shimming bases/rings

[stu_bear2002]

G'day all, i would like some verifacation on shimming bases/rings to settle an ongoing arguement between 2 friends of mine, to which i will print out your replies to set the record straight.
i have tried to sort it out for them to no avail even going to the extreme of showing them the difference to each of their theories on one of my rifles.

Joe is under the belief that in order to get to a long range zero that you need to add a shim to the front ring/base to raise the objective lense.

However(and i agree with this one) glen belives that the rear base/ring needs to be raised in order to get full usage of the adjustments for the cross hairs at a longer distance.
please help me on this as im sick of the same arguement everyday at smoko.

[Guest]

Stuart,
To fully utilize the available elevation built into your scope so that you can zero at long range, you need to raise the rear of the scope relative to the front. In other words shim the rear ring ( not a good idea) or mount your scope on a tapered base which is higher at the rear and tapers down toward the front.
The explanation is this. Imagine the rifle clamped in a vice with the cross hair pointing at a target centre, which is also the point of impact.
Now shim the front ring. ( without moving the rifle) The cross hair now points higher on the target than the point of impact. ( target centre) To bring the cross hair back onto the target centre you have to lower the muzzle and thus lower the point of impact to below the target centre.
This is why we use a tapered base otherwise we would not get a zero at long range.
Hope this is clear.
Barry

[Malcolm Hill]

stu_bear 2002. Glen has won the bet as the rear of the scope has to be raised in relation to the front but preferably not by shimming. Shimming the rear ring will induce a misalignment of the ring centres causing stress on the scope tube when they are tightened. Far better to use a tapered rail or base under the rings to prevent this happenning.
Regards Malcolm.

[stu_bear2002]

thanks guys, i knew that if i got it in writing that i would put an end to this "discussion" and i even managed to score a few scotches from glen's winnings once again thanks