Fred Bohl
Retired Engineer
Al,
To refresh your memory, this is from your post of 7-29-07 in that thread about Gene Davis's boosters.
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Jim wrote:
Quote:
To put it in general terms, any lens of "plus power" (normally used for hyperopia or presbyopia correction) does "magnify" to some degree, depending on the power.
Jim, thanks for the professional overview. Being in the medical field myself, this has always intrigued me. A couple of years ago, I worked with a local optometrist on just what you outlined. We had exactly the result you predicted...virtually zero eye relief. Bang...whack...blood...headache...go home...regroup.
Fred wrote:
Quote:
Since you have been a referee at many HBR matches maybe you could give us the actual intent of the rule about 6x only scopes?
I am confused because the scopes I see at HBR matches are the Leupold Competition 6x, Weaver T6 and Burris 6XHBR models. All of these have adjustable parallax (front objective cell) and ocular adjustments (eyepiece focus). In all cases the eyepiece adjustment range is about 3 diopters. These two adjustments will change the actual magnification – note that the Leupold specification page list the actual magnification as 6x minimum.
It is also a confusing choice of power as it is more than adequate at 100 yards (4.3x needed) but insufficient at 200 yards (8.6x needed) to resolve a minimum caliber (.243) bullet hole for a person with normal or corrected to normal (1 MOA) visual resolution/acuity.
Fred, to understand the HBR rules on scope magnification, we have to go back to the original intent of HBR shooting: to have a class where a person could compete in a Benchrest tournament using their normal hunting rifle. Along the way, HBR (in both the IBS and the NBRSA) has morphed into what we see today...nothing less than full blown BR rigs with a few vestiges of the original rules left to keep things on a level playing field. These are the 6 power scope rule, the 2.25" stock forearm width, the magazine cut in the action and the minimum case capacity rule of holding as much powder as a 30-30 (45.0 or 45.5 gr. of water, depending on the rulebook you're reading). There are a few additional differences, but these are the salient points of HBR.
But one rule stands head and shoulders above all else: the 6 Power Scope rule. You could drop the case capacity rule, allow solid bottom actions and increase the stock width to 3"...but the 6 Power scope rule remains the defining characteristic of the HBR rifle.
While you can resolve a .243 bullet hole at 100 yds. with a 6 power scope, it's impossible to see either the 10 ring or subsequent scoring rings. At two different times, I've been able to faintly see the black scoring rings barely fade in and out on a perfect morning with the sun angle just right...but not for very long!
Hope this helps. -Al
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To get to the point, I was helping a shooter mount and boresight his Leupold FX-III 6x42mm Adj. Obj. Competition Hunter and he brought up that thread from last summer. After some discussion we arranged a later meeting to check the actual magnification of his scope. The protocol was to use his digital camera at the equivalent eye position with a macro lens and my collimator as the target. He then took images with out the scope and with the scope in place at various combinations of eyepiece and parallax adjustment. Using pixel count from edge to edge of the collimator grid he reports that actual range of magnification is 6.0x minimum to 7.8x maximum. He also reports that the range reported is due to range of adjustment of the eyepiece.
No intent to start a battle here over the NBRSA HBR 6x rule but since you are a frequent referee at HBR matches, would you care to comment? Also, how is compliance with the 6x rule checked?
To refresh your memory, this is from your post of 7-29-07 in that thread about Gene Davis's boosters.
----------------------------
Jim wrote:
Quote:
To put it in general terms, any lens of "plus power" (normally used for hyperopia or presbyopia correction) does "magnify" to some degree, depending on the power.
Jim, thanks for the professional overview. Being in the medical field myself, this has always intrigued me. A couple of years ago, I worked with a local optometrist on just what you outlined. We had exactly the result you predicted...virtually zero eye relief. Bang...whack...blood...headache...go home...regroup.
Fred wrote:
Quote:
Since you have been a referee at many HBR matches maybe you could give us the actual intent of the rule about 6x only scopes?
I am confused because the scopes I see at HBR matches are the Leupold Competition 6x, Weaver T6 and Burris 6XHBR models. All of these have adjustable parallax (front objective cell) and ocular adjustments (eyepiece focus). In all cases the eyepiece adjustment range is about 3 diopters. These two adjustments will change the actual magnification – note that the Leupold specification page list the actual magnification as 6x minimum.
It is also a confusing choice of power as it is more than adequate at 100 yards (4.3x needed) but insufficient at 200 yards (8.6x needed) to resolve a minimum caliber (.243) bullet hole for a person with normal or corrected to normal (1 MOA) visual resolution/acuity.
Fred, to understand the HBR rules on scope magnification, we have to go back to the original intent of HBR shooting: to have a class where a person could compete in a Benchrest tournament using their normal hunting rifle. Along the way, HBR (in both the IBS and the NBRSA) has morphed into what we see today...nothing less than full blown BR rigs with a few vestiges of the original rules left to keep things on a level playing field. These are the 6 power scope rule, the 2.25" stock forearm width, the magazine cut in the action and the minimum case capacity rule of holding as much powder as a 30-30 (45.0 or 45.5 gr. of water, depending on the rulebook you're reading). There are a few additional differences, but these are the salient points of HBR.
But one rule stands head and shoulders above all else: the 6 Power Scope rule. You could drop the case capacity rule, allow solid bottom actions and increase the stock width to 3"...but the 6 Power scope rule remains the defining characteristic of the HBR rifle.
While you can resolve a .243 bullet hole at 100 yds. with a 6 power scope, it's impossible to see either the 10 ring or subsequent scoring rings. At two different times, I've been able to faintly see the black scoring rings barely fade in and out on a perfect morning with the sun angle just right...but not for very long!
Hope this helps. -Al
----------------------------
To get to the point, I was helping a shooter mount and boresight his Leupold FX-III 6x42mm Adj. Obj. Competition Hunter and he brought up that thread from last summer. After some discussion we arranged a later meeting to check the actual magnification of his scope. The protocol was to use his digital camera at the equivalent eye position with a macro lens and my collimator as the target. He then took images with out the scope and with the scope in place at various combinations of eyepiece and parallax adjustment. Using pixel count from edge to edge of the collimator grid he reports that actual range of magnification is 6.0x minimum to 7.8x maximum. He also reports that the range reported is due to range of adjustment of the eyepiece.
No intent to start a battle here over the NBRSA HBR 6x rule but since you are a frequent referee at HBR matches, would you care to comment? Also, how is compliance with the 6x rule checked?
