The Firing Line Forums

Go Back   The Firing Line Forums > The Hide > The Art of the Rifle: General

Reply
 
Thread Tools
Old April 22, 2014, 02:31 PM   #1
Unlicensed Dremel
Senior Member
 
Join Date: February 13, 2014
Posts: 432
Riddle me this: Ballistic calculation

OK, so I use ballistic calculators, but I'm not sure how to answer this question.

My friend wants to know this... looking out his back door, there's a tree - let's say that it's right at 200 yards (it's close to that).

He bought a laser boresighter.

Rifle is 16" bbl, 5.56x45 with M193. Assume 3075 fps and .233 BC, 70 deg F, 800 ft elevation. Assume line of sight starts 2.5" above bore line.

He wants to know, withOUT actually shooting, if he put the laser on that tree in the dark or on a cloudy day, and starts adjusting the reticle, where his crosshair should BE, to make a hit at that range. Should they match precisely or what, he asked.

Now, the answer is no, they shouldn't match precisely; that he will want the crosshairs BELOW the laser dot, to try to zero at that range (ignoring windage for the moment), since the laser represents the bore/ bullet path which doesn't act like a laser much past 75 or 100 yards, but by how much?

Evidently he just wants this as a temp measure until he can get to the range for full sighting....

Lasers boresighters aren't really made FOR that - they're fine for what they're intended -matching up exactly at 25 or 50 to get you on paper. But I don't know how to "make" my ballistic calculators calculate this number.

I suspect, just kind of eyeballing things, that the answer is that he will want his crosshairs to "show" on the distant target about 3.0" to 5.5" below the observed laser dot (would be more if the line of sight was less than 2.5", I think) - just not sure if this is even calculable, or how one would do it.

Edited to add: After some more thought, I came up with this: The Max height of the bullet path is +1.63 at about 125 yards, with a 200 yard zero. With a 2.5" differential at the muzzle, this means the bullet has risen a total of 4.13" in 125 yards (62.5% of the way to the target), before it starts dropping. Now *assuming* the bullet has acted EXACTLY like a laser to this point of 125 yards (which is hasn't but it's fairly close so let's pretend), or 62.5% of it's way to the target, if it would *continue* to act like a laser from there on out the last 37.5% of it's way to the target (75 yards), then it (the actual laser or laser-bullet) would rise another 2.608". Now, it's only 1.63" above the line of sight at 125 yards, not 4.13", so add 1.63 to 2.608, you get a 4.238" height over line of sight at 200. I'd say, round it UP to 4.5" in order to account for the gravity "drag" on the bullet from 0 to 125 yards.

Sound about right? Thanks. More of a hypothetical exercise than anything.

Of course I told him to "get thee to the range", but ya know....

Last edited by Unlicensed Dremel; April 22, 2014 at 03:03 PM.
Unlicensed Dremel is offline  
Old April 22, 2014, 02:48 PM   #2
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 17,502
The trouble is that the laser is not perfectly centered or straight. If it were, you could come very close to getting your scope perfect at the close zero. Remember, the bullet is dropping and the laser is not, so the laser will be higher and higher above the bullet as range increase, even under perfect conditions.

In the real world, if you insert the laser, note where it is compared to the reticle, eject it, turn it 90dg and try again you will find that it is not in the same place relative to the reticle.

Once you find the point where the laser shoot straight high (or low), you can mark the body of the laser and make sure you insert it the same every time. Once the gun is sighted in, you can then use the laser to verify, as it should be at the same relative position each time at that range.
__________________
Still happily answering to the call-sign Peetza.
---
You do not HAVE a soul. You ARE a soul. You HAVE a body.
-----
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.
Brian Pfleuger is offline  
Old April 22, 2014, 03:01 PM   #3
Unlicensed Dremel
Senior Member
 
Join Date: February 13, 2014
Posts: 432
Quote:
Once you find the point where the laser shoot straight high (or low), you can mark the body of the laser and make sure you insert it the same every time. Once the gun is sighted in, you can then use the laser to verify, as it should be at the same relative position each time at that range
Great idea; thanks!
Unlicensed Dremel is offline  
Old April 22, 2014, 05:06 PM   #4
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 3,873
Bore sighters do help get things close. But they're not exact. The way each of us holds the rifle to shoot in will differ a little bit. The bore axis won't point exactly at a point above the target equal line of sight above bore axis plus bullet drop at target range when we fire the rifle. But it will point there when the bullet exits.

