370
Joined: Sun Jun 19, 2005 4:07 pm

Re: canting reticle to allow for spin drift

#11 Postby bruce moulds » Sun Feb 18, 2018 5:20 pm

given that most of our rings relate to moa, it is a no brainer.
60 minutest to a degree, 360 degrees in a circle.
a fundamental of nature.
many old vernier sights were in minutes and degrees.
it just so happens that with the correct sight radius, i minute, or 1/60th of a degree is 0.010" adjustment = 1 moa.
1/8 moa is a fine enough adjustment to get an elevation setting centred so that vert is best allowed for.
keep safe,
bruce.
"SUCH IS LIFE" Edward Kelly 11 nov 1880

http://youtu.be/YRaRCCZjdTM

sungazer
Posts: 120
Joined: Fri Mar 17, 2017 7:58 pm

Re: canting reticle to allow for spin drift

#12 Postby sungazer » Sun Feb 18, 2018 7:50 pm

I was reading an article today about a particular gun and in it was mentioned that not all bubble levels are created equal. This really surprised me due to the simple way they work. I suppose the tube the bubble is in could have non parallel walls or have a bend in it. How to determine the specs for a bubble level and have a calibrated one. My Chinese cheap ones that I use one on the rail and one on the scope may be misleading me.

williada
Posts: 729
Joined: Wed Jul 22, 2009 12:37 am

Re: canting reticle to allow for spin drift

#13 Postby williada » Sun Feb 18, 2018 8:14 pm

Some of the old timers might remember Brian Titcombe. Quality bloke who when I started gave me a tip when leveling up a zero target for fullbore or TR as we now know it. He said the bubble moves towards the sun, put your hand over it.

With regard to permanent canting of the scope for spin drift with a modern FO setup. DON"T. Too many variables which may lead the novice to mistake cause and effect when analyzing groups. Keep this one simple on the small target we now shoot at. Atmospheric density, light effects and magnification are some variables that a fixed cant setting does not cut the mustard with these days i.e. the drift may vary both in windage and elevation.

6.5x55ai
Posts: 176
Joined: Mon Mar 18, 2013 4:21 pm
Location: Waikato NZ (Ex Tyabb)

Re: canting reticle to allow for spin drift

#14 Postby 6.5x55ai » Mon Feb 19, 2018 5:53 am

sungazer wrote:I was reading an article today about a particular gun and in it was mentioned that not all bubble levels are created equal. This really surprised me due to the simple way they work. I suppose the tube the bubble is in could have non parallel walls or have a bend in it. How to determine the specs for a bubble level and have a calibrated one. My Chinese cheap ones that I use one on the rail and one on the scope may be misleading me.


Reminds me of the time I fitted an expensive Anschutz adjustable level to my smallbore rifle's foresight - I shoot with a substantial cant. And yes, elevation changes did affect windage to a degree as well – I didn’t have the cant adjustable diopter rear.

The first shoot and the level drove me nuts. As I set up the rifle to my cant and the bubble approached the centre it would madly rush to the one side. Bring it slowly back to centre and it would disappear to the opposite side.

Was able to remove the level phial without breaking it and by rolling it around on a flat surface was just able to determine it was made with a slight curve. It had been fitted incorrectly with the curve downwards hence my “racing” bubble. Refitted it with the curve upwards and it performed perfectly ever after.

KHGS
Posts: 633
Joined: Thu Oct 20, 2005 12:46 am
Location: Cowra NSW
Contact:

Re: canting reticle to allow for spin drift

#15 Postby KHGS » Mon Feb 19, 2018 8:22 am

williada wrote:Some of the old timers might remember Brian Titcombe. Quality bloke who when I started gave me a tip when leveling up a zero target for fullbore or TR as we now know it. He said the bubble moves towards the sun, put your hand over it.

With regard to permanent canting of the scope for spin drift with a modern FO setup. DON"T. Too many variables which may lead the novice to mistake cause and effect when analyzing groups. Keep this one simple on the small target we now shoot at. Atmospheric density, light effects and magnification are some variables that a fixed cant setting does not cut the mustard with these days i.e. the drift may vary both in windage and elevation.

I remember Brian Titcombe, I fitted a couple of barrels for him at one time or another. For me spindrift gets "lost" in the wind & only becomes evident in perfectly still conditions (I am not sure when that happens at the longs) I have rarely seen it still at those ranges & in apparent zero conditions bullet drift will often be against the spindrift which means that zero conditions aren't zero at all, so the spindrift is lost in the condition & impossible to calculate, for me anyway!!!
Keith H.


Return to “Equipment & Technical”

Who is online

Users browsing this forum: Google [Bot] and 7 guests