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View Full Version : Height Over Bore on ACOGs? (Conflicting reports)


GettoPhilosopher
03-16-2012, 5:39 PM
Ok, yes, I have Google'd the hell out of this, and read all the prominent threads/articles/posts/etc. The problem is, I KEEP GETTING CONFLICTING REPORTS.

The famous Bartholomew Roberts chart on ACOGs says the flattop models are calibrated for an AR15 with a 2.5" height over bore (HOB for the rest of this post)....but my measuring tape looks more like 2.75", and I've read other places online that its' 2.77", but then there are still plenty that say it's 2.5"......

Someone wanna help me out on this one? :P I'm thinking the chart was wrong, everyone copied it wrongly, but it's really 2.75"/2.77". That sound right to you guys?

Dhena81
03-16-2012, 6:08 PM
That's a damn good question I'd assume the BDC would be calibrated from a standard flat top and M855 with a 5.56 BDC at unknown altitudes, air density, and temps ect.

Is there a reason your wondering about this? I'd call or email Trijicon if your that concerned about it.

SuperSet
03-16-2012, 6:10 PM
I'd go with your measuring tape, assuming 1.2" for the bore to top of picatinny rail. And I always used 2.75" for a TA31 on TA51 mount. Who knows if it was exact but it was close enough.

GettoPhilosopher
03-16-2012, 6:13 PM
That's a damn good question I'd assume the BDC would be calibrated from a standard flat top and M855 with a 5.56 BDC at unknown altitudes, air density, and temps ect.

Is there a reason your wondering about this? I'd call or email Trijicon if your that concerned about it.

I have a TA11F, which is calibrated for M855 out of a 14.5" barrel. It's attached to a 20" SDM-R shooting M193 at a range calibrated in yards. The barrel length and bullet change don't make THAT much of a difference, but yards v. meters do, so I've written out drop charts using JBM's ballistics calculator (i.e. to hit the 300y target, use the 300m line and your point of impact will be X" high). So having the wrong height over bore would bug me.

But the more I look at it, the more it looks like actually a 1/4" change in HOB doesn't really affect it that much, even out to 1000y (I'm shooting out to 600y)....so I'll probably just not worry about it.