But adjusting the scope to be at a point below the laser beam spot down range a distance equal to bullet drop plus scope height is darned close for most folks.
__________________
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master
Bart B. is offline  
Old April 22, 2014, 05:11 PM   #5
pete2
Senior Member
 
Join Date: May 15, 2012
Location: Texas
Posts: 724
Laser bore sighter should get it on the paper at 50 yds. If he wants it on the mark, he's gotta shoot it.
pete2 is offline  
Old Yesterday, 04:26 AM   #6
Blindstitch
Senior Member
 
Join Date: April 14, 2013
Posts: 155
Just because the laser says one thing doesn't mean that the bore won't throw a certain projectile differently. They're just to get you on paper.
Blindstitch is offline  
Old Yesterday, 09:06 AM   #7
kraigwy
Senior Member
 
Join Date: June 16, 2008
Location: Wyoming
Posts: 9,215
I'm not sure I understand the question completely so I'll add my experience with laser bore sighters.

The one I have came with a little target ( you can make one) for using the Laser. My laser goes in the barrel.

Regardless of range, 25 feet to 100 yards, you measure the sight height, be it scope or iron sights. Aim the laser at the center of the target then adjust the cross hair or front sight until it is above the red dot the same distance as the height of the bore.

For example, if your sight height is 2.5 inches above the bore, then the crosshairs need to be 2.5 inches above the red dot on the target.

Now when you go to the range you'll only be 2.5 inches off at 100 yards, so then fine tune your sight to get your desired impact.

I found this to be pretty dern accurate.

Here is a (poor) picture to give you an idea. The numbers on the right are the distance above the cross hairs need to be above the dot to match the sight height.

__________________
Kraig Stuart
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071

Last edited by kraigwy; Yesterday at 09:16 AM.
kraigwy is offline  
Old Yesterday, 09:36 AM   #8
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 3,873
Quote:
if your sight height is 2.5 inches above the bore, then the crosshairs need to be 2.5 inches above the red dot on the target.

Now when you go to the range you'll only be 2.5 inches off at 100 yards
What about bullet drop and how much the bore axis moves while the bullet goes down the barrel which varies with recoil and how the rifle's held?

My rifle's bore axes never point to sight height distance above their 100 yard zeroed group centers. They're all a few inches off from it in both elevation and windage. Nor has anyone elses on dozens I've measured.
__________________
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master
Bart B. is offline  
Old Yesterday, 09:56 AM   #9
kraigwy
Senior Member
 
Join Date: June 16, 2008
Location: Wyoming
Posts: 9,215
The theory of using a laser sight as I use it, is the light (laser) is line of sight, the bore (which the laser emits from) is also line of sight, therefore, the sights and laser should not meet, there needs to be a difference matching the offset of the scope, be it scope height or side offset (as in the scopes on M1C/D sniper rifles.

Laser and other bore sighters DO NOT allow you to sight in your gun, all they do is get you close, on paper, so you don't waste ammo getting close.

Bore sighters don't know where the shooter wants to sight in his rifle. But they will get you close so you can sight in the rifle to meet your needs.
__________________
Kraig Stuart
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071
kraigwy is offline  
Old Yesterday, 01:06 PM   #10
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 3,873
I've always called, as do most entities, the bore as the line of fire; the axis the bullet departs on. And the line of sight is between the sight and the target That way, there isn't any confusion as to which line of sight one's referring to.
__________________
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master
Bart B. is offline  
Old Yesterday, 01:33 PM   #11
SSA
Senior Member
 
Join Date: August 1, 2010
Posts: 322
From a horizontal barrel, the bullet would drop 9.4" over 200 yards, so he'd want the reticle aimed 9.4" below the dot.
SSA is offline  
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 03:46 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2013 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent: thefiringline.com
Contact Us
Page generated in 0.08791 seconds with 9 queries