Another question for TR shooters

[Dougan]

The true minute of 1.047 inches at 100 yards is based on a sight base of 36 inches

Thanks for your input S Guy :cheers:

[Laurie]

The OP also mentions MOA in relation to scope adjusters. Nearly all modern US supplied target scopes use fractions of an MOA, quarter or eighth the norm. (Specialist tactical scopes aside calibrated in MILs.)

While the sight radius affects the accuracy of iron sight 'clicks', most scope users make the mistake of assuming that their scope adjusters are 100% accurate which they only rarely are. This often shows up in the amount of elevation rise needed for long range, say around the 30-MOA mark for .308 Win going from 100 to 1,000 yards with people saying their MVs must be considerably lower or higher than they believed as they get a significant discrepancy between what the ballistics program predicts and the actual. (Use of G1 v G7 BCs is another contributor here.)

Another significant factor that not too many people take into account is rifle / sights cant. Sight adjustments aren't accurate in a scope for example if the beast is mounted with the reticle lines out of true vertical / horizontal. Having seemingly spent a large part of my recent life changing scopes or fitting new ones, I've discovered just how inaccurate relying on what 'looks right' is as the target frame / backstop view is very often out of the true and one's brain automatically tends to 'correct it'. I now mount the scope having first got the rifle / scope mount position right using an anti-cant device (bubble level) on the scope rail and a plumb line on the target frame turning the scope body until the vertical reticles match the plumb line string. It's instructive how often that true vertical looks horribly 'wrong' in a short-range sight picture. It's also very important in disciplines like F/TR and F-Class to have a level on the rifle or front-rest and check the bubble remains reasonably central throughout a comp. F/TR rifles are very prone to have recoil / torque gradually induce cant over a series of shots as one bi-pod foot sinks more than the other.

If the scope is mounted out of true alignment, it makes both elevation and windage adjustments inaccurate on top of any inbuilt inaccuracies in the scope adjusters. The only way of identifying these is an 'around the box' test using a very accurate rifle / ammo set-up (to minimise any error caused by group dispersion) starting with an aiming mark at the bottom right of a large clean sheet of paper on the target frame. Shoot at the aiming mark (one shot), screw 25-MOA left on / shoot / 25-MOA up / shoot / 25-MOA down / shoot and back to the original aim still using the original aiming mark. The result should be a true square comprising five holes, three single and a twin marking the corners with equal length vertical and horizontal sides when the shot-holes are joined up. Each side of the square should be the MOA adjustment times 1.047" if done at 100 yards or 25 x 1.047" = 26.2 inches in this example, with the final shot touching or nearly touching number one.

Not so many years ago, many top price scopes were up to 10% 'wrong'. Today, errors are virtually 0 to maybe 5%, some makes better than others. A 5% inbuilt error is neither here nor there at short ranges, but has an effect at 1,000 yards.

[Dougan]

Thanks Laurie - some more good info there

[Gaz]

Would a sight base extender a la smallbore TR work on a fullbore rifle?

[Mattnall]

I know a couple of shooters who have a tube extension on their full-bore rifles to increase the sight radius. One is using 6XC, but I'm not sure of the other cartridge.

Similar to this:

[GazMorris]

Would a sight base extender a la smallbore TR work on a fullbore rifle?

Yup. Never really caught on in the UK, but there are a few shooters in the US and Canada who use them.

Cheers,

Gaz