Sight changes between 25 and 50 yds

[Bryan996]

Can anyone help me get my head around this? It's probably a simple answer.

When I change from 25 yds indoor to 50 m/yds outside I not only have to adjust by the expected elevation but also windage by a similar amount. So why do the shots drop low and left at the longer range rather than just low? I do shoot with a lot of cant but gravity only acts in one direction!

Thanks

[jhmartin]

You probably have a cant when you hold your rifle.
On some of the high end sights you can adjust them for that.

[BigAl]

If you shoot with lots of cant the sight aperture moves to the left (RH shooter) as you adjust the elevation. This will move the shots to the left as well. I am surprised you need the same movement on the windage as you do the elevation though. Normally 25 to 50 is not that much of an adjustment, only around 8 clicks or so on the older style 10 click sights. Of course if you also have a significant prevailing right - left wind that will add to the change needed to get in the 10. The big elevation change is from 50 to 100. Thats around 40 clicks on the 10 click sights, although probably better to think of it as four full turns of the knob. This is why a lot of Dewar shooters will have two rear sights, one for 25/50 the other for 100. You also really need to adjust the cheek piece for the 50 to 100 change as well. The other option here is an adjustable foresight, they are very useful as then you do not have to change the cheek piece. When I shot Dewar matches I used to use a Kelly adjustable foresight unit, which you rotated to change the elevation from 50 to 100 yards. Here in the UK we usually shoot 50m (on the ISSF target) and 100 yards on a target that is proportional to the ISSF 50m. With the Kelly I usually needed to add an extra 8 clicks of both windage and elevation to allow for the cant that I used.

Alan

Sorry Brian I did not read your location. One of my favorite ranges for shooting smallbore is on the Ash Vale military ranges, they have (had?) a really nice 50m/100 yard range there that I used to shoot on at the RAF and Inter-Services Smallbore Long range Championships.

[KennyB]

I do shoot with a lot of cant but gravity only acts in one direction!

Gravity may only work in one direction but as Alan says - if your are canting the rifle over, then your sights are moving up AND across as you turn the elevation knob.

For a 20 degree cant - fifteen clicks elevation will actually move your POA 14 clicks up and 5 clicks left.

K.

[Bryan996]

Thanks for all the comments, I should have stated I have the 20 click anschutz rear sights with adjustable cant so adjustment is true up/down & left/right. It may be 1 or 2 degrees off but no more.

My thought was maybe the rifling in the barrel may cause the bullet to have a spiral path down the range.

BigAl, I've not had the pleasure of shooting on the military ranges, although it would be very handy if I could as I have to drive past two military ranges plus Bisley to get to my club range! I could walk to the ash range.

Cheers

[KennyB]

I should have stated I have the 20 click anschutz rear sights with adjustable cant so adjustment is true up/down & left/right.

In that case I can't account for an error more than the displacement of the foresight from the boreline (when viewed from above).
I guess if you cant a lot and use raising blocks under the foresight this could be enough to put you in the 9 ring.

Again for a 20 degree cant - if the foresight ring is 30mm above the boreline then you will have a 10mm horizontal displacement between the boreline and sightline at the foresight.
So a triangle is formed by the boreline, sightline and foresight height - with it's apex at the target (again seen from above).

If you then double the distance to the target the horizontal error will be equal to the horizontal displacement of the foresight from the boreline (by the rule of "similar triangles").

I think that's right, I could do drawings if that would help - visualizing it all in 3 dimensions can be tricky first thing in the morning...

Cheers and thanks for making me think about this kind of stuff.
Ken.

[Tim S]

Perhaps the foresight mount isn't exactly at 12 o'clock?

A Devon shooter had great problems zeroing after having his rifle rebarreled a few years ago. After measuring the froesight mount was found to be slightly off centre. The discrepancy was almost invisible to the eye, but enough to cause real problems. I had a similar problem about ten years ago when I fitted a two-step foresight base - my group moved 2in sideways using the lower mount.

The effect of the rotational spin on the bullet is called the magnus effect. It's why bullets move vertically as well as horizontally in sidewinds. Wind aside, I understand it has to be taken into account with long range shooting (including artillery), but I'm not sure it would be responsible fot the effect you see at 50m.

[BigAl]

I've certainly seen the change in elevation with a side wind effect shooting at 100 yards. The real issue with seeing the effect is that by the time the wind is strong enough to see it you are into other problems. I cannot remember the exact year (it was the year the when the Fastnet yacht race had a lot of problems) the wind on the Thursday of the NSRA meeting was so bad that at 100 yards one gust took my target, complete with backing board out of the frame and off down the range. I had to reshoot that detail, but in conditions like that you're happy when the shot has stayed in the 8 ring. On an open range though the wind is buffeting the shooter too. My local club range is open along one side, with an enclosed firing point, and the wind will often rise and fall quite smoothly. Then you do see the shots seem to move from 10 to 4, but the seeing elevation change really needs a significant change in wind for it to be really apparent.

Alan

[Bryan996]

Thanks for all the comments, I think KennyB has hit the nail on the head. The action is a 1913 so although it could be off centre it's less likely. I seem to be alternating between the two distances regularly at the moment and wanted to establish a quick "20 clicks up" type of approach when changeing, instead I have a 20 clicks up and 18ish clicks right, depending on the wind. And more sighters than I would ideally like.

It looks like the two answers (both of which I was trying to avoid) is a) remove the cant or b) buy a second set of sights!

Or c) put up and shut up!

Thanks guys

[RobStubbs]

I would think that switching between 25Y > 50M you should be able to come up with a rule that gets you 95% there, certainly close enough that a few sighters get you into the middle. Changing from 50 to 100 is slightly more difficult but even then you should be able to get pretty close if you write things down. I'm probably as guilty as anyone else in not writing it down but I know for me 25Y > 50M is baout 8 clicks, 50M > 100Y is about 45. I just have to think for a minute which way to change the sights ;) But I don't generally change distances mid season - winter indoors 25, the rest of the year = 50M outdoors.

Rob.

[KennyB]

Just to add some pictures for clarity:



20 degrees of cant is about all that the 7020 sight can compensate for - so if you're canting more that that, the elevation knob will be taking you left too.

10mm error on the target is about what a 3 MPH (R to L) crosswind would move the bullet - probably of the order of 9 to 10 clicks on the 7020. 18 clicks would be cutting the the 9 ring line by my reckoning.

Ken.