AR optics- HEIGHT OVER BORE issue

Half-cocked · Jan 23, 2014

erwos said:
I need those graphs for 7.62x39, 7.62x51, and 9mm.

What height over bore? And any specific loads? (bullet MV and BC are all that's needed to calculate trajectory; bullet weight is also needed if energy is to be calculated.)

Pinecone · Jan 23, 2014

erwos said:
I need those graphs for 7.62x39, 7.62x51, and 9mm.

JBM - http://www.jbmballistics.com/ballistics/calculators/calculators.shtml

Magnumite · Jan 24, 2014

"How about old school guys that have carry handle mounted optics?
Zeroed at 25, it was almost 11" high at 100"

Why would anyone zero a scope at 25 yards which would yield that type of trajectory? I'd think a good rifleman would use a feature on the scope for closer in hits and zero at more realistic ranges of cartridge effectiveness. That is how I run my scoped carry handle 20" gun.

The 16" carbine 4" dot is zeroed at 50 yards. To make a reasonably accurate hit with the dot 100 yards I set the desired impact point on top of the dot. To be honest, anything past close-in is done with BUIS. Why shoot at a 200 yard target with a dot covering 8"? Shooting 3 gun matches this works out very well.

Both of these guns iron/BUIS are zeroed to 50 and 100 yards using the improved BSZO (2 and 3 clicks down from zero). Subsequent long range trajectories were indicated earlier. They shoot low at 25 yards/meters. All these adjustments make the gun more predictable and pleasant to shoot than the GI type trajectories.

Raising the front post on the GI FSB 3 clicks yields the GI 25 meter POI and long range zeroes.

erwos · Jan 24, 2014

Half-cocked said:
What height over bore? And any specific loads? (bullet MV and BC are all that's needed to calculate trajectory; bullet weight is also needed if energy is to be calculated.)

I've got like a hojillion different guns in a bunch of calibers, so that's a thinker.

Pinecone said:
JBM - http://www.jbmballistics.com/ballistics/calculators/calculators.shtml

Thanks - I'll check it out soon.

SCARCQB · Jan 24, 2014

Why would people zero at that range?
Because they only have access to 25 yard ranges. most are also under the impression that a 25 yard zero is good enough for everything.

" the military uses a 25 yard zero and if its good enough for them, it should also work for me"
This is also the reason why I started this thread.

photoracer · Jan 24, 2014

Pinecone said:
My thing is, for HD, you are not going to be taking shots at longer ranges. And the difference in the various zeros, out to 50 yards, is fairly small.

And if you are going to take a longer shot, why not set up for a maximum point blank range that will work from muzzle to 280 yards?

If you set the vital zone to a +/- 2.6", a 45 yard zero works out to give you a hit in that zone out to 280 yards.

Here is the impact point to point of aim at 0 - 50 yards for 55 grain FMJ at 31500 fps MV.

Range 45 yard zero 100 zero

0 -2.5 -2.5
10 -1.9 -2.1
20 -1.3 -1.7
30 -0.7 -1.3
40 -0.2 -1.0
50 +0.2 -0.8

(forum formatting sucks)

I am not saying it is wrong, but saying I can see the case for other options. I sort of see his point out the hold over staying the same, but again, at HD distances, you are talking about differences of a a few tenths of an inch.

And I am a BIG fan of doing training or competitions to add stress to the situation to learn to work with that stress.

I agree. While my 3-gun AR is zeroed at 100 to correctly use the MTAC reticle in my Burris with 55g ammo. My 14.5" barreled HD/SHTF carbine currently has a 50 yard zero with its scope so I can keep the impact in center-of-mass as close to the center of the reticle as possible from CQ out to a max of 100 yards give or take a little. It stays loaded with PMC 62g X-Tac green tip penetrators. As it happens 55g and 62g ammo has similar ballistic profiles out to almost 600 yards anyway so I can swap mags without too many worries about the optics being off. That is with a HOB of about 2.3".

I think 31500 FPS is a little fast.

Magnumite · Jan 24, 2014

Scar, I,ve read that about hunting rifles, too. I never found it practical as well. Might have to shoot a group at 25 and record POI for 50 and 100 yard zeroes.

Pinecone · Jan 24, 2014

photoracer said:
I think 31500 FPS is a little fast.

Haven't you seen that new round? It is a wildcat, a Naval 5"/38 round necked down to .223. About 16 pounds of powder.

SCARCQB · Jan 24, 2014

My SHTF ARs are zeroed at 50 yards. That gets gets me very close to POA at distances up to 250 yards without too much holdover/ under.

However, my target guns are zeroed at 75 yards to be within 1" of POA at 100. While not a combat zero, it helps in more consistent hits at target/ plinking distances.

Same goes for any 5.56 that has a tall mounted sight, like the Tavor, PS90, AUG, ARs with carry handle mounted optics and the like. i compensate for the tall HOB by zeroing at a longer distance to account for the offset.

SCARCQB · Jan 24, 2014

31500 FPS? Now, we need to get a group buy rolling on man - portable RAIL guns.

Minuteman · Mar 5, 2014

SCARCQB said:
How about old school guys that have carry handle mounted optics?

Zeroed at 25, it was almost 11" high at 100

Yep, it also puts you back on target at around 300m.

SCARCQB said:
Why would people zero at that range?
Because they only have access to 25 yard ranges. most are also under the impression that a 25 yard zero is good enough for everything.

" the military uses a 25 yard zero and if its good enough for them, it should also work for me"
This is also the reason why I started this thread.

Agreed, this is the way I was originally taught, it worked then and still works now. I'm more interested in what happens and how to setup for shooting 308nato from anywhere between 100 and say 800. This is where I think KevP and Pinecone could help guys like me would just want a little more distance with a little more rifle. Why I need more training I guess.

AR optics- HEIGHT OVER BORE issue

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