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Max Point Blank Range

Old 09-11-2006, 02:27 AM
  #1  
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Default Max Point Blank Range

I was reading online about how scope height can affect max point blank range. First of all, can someone explain the term to me? What is max point blank range and how is it determined? Second, how does scope height affect it?

Thanks,
PigDog
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Old 09-11-2006, 03:43 AM
  #2  
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Default RE: Max Point Blank Range

I'd suggest you visit Chuck Hawks guns and shooting online website (www.chuckhawks.com) as he has a wealth of info there. There is a free side that should help alot.
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Old 09-11-2006, 06:30 AM
  #3  
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Default RE: Max Point Blank Range

Point blank range is a term describing the range to which you can hold on a target without compensating for bullet drop. The mounting heigth (within reason)would have little effect. The Sierra loading manuals goes into this subject in depth.
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Old 09-11-2006, 01:57 PM
  #4  
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Default RE: Max Point Blank Range

Yeah, what James said! For a 3" Max PBR, with whatever the particular load you are shooting will never be more than 3" high or 3" low out to XX yards!
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Old 09-11-2006, 04:16 PM
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Default RE: Max Point Blank Range

I will give you an example.

I have an inline muzzle loader that shoots Precision Rifle sabot bullets. They weigh 350 grns and have a BC of .376. My gun pushes them at around 1600 fps muzzle velocity.

I want to hunt at ranges out to 150 yards on deer sized game. A deers vitals are pretty large, so if you can place a bullet in a 8 inch circle you will probably kill it. To be sure we will go with 6 inches, this is what PBR on deer sized game is figured at I believe.

Now I want to know what range to sight my gun in at to achieve this so I don't have to guess on hold over at long shots and hold under at closer ones. What that means is my bullet can't rise more than 3 inches or drop more than 3 inches on the way to the target at the distances I want to hunt at.

For my gun the Max Point Blank Range is 160 yards if I zero my scope at 136 yards. So to make it easier I just sight in for 130 yards. This was with a 1 inch sight height. Which basically means the center of my scope is one inch above the center of my bore.

This puts my trajectory like this.

2.8 inches high at 75 yards and 2.6 inches low at 150 yards. At 175 yards I am 6.3 inches low and at 200 yards I am 11.5 inches low.

If I change the sight height to 2 inches it will effect my trajectory at the above distance by about .5 inches, less in most cases. It actually makes it seem as if your gun shoots flatter at longer distances, but makes it shoot higher up close.

What all this does is lets me put my cross hairs right in the center of a deers vitals out to 150 yards and pull the trigger. I don't have to guess how much to hold over or exactly what the range is, as long as I know it is within range. Will the bullet hit exactly where I aimed at 50 yards and 150 yards? No, but it will still strike the vitals and kill the deer cleanly.

Keep in mind this is assuming your gun can shoot good groups at these distances. If your gun is only capable (or you) of 4 or 5 inch groups at farther distances you best take this into account. If you know your bullet is going to strike the target 3 inches low at 150 yards and it is only capable of 4 inch groups then there is a good possibility it could actually strike 7 or 8 inches low instead.

Did that help, or just confuse you more?

Paul


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Old 09-11-2006, 04:24 PM
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Default RE: Max Point Blank Range

You can figure it yourself if you know the ballistics of your gun/cartridge and distance from center of scope to center of bore. But there are several ballistic programs that will do the figuring for you. Sierra sells a good program.
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Old 09-11-2006, 04:46 PM
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Default RE: Max Point Blank Range

Yeah, I understand what MPBR is and what the term means. I just don't understand how scope height affects the arc of the bullet. I would rather understand the concept instead of just plugging numbers into a formula and seeing the results change.
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Old 09-11-2006, 08:08 PM
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Default RE: Max Point Blank Range

Simply put, If you line your 7mm mag up at 250yds, the bullet will rise above the line of sight 2.5in at 175yds, and drop 2.5 in at 350yds. Your point blank is 5in and your max point blank range would be 350yds. Juat an example.

Mounting your scope at 1.5in and lined up at 150 will make your bullet rise above the line of sight 3/4 in at 100. If you had high monts and your scope was 2in above the bore, it would make the 100yd rise about an in maybe. It dont throw it off much, but it can a little. another example

Pretty much, the higher the scope, the higher the bullet has to come up to meet the scope, and that makes it creep over the line of sight more also. making your pont blank different. Thats the best I can do.
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Old 09-12-2006, 07:54 AM
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Default RE: Max Point Blank Range

ORIGINAL: pig dog 02

I was reading online about how scope height can affect max point blank range. First of all, can someone explain the term to me? What is max point blank range and how is it determined? Second, how does scope height affect it?

Thanks,
PigDog
The term "maximum point blank range" (PBR) is the distance at which a bullet will strikeat the bottom edge of the vital zone of the game you are hunting, when the rifle is zeroed so that it's maximum rise above the line of sight will not be higher than the TOP EDGE of that vital zone, IF you are aiming in the middle of the vital zone.

Let us use a big mule deer, for example. It may have a vital zone of, say, 6". You can zero your rifle so that the bullet will rise no more than 3" above the line of sight (LOS) from the muzzle out to where the bullet crosses the LOS againon the descending arc of its' trajectory. Beyond that point, the trajectory will continue downward in relation to the LOS, and at some point will be -3" from the LOS. That range is you MAX point-blank range, assuming you aim in the center of the 6" vital zone.

Now, let us say your quarry is elk, and you decide that the vital zone for a typical elk is 12", rather than 6". You canrezero your rifle so that the bullet willrisea maximum height of 6" above the LOS and still hit the vital zone with a center hold. It willalso still strike the vital zone after dropping as much a 6" below the LOS, again with a center hold.

The range at which it strikes-6" is now your new point-blank range for elk shooting. Note that the max PBR for shooting elk is a lot longer than the PBR for a big mule deer. If you are going aftersmaller animals or varmints with the same rifle, your PBR's are going to be a lot shorter.

So, as you can see, the PBR of a rifle depends on two things:

A. The flatness of its' trajectory
B. The size of the vital zone of the game you're pursuing

If you have a scope mounted say 3" above the center of the bore of a rifle, the bullet must travel downrange further before itcrosses the line of sight of that scope on the ascending portion of its' trajectory than it would if you had the scope mounted, say 1", above the line of the bore. This higher mounting plane makes itseem as if the trajectory of the bullet is flatter than it would appear to be if the line of sight and line of bore were closer together. The angle of departure of the bullet from the bore in relation to the earth is steeper. The bullet thus will be somewhat farther downrange before it falls enough to cross the LOS the second time as itdescends on the trajectory arc. This makes it look like the gun is shooting flatter, but the trajectory arc is actually unchanged.

Does this make sense??
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Old 09-12-2006, 11:19 AM
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Default RE: Max Point Blank Range

Makes perfect sense, and I agree completely.

Thanks
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