Sight in for zero at 172 yards...

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Sight in for zero at 172 yards...

Postby MR 450BUSHMASTER » Thu Apr 07, 2011 11:15 am

Gentlemen,

Because I've got a few free minute over lunch today I wanted to get some discussion going on sighting in the 450. Specifically related to what I've read from Chuck Hawks on MPBR (Maximum Point Blank Range). Take a look:

http://www.chuckhawks.com/ultra-long-range.htm
http://www.chuckhawks.com/rifle_trajectory_table.htm

Here's the quick reference: "The Maximum Point Blank Range (MPBR)... ...[is] the distance at which the bullet falls 3 inches below the line of sight. Thus between the muzzle and the distance given as the MPBR, the bullet never strays more than 3 inches above or below the line of sight."

So just looking at the factory information for the 250 grain hornady ammo (2200 ft/sec, BC=.210):
http://www.hornady.com/store/450-Bushma ... evolution/

And then going:
http://www.hornady.com/ballistics-resou ... calculator

And playing with the zero in range, if you sight in at 172 yards you'll be 2.9 inches high at 100, and 2.9 inches low at 200.

This would obviously change for handloads, and maybe someone who has done some handloads and has some good stats on velocity and trajectory for them could add what their MPBR is.

To me MPBR is a pretty interesting concept. Thoughts?!

Bill
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Re: Sight in for zero at 172 yards...

Postby Jim in Houston » Thu Apr 07, 2011 11:18 pm

I don't know where the 3" criteria comes from or more exactly, why use a 3" criteria.

Wikipedia (whatever you think of its veracity) defines Point Blank Range as, "...the distance between a firearm and a target of a given size such that the bullet in flight is expected to strike the target without adjusting the elevation of the firearm. The point-blank range will vary with the firearm and its particular ballistic characteristics, as well as the target chosen. A firearm with a flatter trajectory will permit a nearer minimum and further maximum point blank range for a given target size, while a larger target will allow for a longer point blank range for a given firearm.

If you define your target as a 6" circle, then I suppose you could be 3" high at the Minimum Point Blank Range and 3" low at the Maximum Point Blank Range, but the Wikidefinition seems to imply that Point Blank Range depends on the definition of the target. Therefore if you were sighted in at 300 yards and your target were a one inch circle, then the Max PBR would be 300 yards, since that is the maximum distance at which "the bullet in fight is expected to strike the target without adjusting the elevation of the firearm."

I'm not sure what the usefulness of defining Maximum Point Blank Range is, given the variables.

Coloquially, Point Blank Range means the range at which you can't miss the target, but that is probably the Minimum Point Blank Range, not the Maximum.
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Re: Sight in for zero at 172 yards...

Postby FirstNation » Fri Apr 08, 2011 5:20 am

Makes sense that there could be no "standard" maximum point blank range since it's dependent on the size of the animal's "kill zone" that you're hunting, the caliber, load and distance the animal is at.
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Re: Sight in for zero at 172 yards...

Postby dcbeattie » Fri Apr 08, 2011 10:32 am

Sort of interesting.... a six inch circle is sort of a nine inch paper plate with the ridges cut off.... :roll:
Average kill zone of N. American game...
http://www.chuckhawks.com/kill_zone_game_animals.htm
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Re: Sight in for zero at 172 yards...

Postby MR 450BUSHMASTER » Fri Apr 08, 2011 10:38 am

To expand on this, and discuss why the 3" high and 3" low idea...

If you make the same calculations but change the sighted in at 100 yards to be your zeroed in point, then you would be looking at -9 inches from line of sight at 200 yards. The 3" high and 3" low allows you to be pretty confident that in a field/hunting situation you are pretty good for anything up to 200 yards without having to make much of a change in your target point out to 200 yards - which is good if you end up mis judging your range! So only compensating for 3" at 200 yards vs 9" at 200 yards.

I think that is where the author of those articles comes up with the 'point blank range' term. You'd be pretty much OK shooting dead on sight, or at least with relatively little adjustment, for anything out to 200 yards assuming anything within a 6 inch diameter would give you a good kill shot.

Now if you've got time to pull a range finder (or have better ranging skills than most) and a mill dot scope it probably wouldn't matter all that much. But to know you can hold nearly dead on a monster hog from 50 to 200 yards without much thought is nice concept.
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Re: Sight in for zero at 172 yards...

Postby wildcatter » Sat Apr 09, 2011 7:00 pm

You guys have this down pretty well. I use pbr for my hunting rigs, religiously. As I generally shoot things with an 18" kill zone, I use a 10" circle, for that
moment when I screw up the shot, I'll have a margin of error. Then if you have enough gun, the marginal shot, and most of the shots are indeed marginal, that ops turns out to be a kill anyway you cut it. Hence the need for the 450b. I don't have to know the exact range, all I've got to do is be inside the maximum PBR, shoot for center mass and then get the knife out..t
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Re: Sight in for zero at 172 yards...

Postby Escape Pod » Sat Jan 08, 2022 10:45 pm

An oldie but goodie here. I have been, ahem, hashing out whether to go BDC or not. This thread helped show me another trajectory to go on.

Okay I’ll stop ;)
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Re: Sight in for zero at 172 yards...

Postby Smokinbubba » Mon Jan 10, 2022 5:00 pm

All this math makes my brain hurt... I will stay with my nikon BDC and spot on app. It has not let me down yet. Plus, three inches either way is not acceptable in my book. Just sayn...
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Re: Sight in for zero at 172 yards...

Postby plant_one » Tue Jan 11, 2022 12:50 pm

Smokinbubba wrote:All this math makes my brain hurt... I will stay with my nikon BDC and spot on app. It has not let me down yet. Plus, three inches either way is not acceptable in my book. Just sayn...


you can always adjust your zero to a smaller +/- range. you wont end up with as far of a MPBR, but the result is still the same.


i'm with you, i run BDC's and just have data in strelok for that reason. works dandy, but you also gotta have a rangefinder handy, or have good known distance markers as well as a signal to use it.

yes a pre-printed cheater can be done too to put in scope caps
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Re: Sight in for zero at 172 yards...

Postby Smokinbubba » Tue Jan 11, 2022 3:53 pm

Rangefinder never leaves my neck. It is crucial. That reminds me, I really need a newer more reliable one. The elcheapo simmons takes a few tries on game past 200yds.
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