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Old November 13, 2017, 07:57 AM   #1
DMK
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Calc trajectory for a bullet different than zero'd?

I'd like to find a common point blank zero of about 2" out to about 200y, for both Mk262 and M193 out of a 16" barrel.

Is it possible to do this with one zero?

How does one go about calculating something like this?

Here's the bullet data:

Magtech CBC 5.56 77gr Sierra Matchking:
2630fps

BC .362

Federal 5.56 XM193:
3100fps

BC .246

Last edited by DMK; November 13, 2017 at 08:16 AM.
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Old November 13, 2017, 08:46 AM   #2
Jim Watson
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I don't think it is calculable.
Sure, you could figure the trajectories and find out if they intersect twice, but that would not account for barrel vibration that would affect the angle of departure.

I would set the sights for the cheap stuff and then see where the high priced spread went. It might be possible to adjust from there.
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Old November 13, 2017, 09:09 AM   #3
DMK
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Thanks Jim. You may be right.

What I may have to do is zero the 77gr a bit high at 100y, shoot the 55gr and see where it prints, adjust the zero of the first round again until I can get them both close to +/- 2" of POA at 100y. The 77gr should print higher with the 55gr somewhere below that. If I'm lucky, they'll be within 2" of each other, otherwise this won't work.

In theory, 100y should be the high point of a ballistic curve on the way to a 200y zero.

I was just hoping to get a calculated theoretical curve first to starting point and save some time/ammo.
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Old November 13, 2017, 09:48 AM   #4
Jim Watson
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You can. Just plug your figures in at
http://www.jbmballistics.com/cgi-bin...j_simp-5.1.cgi
and tinker with the zero range until the two get close.
But you will have to confirm and maybe correct with live fire.
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Old November 13, 2017, 01:45 PM   #5
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That's a nice online ballistic calculator. I hadn't seen that one before.

Thanks for the link!
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Old November 13, 2017, 08:51 PM   #6
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Calculating ballistics won't make much diff if the POI of the two bullets doesn't coincide. For instance, the AR I currently use for a suppressor host shoots 60 grain soft point bullets about 2" higher @ 100 yards than the 50 grain polymer tipped bullets I use for predators. If I want to use those tougher 60 grainers, I must remember to hold low unless the range is 250-300 yards.
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Old November 13, 2017, 10:18 PM   #7
Don Fischer
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A point blank zero of two inch's doesn't say anything. If what you want is MPBR it is determined on the size of the target! Bullet velocity and B.C. can give you the answer pertaining to the size of the target. At one time I saw a coyote head, all I could see in the grass. Didn't know the range and the rifle was sighted in on a 6" target. Would have been very easy to miss. When I got back home I recomputed the MPBR but dropped to a 3" target. Made a huge difference. I don't know the range of that coyote but it as well within the ability of my 243 MPBR zeroed on a 3" target.

OK
With the velocity you noted and the BC with a 2" target at 200 yds you would be -2.6 inch's. Ath 100 yds you would be +.9" high MPBR would be 170yds and zero would be 150 yds.

Same thing but change the target size to 3".

You would be _1.6" low at 200yds and at 100 yds you would be=1.5".

If you want to check this, I would, simply find a range to 200 yds and shoot against it. Keep in mind that the two inch target mean's your bullet, If fired throught a 2" pipe would never go above or below the number's printed. With the 2" target it means a MPBR of the bullet being used at that velocity, would fall out the bottom of the pipe. To adjust at that point, raise the sight's.The drop of 2.6" at 200 yds is 1.6" below the line of sight.
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Old November 13, 2017, 10:31 PM   #8
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DMK,

Unfortunately the calculations are useless without real zero data. The problem is that the slower, heavier bullet gives the gun more time to recoil and flex the barrel, so you can't really say where the muzzle will be pointing exactly at the moment the bullet exits until you try. Once you have 100 yard group centers with both loads and chronograph readings for both from your chamber and barrel length, you will be able to fiddle with the trajectory program to see what sight settings you'll need to get the two as close as you can to your objective.
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Old November 13, 2017, 11:09 PM   #9
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It is not possible to calculate for a common zero for a known distance because the bullet trajectory will be different for each of your bullets. The angle at which you aim above the horizontal will be different for each bullet being that the velocity and the friction the bullets create are different. Euler's Method in mathematics can guide you in your search.

Your best bet is to calculate trajectories for each of you bullets.
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Old November 14, 2017, 08:11 AM   #10
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Quote:
Once you have 100 yard group centers with both loads and chronograph readings for both from your chamber and barrel length, you will be able to fiddle with the trajectory program to see what sight settings you'll need to get the two as close as you can to your objective.
That's what I'm going to have to do I believe. Once I see where each round actually prints at a known distance (say the 77gr is optimally 1" high of POA and the 55gr is 1" low of POA), I can use that data along with velocity to plot the two curves with the same sight setting.

I'm not going to be able to change the ballistic curves of the rounds themselves, but the intent is to find a zero that's halfway between them. Obviously, the curves will be different and that difference won't hold true at all ranges.

Last edited by DMK; November 14, 2017 at 08:19 AM.
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Old November 14, 2017, 11:39 AM   #11
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Yes. That's what you are stuck with. On rifles with longer and thin whippy barrels, I've actually seen a heavier bullet load print lower at 100 rather than higher, as is more normal. Everything depends on the timing of the muzzle deflection, which will just be whatever it is for your ammo.
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Old November 14, 2017, 01:49 PM   #12
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one of the pieces of ballistics software I use has a point blank range calculator. You put in all the pertinent information and the diameter you have as a target. It calculates the trajectory and gives you the zero range and the range that hits the low point and high point. It is old DOS software but it still works.
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Old November 14, 2017, 02:04 PM   #13
Don Fischer
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That's is the deal. It's where my number's came from. I always shoot against them to confirm them. Pact Chronograph!
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Old November 14, 2017, 03:47 PM   #14
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I just plug mine into the Strelok Pro app.

Have it set up to tell me poi change. For example 257 Roderts. 90 gr Blitzking zeroed. If i want to shoot 115gr Berger VLD will tell me how much up or down to start my new zero.
Trajectory started from there.
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Old November 14, 2017, 07:28 PM   #15
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This JBM on-line calculator has an argument for vital zone radius at the bottom that serves to define the PBR circle.
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Old November 14, 2017, 07:54 PM   #16
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Calculations won't help if the POI of one or the other bullet is higher/lower AND left/right some random distance from the other.
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Old November 15, 2017, 10:47 AM   #17
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Quote:
Calculations won't help if the POI of one or the other bullet is higher/lower AND left/right some random distance from the other.
That's a good point. I was initially just thinking about different ballistic curves based on velocity and bullet weight differences between the two rounds.

However, I forgot to consider that many (most?) times shooting different bullets will move the grouping laterally. Even shooting rounds will similar velocities and bullet weights.
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