Elevation Issue with mounting RDS on carry handle?

[Doc Safari]

Most of my AR's are flat tops but one does have the fixed carry handle.

Although I keep that one strictly to stay in practice with iron sights, I've decided to sight in at least one of my micro red dots on the carry handle.

Is there a significant difference in elevation adjustment when mounting on the carry handle versus a co-witness mount on the flat top rail?

[titsonritz]

Yes it quite a bit taller. You might want to see if you can locate an A.R.M.S. #39, which has been discontinued.

[Doc Safari]

So first thing this morning I went out into the desert and answered my own question.

My Sig Romeo 5 was zeroed using its factory mount on a flat-topped Colt 6720.

I wanted to see if moving the RDS to the fixed carry handle 6520 would make a difference in the zero.

Not only did it not make any difference, I'm actually convinced it was sighted in better for the fixed carry handle!

Of course, at closer range the difference between mount heights and the bore would become a factor, but at 50 yards I didn't have to make a single adjustment transferring from a flat topped rifle to a rifle with a fixed carry handle.

[GH41]

It will change the distance between where the bullet crosses the line of sight going up and coming down. Is it enough to matter? I don't know. You were lucky the zero didn't change between the 2 rifles. It does prove the mounts/rails are not consistent between the two.

[26 Inf]

You left some info out - which base did you have on the Romeo - measuring the one I have sitting on the bench with a tape measure the low-mount would place the Romeo dot about .75" above the carrying handle mount; the co-witness mount about 1.5" above.

Assuming you are using a see through rail, that probably adds .5 inch to the height of the sights.

Call it close to either 1.25" or 2.00" higher.

If you were shooting at 25 off a bench or a barricade, vertical shift with either Romeo base should be noticed on a zeroing type target; standing unsupported, probably not. At 50 to 100 with a red dot, not so much IMO.

I'm more surprised that it lined up acceptably windage wise.

[Doc Safari]

You left some info out - which base did you have on the Romeo - measuring the one I have sitting on the bench with a tape measure the low-mount would place the Romeo dot about .75" above the carrying handle mount; the co-witness mount about 1.5" above.

Assuming you are using a see through rail, that probably adds .5 inch to the height of the sights.

Call it close to either 1.25" or 2.00" higher.

If you were shooting at 25 off a bench or a barricade, vertical shift with either Romeo base should be noticed on a zeroing type target; standing unsupported, probably not. At 50 to 100 with a red dot, not so much IMO.

I'm more surprised that it lined up acceptably windage wise.

I installed the low mount that came with it and attached all of that to a Tapco polymer carry handle rail (see-through). I also used a CAA 2.6 snap-on cheekpiece. I knew it was supposed to get really windy today, so I was in a hurry because the breeze was already starting. Therefore I didn't set up a paper target to get a precise group. All I really wanted to do was see how close it was already. I just set up some aluminum cans at 50 yards. I won't say it might not be an inch off give or take but I hit every target every time shooting off-hand with my elbow resting on a bench at the range and my off-hand wrapped around the mag well. I may at some point go back out when it's not windy and get a more precise zero but right now it's close enough for government work. As I said earlier, I shot slightly better with this setup than I did with it on the Romeo's tall mount that it was originally zeroed for. The one thing that bothered me was that I couldn't see the FSP in the optic to use as a point of reference. I always try to center the dot over the FSP because of the write-up on parallax issues with RDS sights.

So, bottom line is that it needs to be properly re-zeroed because it may very well be an inch or two off, but it's not going to be wildly off like I thought it might be.

[26 Inf]

Doc, if you would have hollered 'Helllooo Dan!' this morning I probably would have heard you this afternoon. Pretty good south wind all day here.

[Doc Safari]

I'm taking it out again this morning. The Tapco mount had loosened just from shooting yesterday.
I just put everything together from junk in the parts bench so I guess I shouldn't expect much.

[Doc Safari]

I'm back. The windage was a little off. No big deal. This RDS originally sighted in on another rifle. Dang wind cut my visit short again.

[Pappabear]

In my experience , it makes the downrange effects dramatic. So you might be zero'd at 50, but your 100 yard should be way off. YMMV

PB