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Negative sight heights?
Help / Support / How-to's / Discussion related to PointBlank Software and External Ballistics in general
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Pumpkinslinger
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PostPosted: Sat Jul 30, 2016 1:59 pm    Post subject: Negative sight heights? Reply with quote

I have a laser under the bore of one of my guns so I tried to enter a negative sight height to see what the trajectory might be. Didn't work, it automatically corrected it to a positive number. I'm not sure how many of us would use this feature but I thought I'd throw it out there.

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PaulS
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PostPosted: Sat Jul 30, 2016 5:32 pm    Post subject: Re: Negative sight heights? Reply with quote

There is always one guy doing something the programmer never thought would come up...

Nice going Pumpkinslinger! Smile
(if I had written the software it would have crashed, out of spite.

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Pumpkinslinger
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PostPosted: Sat Jul 30, 2016 5:37 pm    Post subject: Re: Negative sight heights? Reply with quote

I try Paul!

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Elvis
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PostPosted: Sat Jul 30, 2016 6:11 pm    Post subject: Re: Negative sight heights? Reply with quote

Pumpkin......... you can use that as a rangefinder of sorts. if you get lazer to intersect with cross hairs at a known distance,,,say 100 yards if beam/dot is below crosshair so is range and vice versa...works a treat on rifles.
as to your issue.......beats me why you would need it????? surely your scope is where you need the info for and dot is mearly to speed up process,aid in poor light???

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PostPosted: Sat Jul 30, 2016 6:13 pm    Post subject: Re: Negative sight heights? Reply with quote

And you are very good at it! Smile

a sight line lower than the barrel.... I think that will pose some unique situations.

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Pumpkinslinger
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PostPosted: Sun Jul 31, 2016 2:26 pm    Post subject: Re: Negative sight heights? Reply with quote

The laser sits almost 3" below the centerline of the bore. The current sight is about 2.5" above the bore.

Elvis, I have a red dot sight. I looked and it's pretty hard to tell which dot is the sight and which is the laser! I'm looking for another sight that doesn't require a battery, but can be illuminated. So far I haven't made up my mind.

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Elvis
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PostPosted: Sun Jul 31, 2016 10:21 pm    Post subject: Re: Negative sight heights? Reply with quote

maybe look for one with green illuminated reticle/dot...then there be no confusion
it works just as well from one side of scope too,eg if on left of cross hair under 100,right of it more than a hundy.as for your red dot...that's just got to be an issue in the making,sooner or later you will use wrong dot and go high or low.
the bit about them that really impressed me was out on edge of spotlight the beam helped illuminate a bunny rather well,making it easier to see in scope.

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PaulS
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PostPosted: Mon Aug 01, 2016 3:44 pm    Post subject: Re: Negative sight heights? Reply with quote

A straight line from under the bore is going to cross the line of fire exactly once. The trajectory of the bullet will be above that line except at the sight-in-range, and at all ranges longer, the trajectory will be increasingly below the line-of-sight. The deviation from the laser line will be exaggerated due to the line going up and the bullet falling faster at each yard.

Although the ballistics will remain constant the "line-of-sight" is at an angle away from the ground from below the bore line instead of more parallel from above the bore. The math may be similar but the results are going to be very weird.

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Pumpkinslinger
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PostPosted: Mon Aug 01, 2016 11:35 pm    Post subject: Re: Negative sight heights? Reply with quote

Paul, yep, that's why I was hoping to see the trajectory "on paper".

I ordered this to stick on the carbine. It has an etched reticle so it will work without a battery. And it has the option of red or green illumination.

www.midwayusa.com/prod...icle-matte

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PaulS
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PostPosted: Tue Aug 02, 2016 6:47 pm    Post subject: Re: Negative sight heights? Reply with quote

Well if you mount that under your barrel you won't be able to use it ! Shocked Cool Laughing

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Pumpkinslinger
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PostPosted: Tue Aug 02, 2016 8:46 pm    Post subject: Re: Negative sight heights? Reply with quote

I'll just hold the gun upside down like Jerry Miculek!

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Pumpkinslinger
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PostPosted: Fri Sep 02, 2016 12:36 am    Post subject: Re: Negative sight heights? Reply with quote

Sooo... No negative sight heights?

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slimjim
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PostPosted: Fri Sep 02, 2016 5:36 am    Post subject: Re: Negative sight heights? Reply with quote

Pkslinger, here is an on-line cal that does negative sight height. I tried a couple before I found it as most only did positive.

www.jbmballistics.com/...aj-5.1.cgi

Wouldn't a green laser be better for your situation as it would be brighter and be more usable during daylight?

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Pumpkinslinger
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PostPosted: Fri Sep 02, 2016 6:26 am    Post subject: Re: Negative sight heights? Reply with quote

Thanks Slim!

Yes, green would be better in daylight but red was a LOT cheaper...

At this point I think I'll zero the prismatic sight at 100 yards and the laser at 25 yards. We'll see if this works in actual practice. I've got a few days off here and may actually get to go to the range!

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Pumpkinslinger
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PostPosted: Fri Sep 02, 2016 6:35 am    Post subject: Re: Negative sight heights? Reply with quote

I stand corrected!!! I just went back and played with PointBlank again. I entered a negative sight height and it worked fine! Maybe I fat fingered something before...

The trajectory looks odd but revealing! I'm now thinking maybe a 50 yard zero for the laser.

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