POI with side mounted optics questions

[Bullseye]

I have two scenarios.

Lets say I have a Krag 30-40 sporter with a side mounted scope to the left and sight the rifle in at 100 yds. At 50 yds it would shoot a bit left of point of aim and at 150 yds the same amount to the right of point of aim and at 200 yds perhaps twice as much to the right as it does at 150 yds.
Not taking in any consideration for ballistics, drop, windage, velocity etc.
Just what I see through the scope at 100 yds is what I can expect here. Correct?

See 1st attachment image

The other is a scope mounted rifle with a side mounted Red Dot Sight.
The reason for this set up is for closer encounters with a target using a red dot and also the rifle could reach out to further distances using the scope.
Now at 50 yds with a zero on the red dot to a zero with the scope and closer I should be off target no less of than the distances between my barrel and mounted scope. Less than say 3 inches I assume.

After the distance of 50 yds, I am thinking the trajectory would increase dramatically. Much more than a side mounted scope.


I have attached two images that are not to scale but illustrate my thinking here.
The question is ... am I right in my general calculations?

See 2nd attachment image

Here's why I question this.

 

[strambo]

I probably wouldn't zero it left to right, but maintain the offset. At close range the offset will be there anyway. At long range it will be off in the other direction and another variable to try and predict. If the RDS is 3" from muzzle center-line, I'd zero it at say 50yds vertical and 3" offset still.

So have the bullets hit vertically in line with the POA, but impact 3" to the right. At distance, the 3" right is no big deal, close up the offset would be there anyway to account for (before it intersects).

 

[FL-NC]

These things are a windage nightmare. As seen from above, the line of sight and the path of the bullet for an "X". The US mil side mounted a scope on the M1 Garand (M1C and M1D models). Most snipers of the era preferred the Springfield 03 since the scope was mounted properly. I fired a M1C in training for familiarization, and I'm glad I never had to use one for real.

 

[Arizona_Mike]

1. Good ballistic software will allow you to put a horizontal optics offset.
2. The way such close range backup sights should be used is not by moving the head but by canting the gun slightly so the backup sight is vertical. In that case you can sight it in as usual. Are you left-handed? That looks like a left handed backup setup.

This video shows how such diagonal mounts are supposed to be employed: https://www.youtube.com/watch?v=FCMK2hLRaSo

Mike

 

[Bullseye]

Actually Arizona this setup is easy to look into the red dot sight with just a slightly higher cheek weld. No canting the rifle at all.
Hold straight up and down with head straight and shoot. I can keep both eyes open with the red dot. Dominant right eye and right handed
On these Tavors, you want a riser if using optics like I have here.
The bullpup stock and butt pad is level with the rail so a 1" riser like this is necessary or taller rings.
Yes, I realize that the bullets will hit higher since the barrel is way down there.
It's the same way with the flip up iron sights anyway.

This was an experiment with some stuff I had for my AR.
The red dot would not be used past 50 yds anyway. I just wanted to know where my bullets will end up downrange. I should know that.

I put this together yesterday since I am getting a different scope for the AR.
(I haven't shot it yet.)
Perhaps I should zero in at 50 yds with both where they hit the same distances away from the barrel as where they are mounted.
I could compensate by where I hold on target.
It's a geometry problem the other way.
I thought it might be an interesting project.

The crosshairs are about 4 inches above the barrel and the red dot is about 3.5 inches above the barrel and 2 inches to the left.

Suppose I get on the bench and shoot a 10 inch target at 50 yds and theoretically hit dead center.
Then if I adjust my scope and sights to these same distances on the target as in this image I made.

I can expect the same hold on target at any reasonable distance not taking into account the bullet drop and windage factors.

Sighting in at dead center with the sights and scope would only really affect the red dot trajectory from left to right.
The scope shot always being the bullet hitting 4" higher than my point of aim.
I know .... this requires some thought. I am trying my best to express this in words. I think the red dot would only be used close up.

My bet is that I should sight in at 50 yds with the red dot right on and use the scope wherever it zero's in.

 

[RX-79G]

I don't see why you would do anything but zero with built in offset. In other words, every time you aim at the bull, the hit should be right of the bull the exact amount of offset the scope has to the bore. No matter what range you're shooting, as long as you keep that 1.5" of offset at the target in mind, you won't have to do any other corrections.

In other words, if you always aim for the "right eye", and you'll hit center at every range.

Anything else is a mess where you would have to be guessing range vs whether the bullet is right or left of POA at that range. No thanks.

 

[Arizona_Mike]

Given the way you want to use the sight and the fact that you want to use it for 50 yd, I'd actually zero it at 50 yd. Your average horizontal error within 100 yards would be half of what it would be for parallel alignment.

Scope adjustments on the other hand . . . all that dividing by square roots of 2 . . .

Mike

 

[Bullseye]

I took out the bolt and looked right through the butt stock through the bore and set it on a knot on a tree 50 yds out. I zero-ed both the red dot and scope on the same knot on the tree.
I am ready for the range for final adjustments.
This way, the closer it is using the red dot will just put the point of impact a tad to the right and a tad lower the closer it is. The worse it can get is less than 4 inches off point blank.

As for the scope, at the range, I'll zero in at 100 yds and use the mildots for other distances.
Range report will follow.

 

[Bullseye]

Back from the range this afternoon with this ( rather inexpensive ) optics setup.
The TRS-25 after quite a few adjustment is right on at 50 yds. Even with no magnification I was getting a 5 inch group.
The UTG 3x9x32 scope was giving me 1 moa groups when I did my job with the heavy trigger this thing has. I really concentrated and put 3 consecutive shots right in the bullseye. I shot probably 100 shots give or take and at first I was way off but on the paper and they were landing together.
Both of these units are on 1 inch risers. The red dot is on a 45 degree attachment and therefore a bit tricky to go up and down & side to side with adjustments but this is just how I wanted it to be. Further out to 100 plus yds I can use the scope and mildots or remember/record how many clicks I have to move it up or down for the distances.
I set both turrets at 0 for 50 yds.
For closer shooting and faster acquisition the Bushnell red dot will be fine. I have a flashlight on the opposite side for night time use with the red dot.
All are holding zero so far. Happy with my experiment.

 

[Bullseye]

Oh yeah, one other tweak. The brass was getting dented up ejecting on this unit so I cut a perfect sized little rectangle of the fuzzy side of a velcro sheet I had, cleaned the surface with acetone where the cases would hit and stuck it on there.
No more dented brass. Not a one.