Question For The Avid Riflemen Here?

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the spank

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I understand the concept of trajectory in how your line of sight is above the bore and the sights are adjusted so as to "lob" the bullet to the point of aim in where it crosses the line of sight. My question er problem is compounded in that the rifle I inherited from my Grandfather a decade or so ago is top eject and has a side mounted scope on it. So not only do I have a sight above the bore I have one to the left of the bore. As I understand it once I have set my sights at a given distance my bullet has to cross the line of sight at the distance I have sighted in for? Now I am told that if lets say the bullet is coming from the bore to the sight on the left it crosses over at my zero point and then continues on in the direction of travel it crossed my line of sight from. So if it comes from the right to the left it continues left after the zero point has been crossed. Is there a chart of any kind that can help me determine where on the flight path the bullet is in relation to my sights or bore before and after the zero point? Measuring to the centre of the scope is easy enough to determine for the side trajectory.
For those wondering the rifle is a Model 71 Winchester .348 and I am shooting factory 200 grain silvertips. I have the rifle sighted in 3" high at 100 for a dead zero at 200. I know you are thinking well how far can he be shooting with that to worry about the crossover but I took a Whitetail with the rifle in 2009 at 338 yards. Heightwise I had my holdover figured pretty well and hit the deer dead centre height but the side crossover (windage?) was something I had not considered and I hit the deer about 10" to the left of where I was holding and plugged him a bit far back. He went down after about 25 yards running but after I got to him I was surprised to see how far back I had placed the shot. I heard but could not see the hit through the fixed 4X scope that was on the rifle. I replaced the scope about a month ago with a 3-9 Redfield and have yet to sight it in, heck I haven't even shot the rifle since the day I took the deer with it but I picked up a bunch of ammo and joined a club here with 100, 200 and 300 metre ranges and am anxious to get out, get sighted in and burn a bit of ammo.
 
if your scope is offset say, 1" to the left of the bore, and you zero it at 100m, then by aiming at 200m it will be shooting 1" to the left. At 300m it'll be 2" to the left. At 50m it'll be about 0.5" to the right.
 
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From the swiss rifles forum.
The Swiss Products clamp-on K31 mount places the scope offset about 3/4" to the right of the bore. If we zero so point of aim and group center coincide at 100 yards, line of flight (bullet path) at 0 yards will be 3/4" left of line of aim and will be 3/4" right at 200, since the line of aim crosses the bullet path at 100 yards. The more distant the target beyond the intersection, the more the two lines diverge.

Beyond the zero range, the deviation toward the offset direction equals the target distance minus the zero range; divided by the zero range; times the offset - e.g. for zero at 300 meters, target at 900 meters, offset 19 mm (3/4"), it's (900-300)/300 X 19mm = 38mm at 900 meters - or ~1.5" at 1000 yards. Zero at only 100 meters and it gets more significant (900-100)/100 X 19mm = 152mm at 900m - or ~6" at 1000 yards. If the target is closer than the zero range, the same formula yields a negative number telling how far toward the barrel-side group center is expected. Example - for 100 yard zero, 25 yard target, 3/4" offset, it's (25-100)/100 X .75" = -0.5625". (Notice that the unit used for target and zero distance is immaterial - meters, yards, furlongs, cubits, etc. are all OK unless we mix them. Calculated deviation will be in the same unit as used for the offset.)

Confused yet? Happily, there's an easy way. If we zero a K31 with SP clamp-on mount at any reasonable distance and put the group center 3/4" left of the point of aim, it will be 3/4" left at all distances - because the line of aim and bullet path will be parallel in the horizontal dimension. (If using a mount with a different offset distance/direction, adjust accordingly.) Unless you're hunting horseflies, this should prove a more practical solution to the offset "problem" than trying to remember or calculate divergence at various distances - figuring wind drift and bullet drop is plenty of mental gymnastics already.
 
mgimp said:
From the swiss rifles forum.
The Swiss Products clamp-on K31 mount places the scope offset about 3/4" to the right of the bore. If we zero so point of aim and group center coincide at 100 yards, line of flight (bullet path) at 0 yards will be 3/4" left of line of aim and will be 3/4" right at 200, since the line of aim crosses the bullet path at 100 yards. The more distant the target beyond the intersection, the more the two lines diverge.

