New Posts New Posts RSS Feed - Bizarre Scope Behavior
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Check GunBroker.com for SWFA's No Reserve and No Minimum bid firearm auctions.

Bizarre Scope Behavior

 Post Reply Post Reply   Topic Search Topic Search  Topic Options Topic Options
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/18/2010 at 17:39
Redvole View Drop Down
Optics GrassHopper
Optics GrassHopper


Joined: October/18/2010
Location: USA
Status: Offline
Points: 2
I am hoping someone can help me understand a bit of crazy optics behavior.  I have a Nikon Buckmaster 6-18 side focus on a CZ 452.  The minimum parallax adjustment is 50 yards.  I am shooting in a small area, so I am using Remington CBee ammo (33gr, 740fps).  Currently, I have zero at 20 yards (I don't normally shoot in such cramped conditions).  The mystery is that if I use the same setup, I hit one inch low at 10 yards.  According to everything I have referenced, I should be about .4 inches low at 10 yards.  How is it possible to be on zero at 20, yet mysteriously too low at 10?  Is this some artifact of shooting too close for this scope?  Any guesses???
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/18/2010 at 17:50
supertool73 View Drop Down
Optics Jedi Master
Optics Jedi Master
Avatar
Superstool

Joined: January/03/2008
Location: Utah
Status: Offline
Points: 9521
It is because of the height of your scope above the bore.  Say your scope it 1.5 inches above your bore.  You zero at 20 yards.  So once you bullet leaves that barrel it is traveling up until it reaches the line of sight of your scope zero.  So anything before that 20 yards will appear low.  Once it get beyond that it will impact high until that bullet once again falls below the level of your scope.  Most likely the ballistics you have been looking at are showing the sight much closer tot he bore on your gun.  Like irons or something.  Good ballistic calculators have sight height as an input field because it does make a dramatic difference on your point of impact.

An extreme example is an AR-15.  The scope are usually 3+ inches above the bore.  You zero at 200 yards.  If you take a head shot on a target at say 15 yards if you want to hit the target int he eye ball, you have to aim high near the middle of the forehead to actually hit the eye.  Pretty much that whole 3 plus inches.  At around 50 yards it will cross your scope line of sight and be pretty much zeroed.  At that point it travels up for a ways more and then begins to fall again and at 200 yards it falls once again below your scopes line of sight and continues to fall until it hits the ground

That is one other reason why it is always good to get your scope as close to the bore as you can. 

Hope that makes sense.


Edited by supertool73 - October/18/2010 at 17:54
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/18/2010 at 18:09
Redvole View Drop Down
Optics GrassHopper
Optics GrassHopper


Joined: October/18/2010
Location: USA
Status: Offline
Points: 2
That's it!  I forgot to change the sight height in my ballistics software for this rifle Whacko.  It was set to 1.5 inches rather than 1.9 - that accounts for the problem 100%.  Thank you!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/18/2010 at 19:35
Harriershot View Drop Down
Optics GrassHopper
Optics GrassHopper
Avatar

Joined: May/05/2008
Location: Boynton Beach
Status: Offline
Points: 47
Supertool, please don't take offense but you are wrong. If a bullet is fired from a truly level bore the bullet starts to drop the moment it leave the barrel.
What is happening is the scope looks downward through the bullets trajectorey. On most bolt action rifles with normal mounting heights, most scopes/ rifle combos intersect at 25 to 30 yards and then again at 100 yards with a 100 yard zero. No bullet ever rises after it leaves a truly level bore.
There is a simple physics comparison, if you were to fire a bullet from a truly level bore and drop an indentical bullet from the same height the exact same time as the bullet leaves the bore. Both bullets will hit the ground at the exact same time.
 
Charlie
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/18/2010 at 19:53
Bitterroot Bulls View Drop Down
Optics Master Extraordinaire
Optics Master Extraordinaire
Avatar

Joined: May/07/2009
Location: Montana
Status: Offline
Points: 3380
Actually Supertool is right.  He did not mention the bore being level, he only mentioned the path of the bullet relative to the optical line of sight.  He makes an unwritten assumption the line of sight is level.

Of course, you are right as well, Charlie, except about Supertool being wrong.

