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SFP- POI Question

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/20/2011 at 12:51
coldhunt View Drop Down
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I read first, and contacted the manufacturer, and could not find an answer. I have a Burris 2-12 E1 SFP, with ballistcs reticle. I sighted it in 200 yds at 12 power, but did not try the centre poi at lower powers. Should my centre cross hair poi stay the same or move up or down, at lower power? I want to begin to make a drop chart at 7 or 8 power which gives me a clearer picture in low light. If anyone has already done a similar drop chart, any info would be appreciated. I know I will need to go to the range to tweak it. I am just trying to get a starting point.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/20/2011 at 14:22
Urimaginaryfrnd View Drop Down
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With a second focal plane scope the center X remains the same always zeroed at the same distance irregardles of what power the scope is set on.  With  a 2-12 scope if you zero at 200 yds at 12x then dial down to 2X or 5x or 7x whatever power your bullet should hit in exactly the same place. 
The marks below center will represent different distances at every different power with a second focal plane scope.  This takes some work but it can be done.  Take a large piece of paper the longer the better with the cardboard from a refrigerator box being ideal.   Your rifle is zeroed at 200 go to 300 shoot at the target using the center of the cross hair make a three shot group on 12x now repeat this at 400 and 500 and 600.  You will have a paper with bullet holes below center grouped in clusters that show the true drop of the bullet because you aimed using the center of the crosshair every time and let the bullet strike where it falls at each known distance.  Take a can of Krylon spray paint go up to the target and mark a bold line where each of those three shot groups is on the paper.   Now what you have is the true drop of that particular bullet at those distances and you can return to those distances and look through your scope while decreasing the power to see how the marks in the scope correspond to what is on the paper at the known distance.  Make notes and keep your paper.  It will probably simplify things if you figure that you will only use 2x for stalking so dont need a long range chart  you use 6x in low light so you need to chart that and the 12 x max power of the scopea with a balistic reticle should be close to what your marks on the paper are but note any differences.  Now go back and test the theory. Put the scope on 6x go to 400 yds choose the appropriate mark in the scope and shoot it.  It can be done but it's a pain in the butt compared to having tactical knobs and dialing in correction.
http://www.kentonindustries.com/pics/ttctypes/   you may be able to get knobs from Kenton Ind engraved to dial in correction that system is same same at all powers .
 

Edited by Urimaginaryfrnd - November/20/2011 at 14:26
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/20/2011 at 16:21
coldhunt View Drop Down
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Thanks for the info. I started getting curious when I read somewhere that the centre POI could change with some SFP scopes at different powers, and of course started wondering about mine. Burris would not say one way or another, which made me wonder more. Again Urimaginaryfrnd I appreciate the help. It looks like I am going to need a bit of ammo to set up a second drop chart.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/21/2011 at 07:45
Dale Clifford View Drop Down
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For a sfp scope to have the same point of impact thru the power range would mean that the erector would have to move indentically each time the ring was reset. This is impossible and is a random variable when using a sfp.The better the scope quality the lower the frequency of the random variable. The flip side of this same question is "can I use dial in for elevation changes in an sfp at the variious powers without any poi changes?' In most cases this isn't a problem for two reasons, the shooter doesn't or hasn't shot enough to see these changes, (hunters etc.)  and the degree of prescion needed by that shooter isn't high enough to observe the difference. Another problem is the scope does not have "clicks"or markers to tell the users what power he is on for reapeatability purposes. When using hold overs at reduced power remember that a decrease in power raises the poi, and the wider the power range of the scope the more pronounced this is observed. The easiest way to chart your scope is using a target grid(scope sight in available at any gun store) marked off in 1" increments and at 100 yds looking through the scope at various powers note how many minutes of angle each hash mark has at the power you are using. Then using a ballistic program run a drop chart using the data for your load. Comparing the two will give the moa drop values at the hash mark interval.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/21/2011 at 07:47
Dale Clifford View Drop Down
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No ammo is needed for this chart, except maybe 3 rounds getting the muzzle velocity off the chrono,
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/27/2011 at 09:49
coldhunt View Drop Down
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Just got back from a trip for whitetail, and cow moose. The winter kill was pretty hard on the whities here so I didn't take one, I could have but I didn't see the guy I was after. The mule deer definatly did much better. We saw 5 bull moose, but had a cow draw, go figure. I was sitting one afternoon on a ridge that gave me an active treeline at between 460 to 550 yds. As the sun went down I played a bit at different power ranges, and will definatly be working on the second drop chart at about 7 or 8 power. Thanks again for the info.
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