SCOPE / MOUNT HEIGHT , ZERO DISTANCES AND THE EFFECTIVE PBR

[robs5230]

Bit of info for anyone interested.
I'd like to discuss a pretty boring subject that is actually really important in all calibres (mainly for hunting). This has come about for me with having a renewed interest in my tx's ( the old .177hc and my newly acquired .22hc).
Its the height of the mounts that you use and how this actually changes the effective PBR (point blank range) .
The reason the TX has brought about this is that the barrel is centralised with the cylinder unlike HW break barrel and underlever rifles where the barrel usually sits towards the top of the cylinder. The transfer port is centred with the barrel in all cases or they wont work will they ?
Anyway back to the point.
I've only recently (last 5 years) discovered chairgun but I use it to get an idea how various things affect trajectory in a given combo.
One thing that has a big effect is scope height above the barrel or more specifically what mount height you use.
Now for looks you want the mount height that keeps the scope nearest to the action (it looks better that way) right ?
For useable PBR you need the highest mounts you can get.
Alright PBR what is that ?? PBR is the effective range of nil holdover or holdunder within a given measurement. For me its 1". That means between certain distances say 10 to 45 yards with my .177 hc I aim bang on (point blank) and the maximum the point of impact will deviate from where I aim is only 1/2" high or 1/" low. (basically a clean head shot on a rabbit, as long as you aim under its ears and on a horizontal line with the eye. total lights out)
So I've been wandering , back to scope height.
I've always considered medium mounts (up to 40mm scope) will give a scope height of approx 1.5" on a hw and high mounts will give about 1.75".
The TX is different as the barrel is centralised to medium give about 2.00" and high mounts give about 2.25".
This makes a massive difference to the range at which you can zero and effectively get a PBR.
Ialways used to zero a hw in .22 at 27 yards with medium mounts. Now the tx can be zeroed at 31 yards for the same pellet and velocity at 31 yards using high mounts.
This effectively lengthens the range that you can take bang on shots at.
In reality right out to 35 yards is now bang on aim and only holding over beyond that.
I hope you hunters all use high mounts.
not really expecting anyone to be bothered about any of this but it does make a little difference to the effective range at which I would be certain of killing a bunny with a bang on aim. Its different for those who shoot targets and know the range they shoot at (or indeed anyone who hunts and uses a rangefinder) as all you have to do is know your holdover / holdunder at given ranges. For a hunter who doesn't have a rangefinder and like me shoots walked up rabbits its gold dust
Another last comment I'd like to qualify all the above with is that all of the above only matters if A- you have a chrono, B- you use chairgun, C- You regularly check the performance of your combination with A and change the info in B. I'm aware some folk don't have A and don't use B so can't do C

[Mark112]

Hi,

I understand your comments. The problem I have with high mounts is that it forces my head into an unnatural position in order to get good scope alignment and if I'm not comfortable I am more likely to fluff the shot. I tend to limit my ranges to say 40 yards tops and have learnt aim points for up to that range. If lying in ambush shooting off a bipod I sometimes laser the target to get accurate ranges and may also refer to Strelok on my phone (it has accurate measured data from my various range testing input into it) to give us confirmation of what I think my holdover should be. I'll use various 'landmarks' (grass clumps, molehills etc) as a reference should quarry appear. The other problem I have with high mounts is that it makes close up targets, which may pop out unexpectedly (when your scope is likely to be parallaxed at a further distance and thus not as clear), more awkward to hit as you may have to allow for significant holdover as the pellet has not reached line of sight at that range. I have probably missed more close targets than far ones over the years .

Anyway this is all about what suits the individual shooter and if high mounts suit you well and they improve your hunting then it's a good thing. It also depends on how you quarry presents itself and at what typical ranges. I've actually had a young rabbit sat on my boot as I leant on a gate and scanned a field for it's parents Really. No I couldn't bring myself to kill it....would have ruined a decent pair of boots...and my toes!

Mark

[granville]

map the scope works every time for me .

[woody2906]

I believe FT shooters use high mounts as some of their targets are further away than hunters who usually limit themselves to say 40yds. The benefit of high mounts when shooting longer range targets is that less hold over is needed for targets relatively far away because the scope is looking down the curve towards the end of the pellet's trajectory, well thats the theory.

Ofcourse the drawback is more hold over is needed for closer targets up to the primary zero point. Personally I prefer low mounts to get the scope as close the bore as possible, magazine height allowing. For example rat shooting is usually between 10 and 25 yds so low mounts require less hold over at close range also there is a chance of less cant when holding the rifle but I'm not sure on that one?

[gary martin]

Woody, you are in the right direction but not quite there. In FT as you say we generally have the scope mounted very high in comparison with Hunters or HFT shooters. We don't use hold over though, we re set our zero to the range. The effect of having the scope mounted high is at the longer distance ranges, you require less clicks of the turret between ranges, if you miss range there may be a chance of the pellet just getting into the target due to fewer clicks. You are correct that at the close ranges the opposite is true, greater clicks between ranges, but we tend to have very few very close targets. With the scope mounted very high your PBR is minimal, but as we re zero the scope on the range calculated this is of little consequence.
Gary.

[woody2906]

Gary I realise FTers dial in the range rather than relying on hold over aim points but I was trying to describe the principal involved in explaining why having high mounts favours longer ranges by looking down the path of the pellet after it has reached the top of the arc of its course, I was just thinking aloud really