Zeroing a telescopic sight

[Simbu]

Quick question for the learned PHers.

Why, when i was zeroing my dad's air rifle (BSA Airsporter mk4) scope over the weekend was there a difference in zeroing it for left or right-handed use? I'm one of those deviant lefties, so when i zero'd the scope for me and my dad had a go it was a couple of inches inaccurate. So, i zero'd it again firing righty and it was fine for him. When i tried firing from my left whe it was zero'd for righty usage, it was again inaccurate - so it's not a case of us being bad shots Range was around 10-12 metres.

Perhaps theres an obvious reason and if someone could explain i'd be grateful!

Thanks

[gazchap]

Was the scope rail offset at all, i.e. not directly above the barrel?

[Simbu]

Now that you mention it.. it may well be. That'll do it! Thanks

So, can that be a 'design feature' or is it simply that it's from the 70's and as such possibly not well built?

[dfen5]

Sounds like parallax error. If you look through the scope with the gun fixed in a place such as a bean bag, move your head slightly. does the crosshair seem to move off the target? A lot of cheap scopes aren't adjustable parallax and suffer with this. Other than that, if you're shooting "free hand" (as you would normally with a spring air gun) it's normally down to trigger technique. Make sure you're squeezing the trigger with the pad of your finger, not wrapping it around.

[Lord Flasheart]

It was probably something to do with the angle at which you were looking through the sight and the way you were holding the weapon. Its possible for two people, both firing from the same shoulder, to have a completely different zero. To take that to the nth degree, a serious marksman would not accept a rifle as zeroed if he hadnt done it himself. It may well be "close enough for government work" but the weapon still wouldn't be zeroed to him. Its nothing to be concerned about.

[gazchap]

Depends on the rifle I should think. Sometimes it might be built-in, other times it might be personal choice.

A couple of the guys on our bell target team (pub league) have offset sights because they just find it's more comfortable having their head in that position than it would be if the sights weren't offset.

[Mikeyboy]

It's to do with the angle of the barrel to the angle on the sight. Plus like the lenses on glasses each eye is different too. Or am I misunderstanding you?

[dnomyar]

dfen5 said:

Sounds like parallax error. If you look through the scope with the gun fixed in a place such as a bean bag, move your head slightly. does the crosshair seem to move off the target? A lot of cheap scopes aren't adjustable parallax and suffer with this. Other than that, if you're shooting "free hand" (as you would normally with a spring air gun) it's normally down to trigger technique. Make sure you're squeezing the trigger with the pad of your finger, not wrapping it around.

This..........

Had the same problems, years ago with a .22 rim-fire, and a .222.

[Simbu]

Thanks chaps, i'm pretty sure the rail is off-centre on the top of the barrel, so if i'm understanding correctly this will create a parallax effect?

It's only being used for blatting pigeons at short range (little buggers can't get enough of the parents' cherry trees) and now that we know to only use the rifle right-handed with its current zero it's accurate enough.

Thanks all for the replies and interesting insights!

[tenex]

Lord Flasheart said:

It was probably something to do with the angle at which you were looking through the sight and the way you were holding the weapon. Its possible for two people, both firing from the same shoulder, to have a completely different zero. To take that to the nth degree, a serious marksman would not accept a rifle as zeroed if he hadnt done it himself. It may well be "close enough for government work" but the weapon still wouldn't be zeroed to him. Its nothing to be concerned about.

I agree with this. The variation in zero is mainly due to the way you hold the rifle. All "normal" rifles recoil and you are going to extremes with the pressures exerted on the cheek piece and fore-end by switching from right to left handed. As a result the rifle recoils totally differently and the shots go "off".
It is impossible for someone to sight in a rifle, to a high degree of accuracy,for anyone else.
The only way to minimise your problem is to teach yourself to shoot right handed. It feels strange to start with but it's not difficult. I've done it so it has to be easy.

[Colin 1985]

It could be that one of you (either the OP or your father) is not holding the rifle completely upright, in this case by tilting the gun slightly it would be of zero since the pellets 'rise' is transfered to lateral travel, although at only 12 metres it would need to be quite bad to affect the accuracy in inches.

Believe it or not I have a spirit level on mine.

ETA: I see its a spring gun, could also be that the recoil is knocking the mounts out of place slightly but again the effect would be small and progressive.

Edited by Colin 1985 on Wednesday 26th May 07:41

[itsnotarace]

Parallax error as has already been mentioned. The solution is to buy another gun for the 2nd shooter to use

Edited by itsnotarace on Wednesday 26th May 08:11