Beyond the zero range, the deviation toward the offset direction equals the target distance minus the zero range; divided by the zero range; times the offset - e.g. for zero at 300 meters, target at 900 meters, offset 19 mm (3/4"), it's (900-300)/300 X 19mm = 38mm at 900 meters - or ~1.5" at 1000 yards. Zero at only 100 meters and it gets more significant (900-100)/100 X 19mm = 152mm at 900m - or ~6" at 1000 yards. If the target is closer than the zero range, the same formula yields a negative number telling how far toward the barrel-side group center is expected. Example - for 100 yard zero, 25 yard target, 3/4" offset, it's (25-100)/100 X .75" = -0.5625". (Notice that the unit used for target and zero distance is immaterial - meters, yards, furlongs, cubits, etc. are all OK unless we mix them. Calculated deviation will be in the same unit as used for the offset.)

Confused yet? Happily, there's an easy way. If we zero a K31 with SP clamp-on mount at any reasonable distance and put the group center 3/4" left of the point of aim, it will be 3/4" left at all distances - because the line of aim and bullet path will be parallel in the horizontal dimension. (If using a mount with a different offset distance/direction, adjust accordingly.) Unless you're hunting horseflies, this should prove a more practical solution to the offset "problem" than trying to remember or calculate divergence at various distances - figuring wind drift and bullet drop is plenty of mental gymnastics already.
Click to expand...

So if I read this correctly the further I use as my zero point the less deviation I have at longer distance? So if say my offset is 1" left and I use 200 yards as my zero point I should be within 1" of right of centre under 200 yards and 1" left of centre at 300 yards? So I will have a 2" right/left variance from 0-300 yards? Is that correct?

In your last paragraph are you saying that if I have a 1" offset and I place my group centre 1" left at say 200 yards it will be the same at 100, 300, 400?
 
The line of sight through the scope has to be horizontally parallel with the bore. This will place POI to the left of POA (on the K31 the scope is offset to the right) at any distance. With the example of the Swiss rifle you would zero the scope .75 inch to right of chosen POI.
This is essentially what stampede is saying. The following link includes diagram.

htt p://theswissriflesdotcommessageboard.yuku.com/reply/75003/Quick-Alpine-Sporter-308-Swiss-Products-DT-mount-questi#reply-75003
 
mgimp said:
The line of sight through the scope has to be horizontally parallel with the bore. This will place POI to the left of POA (on the K31 the scope is offset to the right) at any distance. With the example of the Swiss rifle you would zero the scope .75 inch to right of chosen POI.
This is essentially what stampede is saying. The following link includes diagram.

htt p://theswissriflesdotcommessageboard.yuku.com/reply/75003/Quick-Alpine-Sporter-308-Swiss-Products-DT-mount-questi#reply-75003
Click to expand...

Ok so that really cleared things up. Thank's for the link!! Now just to decide sight in zero or parallel? Parallel seems like an easy option though? :confused:
 
I would go with zero.
If zeroed at 200m, it will be a mere "scope offset" at 400m (which is usually less than 1").
Wind and natural bullet spread makes it error prone when aiming "one offset to the left".
It is doable though, the smaller the group the easier.
 
A side mounted scope could be the cause of a 10" miss left, if....
If the scope centre was 1" left of bore centre.
If grandpa zeroed at 30yd, for a second zero at x number of yards.
30/zero, 60/1", 90/2", .....330/10".
Nice shot :).

I'd suggest running parallel, it's just less to think about. If you zero at 100 you're only talking a couple inches at 300, it's no big deal right. If you run parallel your only an inch at any distance. Less to think about.
 
9x19p said:
A side mounted scope could be the cause of a 10" miss left, if....
If the scope centre was 1" left of bore centre.
If grandpa zeroed at 30yd, for a second zero at x number of yards.
30/zero, 60/1", 90/2", .....330/10".
Nice shot :).

I'd suggest running parallel, it's just less to think about. If you zero at 100 you're only talking a couple inches at 300, it's no big deal right. If you run parallel your only an inch at any distance. Less to think about.
Click to expand...

Thanks!!

I discovered when I swapped out the old scope a few weeks ago that the old scope was not sitting on the mount properly, the mount was not not flush to the mounting block screwed to the receiver. It was barely noticeable but I caught it when I was about to remove it to change the scope. First time I ever noticed that. I had resighted the rifle for 100 yards shortly before I took the deer with it in '09. I have had the rifle about a decade or so and had it out maybe 3 times. I made sure it was on zero before I hunted with it the three times I have used it and that's it. Hopefully now that the scope is sitting correctly it will help? Plus the crosshairs from the old fixed 4X scope were covering a fair size chunk of deer at that distance so that would certainly contribute to my aim being off not to mention I don't have alot of long range experience behind me. I am changing that though, lol!!
 
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