Night Vision Halo XRF Optical Center, Height above bore for thermals.

[shields shtr]

Kind of an offshoot to the zero thread, but it got me thinking. I called up Nvision and asked them how they figure out their height above bore. The tech on the phone said that their optical center is 39mm. Of course, I stupidly hung up the phone and then thought after "I wonder if that is the optical center of the unit (which makes the most sense to me", or what they use to calculate the height above bore? Obviously, this measurement will vary.

Any tips or common numbers folks use? I am using the stock mount that came with the unit and have it mounted on a Defiance action with a 20moa rail. Caliber is 6 creed. I am thinking to calculate I use 1/2 bullet diameter+rail thickness+mount thickness+39mm?

 

[Jumper]

I have almost the exact same setup; XRF with Bobro offset mount on a 6mm creedmoor, 20moa rail, ARchimedes action. I just took careful measurements with my calipers. Mine is 2.88" above bore. Interestingly my other rifle with a steiner M7 scope on it is 2.90".

Its not that critical to have exactly accurate measurement. I doubt you would ever notice the difference if you were off by a tenth of an inch.

 

[shields shtr]

I have almost the exact same setup; XRF with Bobro offset mount on a 6mm creedmoor, 20moa rail, ARchimedes action. I just took careful measurements with my calipers. Mine is 2.88" above bore. Interestingly my other rifle with a steiner M7 scope on it is 2.90".

Its not that critical to have exactly accurate measurement. I doubt you would ever notice the difference if you were off by a tenth of an inch.

Awesome. Thanks for the reply, this is what I was hoping for. When I took the measurements I ended up with 2.7". I do know that tenths don't make much of a difference, but I had 1.5" plugged into my ballistic calculator and the 1.2" difference was substantial. I airballed a few coyotes last night at about 300 yards and realized I was shooting about a foot high with the zero I had calculated.

 

[Jumper]

I just zero'd mine a few days ago. I'm shooting a 105gr Barnes matchburner and I zero'd mine 1.7" high at 100yds. At ~ 3000fps, this should let me hold dead on out to 375yds on a coyote. I figure 375 yards and in is a reasonable expectation where I hunt. Also, I recommend using the TH zero method with a good thermal target. Im confident in my zero on this rifle now for the first time. Now I just have to find time to go hunting.

 

[Grizz11]

I have a question on the xrf. According to my ballistic calculator a 50 yrd zero with my setup at a 2.75 sight high should be 1.2 inches high at 100 yards. Verified at 100 and am 6 inches high. Same issues with a friends gun who runs the xrf also ? At every distance from 100 out to 600 yards this scope shoots way high according to my calculator. Have verified mills with day scope. Also reverified our 50 yard zeros multiple times from a bench and rest. Any guesses ?

 

[Max_R]

... I called up Nvision and asked them how they figure out their height above bore. The tech on the phone said that their optical center is 39mm. ... "I wonder if that is the optical center of the unit (which makes the most sense to me", or what they use to calculate the height above bore?

It is the height measured from the top of the Picatinny rail to the optical axis of the unit with a standard weapon mount.

 

[Grizz11]

It is the height measured from the top of the Picatinny rail to the optical axis of the unit with a standard weapon mount.

Max why would the optic be shooting 6 inches high at 100 with a 50 yard zero ? Is there something im doing ? It does not do this with a day optic ?

 

[Max_R]

Max why would the optic be shooting 6 inches high at 100 with a 50 yard zero ? Is there something im doing ? It does not do this with a day optic ?

This puzzles me. Are you sure that the 2.75" sight optical axis height over the bore is accurate (it probably is) and correctly entered into the calculator? This is just a check. 1.2" high at 100' sounds reasonable. Somehow, however, you are getting 5" extra at 100'. It is 5 pixels if you are at 1x digital zoom. The reticle is 1 pixel thick (unless you use round reticle 6 with a center dot, which is 2 pixels). Assuming that you use 1x digital magnification, if, for whatever reason (targets looking slightly differently at 50' and 100', etc.), you aim right below the reticle when you zero and right above the reticle when you shoot 100', you will have something like a 2.5" error but not a 5". As I said at the beginning, your problem puzzles me, and I don't know the answer to your question. I'm giving you my considerations, hoping that you can use them to figure out what is going on. I would start by analyzing the zeroing target and ensuring that it does not play any tricks, like appearing somewhat brighter on the bottom than on top and so on.

 

[Grizz11]

This puzzles me. Are you sure that the 2.75" sight optical axis height over the bore is accurate (it probably is) and correctly entered into the calculator? This is just a check. 1.2" high at 100' sounds reasonable. Somehow, however, you are getting 5" extra at 100'. It is 5 pixels if you are at 1x digital zoom. The reticle is 1 pixel thick (unless you use round reticle 6 with a center dot, which is 2 pixels). Assuming that you use 1x digital magnification, if, for whatever reason (targets looking slightly differently at 50' and 100', etc.), you aim right below the reticle when you zero and right above the reticle when you shoot 100', you will have something like a 2.5" error but not a 5". As I said at the beginning, your problem puzzles me, and I don't know the answer to your question. I'm giving you my considerations, hoping that you can use them to figure out what is going on. I would start by analyzing the zeroing target and ensuring that it does not play any tricks, like appearing somewhat brighter on the bottom than on top and so on.

I use a 1 inch square piece of reflective tape that shows up very clear. I can put all 5 shots in a good group inside this 1in square tape at 50. Group At 100 is same size target and all shots are 6 inches high although a very tight group from bench. At 600 yards according to my ballistic calculator I should be 2 mills. I have verified this with same gun and day optic. I’m roughly 12-14 inches high at 600 yards With a 2 mill hold. The crazy thing is my buddy runs a halo xrf as well and his results are almost identical to mine? Is there any chance at all that with the digital zoom it’s causing poi shift ? I’d assume not as this would be unusable. I’m just having a hard time putting together a ballistic table and have tried to insert some crazy numbers to get the calculator to show 6 inches high at 100 with the 50 yard zero. It would have to be like an 8 or 9 in sight height to come close to that. I’m going to shoot tomorrow from 100 out to 800 and figure out the dope but if I’m 6 high at 100 that should in theory put me at roughly 13 high 175-200 as this is my highest peak. All of the coyotes I’ve shot have been very high impacts at yardages from 100-300 with a good amount of misses. I’m using reticle 7 btw. I like to take my longer shots at 4x zoom. My understanding is each hash at 4x zoom should be .5 mills. I’m just having a hard time quantifying this as it makes my calculator impossible to use. I’ll have more data tomorrow after the range session each yardage out to 800 and maybe we can figure out why.