Scope cross over

[ARWP]

Evening all, please can you folks help me with the above, apologies if this is a numpty question?.

If I understand this effect correctly, cross over occurs when the axis of the scope is not parallel to and in line with the axis of the barrel bore, resulting in accuracy that may be on zero at a given range but low and off centre at lesser ranges and off centre the other side and either high or low (depending on the distance to the target) at longer ranges (hope i've articulated this correctly)

Have I got this right?. If so, can you give me some guidance on how to align the scope to overcome it?. Trial and error is proving frustrating!!.

Regards

A

[Chewy]

There was a MASSIVE thread on this a few months back. If i remember correctly the outcome of it was that it just doesn't come into play at airgun ranges.

[Gwylan]

As you say.... if the scope and barrel are not parallel when you zero at, say, 35 yards, you may hit left at distances closer, and right at distances greater than zero. (or vice versa.) There shouldn't be any vertical variation, unless the rifle/scope is tipped to one side, so they don't coincide in the vertical axis. This is called "cant." Cross-over is usually caused by cheap mounts, poor barrel alignment, or the scope rails being out. Try setting the scope to optical centre by getting the turrets in mid position, then put the scope in a mount such as a cardboard box with two "V" cuts in it. You need to be able to rotate the scope, and look at a distant object. If the cross-hairs rotate around the distant object, you haven't got the optical centre right. Mount the scope on the rifle, and if you still have cross-over, you need adjustable mounts, made by either B-Square, or BSA.

Gus

[Fluffybuck]

Imagine that your scope is 2mm offset to the right.
You zero at 35yds.
To land on zero, the pellet will have gone left by 2mm in 35yds, or 1mm in 17.5yds.
....and your scope will be landing 1mm left of zero at 52.5yds (extreme range for any 12ftlb/.177 airgun) and landing 2mm left of zero at 70yds (which is well beyond the range of a 12ftlb gun, but not FAC stuff).

Crossover in airguns is a much-exagerated problem that - as Chewy says - is barely worth worrying about.

Most of what is blamed on crossover is due to barrel aligment, parallax, or loss of zero. The much-blamed HW100 two-piece action is not the problem; it's the kinking of the barrel band when people tighten the forend figure-8 clamp, that presses on the barrel and moves the barrel sideways, or poor stock inletting that causes the barrel clamp to sit too high or low and press on the barrel (movement which varies with temperature). Remove the barrel-gripping O ring and problem solved.

[Swiss Tony]

I had this problem on my old HW97. If I zeroed at 25 metres my groups at 15 metres would be 1/2 inch to the left, and at 35 metres a 1/2 inch to the right of zero. This happened with a variety of scopes and mounts.
In the end I shimmed the left side of the rear mount and the fault disappeared.
I assumed the dovetailing for mounting the scope wasn't aligned exactly with the bore.

[Cheshire_cat]

I had this problem on my old HW97. If I zeroed at 25 metres my groups at 15 metres would be 1/2 inch to the left, and at 35 metres a 1/2 inch to the right of zero. This happened with a variety of scopes and mounts.
In the end I shimmed the left side of the rear mount and the fault disappeared.
I assumed the dovetailing for mounting the scope wasn't aligned exactly with the bore.

surely that would mean the scope would have to be more than an inch off the line of the bore!!!????

or am I missing something here?

[magicniner]

Is everyone here assuming a barrel spits out the projectile on an initial line bang on the barrel's longitudinal axis?
Nick

[hareng]

Seen it time and time again Upstix, usually its the mounts not suited to the rifles dovetail and of course there is no standardisation every manufacturer has different sizes even on the same £2k gun.

[Bellycrawler]

There was a MASSIVE thread on this a few months back. If i remember correctly the outcome of it was that it just doesn't come into play at airgun ranges.

And here it is.

http://www.airgunbbs.com/forums/show...hlight=Bkl%27s

[Bellycrawler]

And here it is.

http://www.airgunbbs.com/forums/show...hlight=Bkl%27s

After you have read tht lot look on youtube for BKL.
Two videos explain about wonky mounts.

