Riser Blocks and accuracy

[Bryan996]

I've noticed that a lot of shooters in the top UK competitions use quite high riser blocks in the prone position so last night I added an additional 8mm to my current setup which is a 8mm rear and 4mm front on a starik tube.

The results seemed to be a clearer sight picture, which is obviously a positive, however I was unable to create 5 shots groups as tight as I could without the additional blocks.

So my questions are, 1) are there any down sides from having the sights so far above the barrel (more sensitivity for error from inconsistent cant angle for example) And 2) is the only advantage to additional risers the improved head position?

thanks

[gstarik]

Bryan,
Sight blocks will not effect sensitivity to cant at all!
Yes,it might give you better eye elignment.
Maybe you didn't rase your cheek piece to the correct hight,or maybe your neck muscels are strained now.
Guy.

[Tim S]

Bryan,

To my mind how many raiser blocks you should use comes down to individual physique, stock shape, and position. I ude 18mm rear and 16mm front (short tube and 22mm foresight), but I'm 6'1'' and my Gemini stock is very shallow. Someone shorter using a deeper stock may need none. Perhaps 8mm is right for you and your rifle. Although it wouldn't hurt to try again to check Guy's suggestion.

[Cumbrian]

This strikes me as an interesting subject that deserves more responses, though, with any luck, from better qualified people than I.

I would have thought that the risk from high riser blocks is that any tremor in the hold, however slight, will be magnified because the sights will now be further away from the axis of the barrel. That might account for looser groups.

On the other hand, I have often felt that my own position, with only a 4mm block at the rear to compensate for a 22mm foresight, was rather cramped and uncomfortable for my neck, which is moderately long but not excessively so (I am only 5' 10'').

Do the top international shooters use quite high riser blocks in prone? I know that Debevec uses a whole stack of them, but presumably that is very unusual. He must have an impeccable hold.

[TenMetrePeter]

There is an ISSF 60mm max allowed distance from barrel centreline to foresight centreline on some disciplines which suggests excessive risers may offer an advantage to some shooters, or there's some history to it. A lot of their rules come from curbing imaginative designs!

[Piefou]

Just for fun : Petar GORSA in kneeling position

[Bryan996]

This strikes me as an interesting subject that deserves more responses, though, with any luck, from better qualified people than I.

Exactly what I thought, but you cant get any more qualified than Mr Starik! (thank you Guy for your input)

I would have thought that the risk from high riser blocks is that any tremor in the hold, however slight, will be magnified because the sights will now be further away from the axis of the barrel. That might account for looser groups.

Again I thought the same but in relation to the cant angle errors being magnified by the increased distance between the sights and the centre of the barrel. I wound have thought tremor would be exactly the same though as you just displacing it to another point, unless perhaps that new tremors are being caused by strained neck muscels. I'm at the club again tonight so I'll be testing this theory more.

[Tim S]

There is an ISSF 60mm max allowed distance from barrel centreline to foresight centreline on some disciplines which suggests excessive risers may offer an advantage to some shooters, or there's some history to it. A lot of their rules come from curbing imaginative designs!

Peter, the restriction applies to 10m air rifle, 300m Standard rifle, and formerly to 50m Standard rifle. These all derived some of their rules from Service Rifle competitions, and followed a one-size-fits-all ethos. All heavily restricted most aspects of the size and shape of the rifle.

[Cumbrian]

Correction: I cannot find the photo of Debevec that I think I once saw. It must have been in kneeling or standing, I now conclude. The photos that I can find do not show him with anything except fairly normal sights for all the positions. Apologies.

[Bryan996]

Alternatively Matt Emmons uses a fore end riser to get the same head position (although I know this is an old photo) raising the whole rifle as opposed to just the sights. I also like the idea of raising my right hand up too.

[Cumbrian]

Alternatively Matt Emmons uses a fore end riser to get the same head position (although I know this is an old photo) raising the whole rifle as opposed to just the sights. I also like the idea of raising my right hand up too.

His eye is impressively well aligned, I must say, and he looks comfortable. And, yes, now that I think of it, a slightly more vertical right hand would seem to have the advantage of increased stability and control for the trigger finger.

The fore end riser does, however, rather negate the advantages of a shallow fore end, but perhaps the modern fashion for the latter is overdone? If so, what am I doing with an expensive Gemini stock? It's all getting a bit confusing for the poor old, very average club shooter.

Roger

[Tim S]

Roger,

I wouldn't go jacking up the fore-end immediately. Matt Emmons is almost the only male shooter at that level who has raised the rifle*. Since he switched to a Bleiker rifle a few years ago he no longer used the raiser. His position as a whole isn't completely textbook; he lies very straight and has pretty high shoulders. It works very well for him, it's not a variation you often see.

