Using a Laser and Scope for Rangefinding

I recently purchased a 5 mW green laser with Picatinny mount, and I have a short Picatinny rail on the bottom of the forestock on my Hatsan AT-44 Long .25 cal PCP.... It occurred to me that with the laser mounted there, about 4" below the boreline, and the scope 1.5" above the boreline, they could be set up to intersect, and by comparing the position of the laser dot to the Mil-Dots, you end up with a rangefinder.... Here is a plot of what I am proposing, using ChairGun....

http://i378.photobucket.com/albums/o...ps157f8bb9.jpg

As per usual, the Line of Sight through the scope is the horizontal red line.... The trajectory is the curved green line, and the laser is the blue line.... I set ChairGun up using 12X with a 1/2 MilDot Reticle, and the light grey lines are the Dots and half-dots.... The Blue dots are where the laser intersects the reticle points, and the idea is to set the laser so that it aligns with 2 MilDots up at 100 yards.... This places the intersects with the other upper Mil-Dots (for this setup) at 78 yards for 1-1/2 up, 65 yards for 1 up, 55 yards for 1/2 up, and 47 yards for the crosshair.... If you are closer than 47 yards, the laser dot will be below the crosshair, which works fine, because with this setup I have a point blank range of 11-46 yards, so if the laser is below the crosshair, I would just hold dead on....

It the laser appears on the target above the crosshair, then I know I need holdover for the shot.... How much holdover is shown by the green dots directly below each blue dot on the graph.... If the laser is between MilDots, then the range would be as well.... My intention would be to make a chart showing the laser dot position, and the corresponding holdover, both in MilDots.... something like this....

http://i378.photobucket.com/albums/o...psba6fd8cf.jpg

Obviously, you can interpolate between those points as well, and it might be useful to do that for the second half of the chart where the gaps in the ranging are longer and the holdover more critical.... I haven't tried this idea yet, but I though I would share the concept with you as I haven't seen the idea before....

Bob

I got some time to mount and align my laser today, and it looks like the concept will work as intended.... HOWEVER, the spot the laser makes on the target is too large for the desired precision.... At 100 yards, at 12X, the spot is about 1.25 MilDots high by about 0.75 MilDots wide.... Yes, you can judge where the center is pretty good, but trying to guess closer than 1/4 MilDot (the way I did up the chart in the first post) would be hopeless.... In addition the rigidity of the mount is in question.... If you apply pressure to the laser in one direction and then release, note the position on the crosshairs, then apply pressure in the opposite direction and release, the spot moves about 1 MilDot at 100 yards.... This would negate any useful rangefinding ability at that distance.... Below 50 yards it would be decent, but below 45 yards I'm within my Point Blank Range anyway and don't need a rangefinder....

Sooooooooooooo.... the concept works, but as with anything like this, the devil is in the details.... IMO for it to be useful it needs a smaller laser dot, and a very rigid mount to be more than a curiousity.... It could be made to work well in FT, but it's probably not allowed....

Bob