[Ian Johnstone]

Its always interests me that the 'No Crossover Exists' theorists say that any small misalignment would be too small to have any noticeable effect & then go on to say that an equally small amount of Cant (no pun intended)...would have a noticeable effect!

I shoot scooped rifles (.22) with both eyes open & this can 'amplify' the perceived Crossover but I have not only seen it but proven it on paper targets at varying ranges.

Take a good close look at a standard mount both on & off the dovetail, they rarely clamp level & they nearly always clamp off the centre (even assuming the right 'jaw' is used).

Bellycrawler & I agree on this & both now use BKLs whenever possible.

[hareng]

Exactly Ian, BKL for unknown dovetails usually shallow no messing as long as they clamp same each side.

Side clamp types need to sit flat on the top part on rifle and pull down on it. Only a couple of rifles are made that this can be done with, Ripley for one with AR5 onwards.
Have seen rifle rails measure 13.3mm wide to 11.8mm and anything from 1.65mm deep to 4.2mm. So not only a matter of being 1.5mm off centre but also up to 2.55mm depth bearing in mind its 60 degree and moving off a lot more than 1.5mm width. Two 3 axis DRO millers, does my fruit in no standardisation but self centring types such as BKL will only open up so much!.

Another alternative is have a rail made up to suit the mounts being used. Always used Sportsmatch but rifles rails are cut within 0.01mm to suit the exact mounts.

[Brian.Samson]

Its always interests me that the 'No Crossover Exists' theorists say that any small misalignment would be too small to have any noticeable effect & then go on to say that an equally small amount of Cant (no pun intended)...would have a noticeable effect!

That'll be because Cant and Crossover are two different things.

You make the "The No Crossover Theorists" sound like the flat earth society, but it's actually the crossover theorists that are unable to explain how offset mounts can possibly make the sort of differences that are being reported.

It's only simple geometry after all - draw it out to scale on a piece of paper.

When the "Crossover theorists" are unable to explain their theory through geometry, they always say pretty much the same thing - "Well, all I know is, I changed the mounts and the problem went away."

Correlation does not imply causation however

Here's the simple geometry - if your mounts are offset by 5mm (which is quite a noticeable amount) and you are zeroed at 35 yards - then the maximum amount of crossover at 70 yards is going to be 5mm... So with a very noticeable offset (5mm) the most you're going to be out at 70 yards is a pellet width.

You may be able to account for a larger effective offset if you had a bent barrel. But... by and large, parallax error and cant will have a far greater effect than crossover ever could.

[Brian.Samson]

Exactly Ian, BKL for unknown dovetails usually shallow no messing as long as they clamp same each side.

Side clamp types need to sit flat on the top part on rifle and pull down on it. Only a couple of rifles are made that this can be done with, Ripley for one with AR5 onwards.
Have seen rifle rails measure 13.3mm wide to 11.8mm and anything from 1.65mm deep to 4.2mm. So not only a matter of being 1.5mm off centre but also up to 2.55mm depth bearing in mind its 60 degree and moving off a lot more than 1.5mm width. Two 3 axis DRO millers, does my fruit in no standardisation but self centring types such as BKL will only open up so much!.

Another alternative is have a rail made up to suit the mounts being used. Always used Sportsmatch but rifles rails are cut within 0.01mm to suit the exact mounts.

If I'm understanding that right Jon, the maximum these mounts in the example are going to be out by is 1.5mm + 2.55mm = 4mm??

So if you're zeroed at 35 yards with a 4mm offset on the mounts, how can the error be any more than 4mm at twice the distance (70yards) - at 55yards, it'll be less than 4mm won't it?

[u.k.neil]

I think two things are getting mixed up here. As has been said, pure offset of scope centreline to bore will not give you a lot of problems, but out of parallel to the centreline of the bore will give you massive problems. For example if the scope is out of parallel by 1/4 of a degree then to zero at 35 yards the reticule will have to be adjusted to move the POI by 5.5 inches.

Neil