The very shallow fore-end of the Gemini can reduce the leverage that can be applied, much like raising the buttplate does. However, the block that Emmons added brought his cast-alu 2313 to about the same depth as his old walnut 2013. The 2013 does not have a very deep stock, certainly much better than the old UIT standard rifles.

*It's more common among women to ensure clearance from the ground and from the sling.

[Cumbrian]

Tim,

Thank you for the further very useful information. I'll certainly stick with my Gemini as is, though I might experiment with taller riser blocks. It's been a most interesting topic.

Roger

[KennyB]

I would have thought that the risk from high riser blocks is that any tremor in the hold, however slight, will be magnified because the sights will now be further away from the axis of the barrel. That might account for looser groups.

Again I thought the same but in relation to the cant angle errors being magnified by the increased distance between the sights and the centre of the barrel.

I used to think this and got into an online argument about it a few years back - but when it was explained to me I had to admit my mistake and apologise...

Essentially, you have two straight lines - the sight line which points at the middle of the target, and the bore line which points somewhere above the middle of the target such that the bullet drops into the centre due to gravity. This is around 100mm above the centre at 50m and around 400mm at 100 yards depending on the flight time of the bullet.

As you cant the rifle the sight line will always point at the centre (if you're aiming correctly) but the bore line will move in a circular arc at a radius of 100mm (at 50m) - I.e. If you put the rifle on its left side and aim at the middle, the bore will be pointing 100mm to the left of the bull.

The difference in height between the sight line and bore line doesn't matter - the sight line always points at the middle and the bore line always points 100mm above the middle if you are sighted in correctly.

This also explains why can't is more sensitive at 100 yards - the distance the bore line moves due to an angular error is four times greater than at 50 yards (square law applies) but the target is only double the size. So effectively angular errors are twice as significant - and similarly they ear half as significant at 25 yards.
I can do some diagrams if that would help....

K.

[Martin H]

I used to think this and got into an online argument about it a few years back - but when it was explained to me I had to admit my mistake and apologise...

Essentially, you have two straight lines - the sight line which points at the middle of the target, and the bore line which points somewhere above the middle of the target such that the bullet drops into the centre due to gravity. This is around 100mm above the centre at 50m and around 400mm at 100 yards depending on the flight time of the bullet.

As you cant the rifle the sight line will always point at the centre (if you're aiming correctly) but the bore line will move in a circular arc at a radius of 100mm (at 50m) - I.e. If you put the rifle on its left side and aim at the middle, the bore will be pointing 100mm to the left of the bull.

The difference in height between the sight line and bore line doesn't matter - the sight line always points at the middle and the bore line always points 100mm above the middle if you are sighted in correctly.

This also explains why can't is more sensitive at 100 yards - the distance the bore line moves due to an angular error is four times greater than at 50 yards (square law applies) but the target is only double the size. So effectively angular errors are twice as significant - and similarly they ear half as significant at 25 yards.
I can do some diagrams if that would help....

K.

That is the best explanation on sight raisers and the effects of cant I have ever read, very nice.
Martin

[Cumbrian]

Well, I have tried out 15mm riser blocks (front and rear, leaving the existing 4mm rear riser block underneath to compensate for the 22mm foresight) on my Anschutz 1800.

The position is certainly more comfortable, but the group has shifted somewhat alarmingly to the very perimeter of the black at 7 o'clock. I estimate that it will require a 15mm move in elevation and a 20mm move to the right to put the group back in the ten ring (this is on a multi bull, 25 yard target). That's a lot of clicks on a '20 click' Hammerli rearsight (i.e. aprox. 0.5mm shift per click). Should I simply adjust the knobs and not worry about the unexpected phenomenon?

I was shooting outside but the wind was negligible.

Any explanation or thoughts from others would be very welcome.

Roger

[Tim S]

Peter,

The change in elevation zero is to be expected. 15mm at 25 yards is just over two minutes, so about 15 clicks on a 10-click Haemmerli; that's only the same as changing from 25 yards to 50m for some rifles. The change in windage is less usual; it makes me think one of the dovetails isn't perfectly in line. I get this, but my sights are offset by 4mm; changingfrom a 6in tube to a 12in tube (same make, same mounting collar) produces the same effect.

[Cumbrian]

Tim,

Thank you very much for the explanation. I can imagine that compound error could creep in with any misalignment of the different blocks underneath the rearsight. I will persevere and simply adjust the windage. The Hammerli, unlike the good old Anschutz Match 54 rearsights (still selling for handsome amounts on ebay, I note), does not have a scale for the windage, only for the elevation. You have peer into the Hammerli from the front to see how off-centre or not the windage might be; even then it's pretty difficult to come up with anything useful.

Roger