Odd trajectory…. Why?

[Distinguished]

Been thinking about an issue I had a number of years ago with a .25 caliber FX not long after the Smooth Twist barrels came out. I mounted a scope I had used previously with good results on the rifle and zeroed it at 20 yards. As I shot further out I noticed the groups were going further and further to the left. Oddly enough it was a linear progression.
Next I remounted the scope where the crosshairs tilted to the angle which followed the groups. By doing this I could leave the rifle upright and as I used the mildots to shoot further out I could hit my target.
Thirdly I tried tilting the rifle so the crosshairs were vertical. By doing this I could also hit the target.
Can someone with more knowledge explain what was going on? Makes you go “Hmmmmmm.”

 

[starlingassn]

the shots were drifting horizontally as you increased the range because your vertical crosshair was not aligned properly to the bore of the gun.

and none of the expensive devices guaranteed to level your scope will work with absolute precision.

do a search for the "mirror method" of aligning a scope to the bore.

then use a plum line to align the crosshair to vertial,
and install a scope mounted bubble level to read.....level lol

step 3,enjoy the fruits lol

""the mirror method i speak of is not to be confused with the mirror trick for finding a scopes optical center""

 

[nervoustrig]

x2 on what starlingassn describes.

Refer to Scott's writeup

Cant errors - Airguns & Guns Forum

Cant errors, Air guns, Airgun Forum

www.gatewaytoairguns.org

and you'll find diagrams very much like your own that describe scope cant and gun cant.

And here for instructions to mount both a scope and level to eliminate both sources of error.

Best way to level a scope

I’m planning on receiving a scope that I had ordered tomorrow and I like to have everything as close to perfection as possible, what is your guys best way to level a scope

www.airgunnation.com

 

[Gerry52]

For the longest time I wondered if it was "spin drift". Once I used the mirror method "spin drift" went away!

 

[Distinguished]

Thanks for the info. This is why I joined! Ha! Until the next ignorant question…

 

[Vana2]

Seen the exact same thing on a friend's rifle.

Turned out that the vertical cross hair was not (vertically) plumb to the bore. We corrected this with a mirror and a plumb-line as mentioned above.

Just wanted to add that you can't rely on a bubble level rested on the receiver, rings or scope turrets to be level enough to ensure that the middle of the reticle (of an optically centered scope) is directly above the center of the bore.

A poorly indexed barrel (or improperly tensioned liner) can also throw the POI to one side.

Cheers!

 

[Distinguished]

Yes, I have two flat sided levels that I have been using, one on the receiver and the other on the scope. It is all making sense now. Thanks!