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Request for Help: Height Above Bore Calculation for Benelli M4 H2O w/ Trijicon RM05 Dual-Illuminated Green Reticle Dot Sight


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I own Benelli H2O shotgun. I recently bought a Trijicon RM05G green dot sight (this one: https://www.trijicon.com/products/details/rm05g) to mount on the shotgun via a Scalarworks Sync Plate (this one: https://scalarworks.com/shop/integral-mounts/sync-01/).  I am attempting to laser boresight the green dot sight. I have a computer ballistics program that assists me with creating laser boresight targets and distances based on several variables, one of which is Height Above Bore (measurement from center of bore to center of sight reticle). I want my calculation to be as precise as possible to reduce initial sight in error prior to live fire sighting. I am curious to know if anyone on these boards has a precise value in inches for any or all of the following:

1. Height from Center of Bore of a Benelli M4 barrel to top of Scalarworks Sync Plate plate where the RMG05G would rest?

2. Height from bottom of Trijicon RMR RM05G sight to center of reticle?

3. Assuming you own a Benelli M4 w/ Scalarworks Sync Plate and Trijicon RMR RM05G and have a Height Above Bore value from your previous calculations, what is the Height Above Bore value you found (in inches)?

4. Assuming you calculated height above bore for the rear iron sight on the Benelli M4, what is that value?  As my Benelli M4 w/ Scalarworks Sync Plate and Trijicon RMR RM05G would be co-witness, the value for question 4 would be close, if not the same, for what I need here.

Note: I sent an email to both Benelli and Trijicon customer service to obtain the answers for questions 1 and 2. After a few days, I have not heard back. If I do, I will share there responses here. In the mean time, I am hoping the forum can help me out.



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What I would do is put the muzzle down on a piece of paper, then trace around the outer diameter of the barrel. Try to keep the muzzle as flush as possible to the paper.  Then without moving the shotgun look thru the optic and plot the dot point onto the paper. The position of this dot will depend on where you have the optic sighted.

Then I’d find the centerline of the barrel to plot your center, then measure the distance from the center to your plot point of your optic. 

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Might be an exercise is measuring with a micrometer, marking it with a piece of chalk and cutting it with a chainsaw.  I like Danger's methodology above but not sure just how gnat's ass it will be or how gnat's ass you'll need it to be at the ranges I'm assuming you will be using the M4.  I might try his method just for giggles.

Taking a quick look, my RMR/Scalarworks combo pretty much co-witnesses with the top of the front blade on my M4.  That's about 1 1/8" from center bore to top of blade.

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13 hours ago, ruger said:

why don't you just set gun in a shooting bench and take a few shots. and adjust to your hits on a target? using a gun rest will allow you to take out any jerking from recoil out of your adjustments.this target is 85 yards from my bench. you can make it out in the trees. i have since these pictures,, changed out the choke tube from the stock modified and seem to have tighter groups.




Dang those pictures are giving me spring fever! 

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Use a standard ruler and measure from the mid portion of the bolt or open breech to the mid portion of the RMR roll cage. Done.

It's not even necessary; as mentioned use 10-15 yards fire a couple of slugs and start turning the elevation dial to get within an 1" or so; then repeat at range you desire your zero.

Edited by benelliwerkes
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