What Height?

[Low Drag]

I figure this group will know the answer or be able to point me in the right direction.

What the height of the rail on an M4 type receiver is over the bore axis? It seems I'm using some ballistic calculators incorrectly. I figure I can add the published height of my mount to the height of the rail and I should have my "sight height" value.

Background:
For the record I've never had good results from ballistic calculations. Even when in the Corps on the BZO range AFTER coming of the rifle range where I shot expert. I KNOW I was hitting at 300 yards, yet on the the BZO range I was hitting way low. Go figure.

Anyway fast forward to today. According to the charts I see I should be .3" low at 50 yards with my zero. I KNOW it hits at 200 yards. I also know the BDC in the optic works out to 400, 500 & 600 yards. Yet I'm over 1.5" low at 50 yards.

So I tried to input a larger value for the sight height and still showed .6" low at 50 yards according to the chart.

I'm trying to keep and open mind about this entire topic but experience has taught me otherwise. Now if it's an issue of having incorrect data for BC, and conditions I might accept it. But again despite my best efforts these ballistic charts come up short compared to my real world shooting.

Any guidance would be appreciated.

[Robb Jensen]

I simply measure the height of the center of the optics tube to the middle of the side of the rail or handguards for height over bore. It's close enough to make the ballistic calculator work very well.

[Low Drag]

Thanks for the reply.

I was afraid you were going to say that.

After being frustrated I came home and measured just what you suggested, from the center of the hand guard to the center of the optic. I came up with 3.2". After running the numbers again the calculator showed I should be .6" low when in fact I was well over twice that at 1.5" low.

[paulosantos]

What optic are you using?

Iron Sights are 2.6"
EOTechs are 2.6"
Aimpoints on a Larue Mount are app 2.715"

[Low Drag]

What optic are you using?

Iron Sights are 2.6"
EOTechs are 2.6"
Aimpoints on a Larue Mount are app 2.715"

At IOR 1.1-4X with a LaRue 1.93" high.

[paulosantos]

At IOR 1.1-4X with a LaRue 1.93" high.


I got 3.295", but 3.2" will work.

[Low Drag]

I got 3.295", but 3.2" will work.

I took GotM4's advice and measured from the seam of the hand guards to the center of the optic. It came out as you said, 3.2 - 3.3 so I'm going with 3.3.

Either way the numbers don't come out using the ballistic calculator with ammo that I know is zeroed for 200 yards and hits all the way out to 600.

The reason I'm trying to work this out is I want to use my reloads in competition. I shot the match in Pueblo last week and I found I need to know better what they are doing and if I can use my BDC in the IOR scope out to 400 yards. If I could get some known ammo to work out with the calculator I could get a good set of dope for my reloads a whole lot faster.

Thanks for all the replies/info you guys. :)

[Nick S]

Height over receiver is 1.215 inches. Add that to your respective ring or mount height for height over bore (HOB) to plug into ballistic calculators etc.

For example a scope mounted in a Larue 1.5 SPR mount would have a HOB of 2.715 inches.

Your scope in a 1.93 SPR should be 3.145 HOB. Unless you are chronoing a decent data set of your respective ammo out of your barrel I wouldn't get too wrapped up around the axis over small variances as your barrel may well produce different velocities than published numbers.

Good luck.

Nick

[Low Drag]

Height over receiver is 1.215 inches. Add that to your respective ring or mount height for height over bore (HOB) to plug into ballistic calculators etc.

For example a scope mounted in a Larue 1.5 SPR mount would have a HOB of 2.715 inches.

Your scope in a 1.93 SPR should be 3.145 HOB. Unless you are chronoing a decent data set of your respective ammo out of your barrel I wouldn't get too wrapped up around the axis over small variances as your barrel may well produce different velocities than published numbers.

Good luck.

Nick

Many thanks!