Wink
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/18/2010 at 20:17
supertool73 View Drop Down
Optics Jedi Master
Optics Jedi Master
Avatar
Superstool

Joined: January/03/2008
Location: Utah
Status: Offline
Points: 9521
No worries Charlie, you are right, and that is good to point out.  I was making that assumption, and yes it does fall immediately from the line of the barrel.

You are making the assumption that a bore is ever truly level with the sights.  It is not.  It would be impossible to hit anything beyond the end of the bore if it was ever level.  The bore will always be pointed up relative to the line of sight of the rifle.

That is how we are able to actually hit a target at distance.  From the gravity standpoint yes the bullet starts dropping immediately.  But from a level line of sight the bullet appears to be going up because the bore is actually pointed up to account for the immediate  drop.  Which in turn will make it cross the line of sight twice, once crossing as it goes above and once as it crosses dropping below.  Where that crosses depends entirely upon your initial zero. In his case it is going to cross that first time at 20 yards.  In the example I stated, it would cross first above at about 50 and then again going below at about 200. 

If you pull up some ballistics charts you will see what I mean. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/19/2010 at 10:28
supertool73 View Drop Down
Optics Jedi Master
Optics Jedi Master
Avatar
Superstool

Joined: January/03/2008
Location: Utah
Status: Offline
Points: 9521
Here are the trajectories of a 223 55 grain fmj going 3200 fps with a 200 yard zero.

0          -3.0
25        -1.8 below line of sight
50        -0.8
75         0.0  around here the bullet rises above line of sight
100       0.5
125       0.8
150       0.8
175       0.5
200       0.0  here is crosses line of sight going down
225      -0.8
250      -2.0
275      -3.5
300      -5.3


Edited by supertool73 - October/19/2010 at 10:33
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/19/2010 at 11:10
Rancid Coolaid View Drop Down
Optics Jedi Master
Optics Jedi Master
Avatar

Joined: January/19/2005
Location: United States
Status: Offline
Points: 7697
Actually, both are kinda right.

When the bullet leaves the bore, it immediately feels the force of gravity, pulling toward the ground; however, it is not actually "falling" as the barrel is not parallel to the ground at the point of departure.

The bullet is most definitely climbing, but eventually plateaus and begins falling - all while being pulled downward by gravitation (atually, sucked downward.)

We've had this discussion/debate for years and, Newton was right, frame of reference is very, very important.

Glad the problem got fixed.

And be advised that sometimes you will see things that ballistic calculators cannot predict.  It is usually a function of some seemingly small variable (unaccounted for) not being as small as you thought (for subsonics, atmosphereic temperature and barometric pressure play much bigger parts than you would think.)

Carry on.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/19/2010 at 12:20
Dogger View Drop Down
Optics Jedi Knight
Optics Jedi Knight
Avatar

Joined: January/02/2007
Location: Ontario, Canada
Status: Offline
Points: 5348
In pictures!
 
 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/19/2010 at 13:04
supertool73 View Drop Down
Optics Jedi Master
Optics Jedi Master
Avatar
Superstool

Joined: January/03/2008
Location: Utah
Status: Offline
Points: 9521
Nice dogger  Excellent
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/20/2010 at 02:12
8shots View Drop Down
Optics Jedi Knight
Optics Jedi Knight
Avatar
Lord Of The Flies

Joined: March/14/2007
Location: South Africa
Status: Offline
Points: 5753
 Post Reply Post Reply
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Similar Threads: "Bizarre Scope Behavior"
Subject Author Forum Replies Last Post
CZ 527 SCOPE/ NO SCOPE? JAMINGRIZZ Rifle Scopes 16
A new comparison with bizarre results Rich Coyle Member's Tests and Reviews 10
what makes a scope a shogun scope? axaviere Shotgun / BlackPowder Scopes 5
Bushnell Elite Spotting scope question? bergbennett Spotting Scopes 1
Cricket Bullpup Scope Mark2Cars Rimfire / Airgun 0
Need advice and new scope wolverine3759 Rifle Scopes 16
Info needed on old Wollensak 4x scope Squirrel Pizza Target 0
Old Redfield Scope Help kevrog1987 Rifle Scopes 4
Affordable long range scope options Mouse Rifle Scopes 5
Scope rings for BSA rifles TRX Rings and bases 1


This page was generated in 0.156 seconds.