My zero is off in warmer conditions. How to apply changes to existing DOPE?

[drew4392]

So, I took a very intense, 3-day long, precision shooting course. It was great training, and my skills improved significantly.

After shooting on my own in the subsequent weeks, I noticed one thing which I don't know how to remedy.

During the course, the temps were in the 50s, BP, and DA were all pretty much static. I had a perfect zero set. Some solid dope for the few days. All was good.

Then... in the later weeks and months, temps were in the 80s, and naturally, BP and DA were different, too. My zero is off. I am shooting about 2 MOA higher, due to the higher velocity of warmer loads.

Instead of resetting my zero (PITA with my Vortex Razor HD), is there a way to apply these environmental changes to my existing zero?

I have a good set of dope, and I know the MOA differences between 100-1000 yards. If I am hitting 2 MOA high at 100 yards (at my zero), can I use a -2 MOA hold for a new 'virtual' zero, and then simply apply my MOA increase for the next farther target?

For example, let's say:

Elevation for 100yds = 0 (zero set)

400 yards = 9 MOA

600 yards = 17.5 MOA

800 yards = 28.5 MOA

1000 yards = 42 MOA


In these new conditions where my rifle is hitting 2 MOA higher at 100 yards, can use a -2 MOA hold for 100 yards, (sorry, thinking through this as I type) and perhaps maintain that same -2 MOA hold for all other ranges, while still applying these turret adjustments??

I hope this makes sense.

Resetting zero on this turret is a complete pain. Hoping there is a workaround. I don't think professionals in the field sit there and reset their zero with a toolkit before taking a shot. I would think there is some modified process involving holds and offsets.

Thanks!

[JMP]

Yes, you can do what you describe. Also, many people use the same rifle and scope and simply swap barrels and bolts when they change cartridges so they just make a notation of the offset. If you have a ballistic calculator, you can plug in an offset to your zero to save the profile.

I'd generally clamp down the scope in the most typical settings you have. You can get a ballistic calculator that is integrated with a weather meter to help with different conditions.

However, 50F to 80F doesn't seem to be that much of a temperature spread to be off by 2 arcminutes at 100 yards. Your scope may have moved, you can be dealing with variable bore conditions, or you may opt to use a more temperature stable propellant. This is why some target shooters will actually warm up their cases if they travel to a place that is colder to shoot.

[highpower790]

Learn how to hold.

[drew4392]

Yes, you can do what you describe. Also, many people use the same rifle and scope and simply swap barrels and bolts when they change cartridges so they just make a notation of the offset. If you have a ballistic calculator, you can plug in an offset to your zero to save the profile.

I'd generally clamp down the scope in the most typical settings you have. You can get a ballistic calculator that is integrated with a weather meter to help with different conditions.

However, 50F to 80F doesn't seem to be that much of a temperature spread to be off by 2 arcminutes at 100 yards. Your scope may have moved, you can be dealing with variable bore conditions, or you may opt to use a more temperature stable propellant. This is why some target shooters will actually warm up their cases if they travel to a place that is colder to shoot.

Great. Thank you for this. Makes sense. I'll have to play around with a few apps. Phew! It was probably a little less than 2 MOA. And the temp difference was probably more like 50 degrees. It could be the scope, I'll double-check.

Learn how to hold.

Hold? Reticle hold?

[CSACANNONEER]

Air density (elevation above sea level, temp, humidity, etc.) does come into play but, powder temperature also comes into play. Were you using a powder know to be very temperature sensitive? I'm assuming that you're using the exact same loads you were using before, right?

[milotrain]

This is why you keep a book. Zero and DOPE are different. DOPE is actually "Data on Previous Engagement". Your Zero is going to change often due to temperature, elevation, light, etc and your DOPE book keeps records of these changes so that when you get to a new range you can estimate what your zero is at this new range and this current condition. You don't rezero the scope, you estimate the change and dial the change, confirm the change is valid, write it in your book and transpose all your range notes to this confirmed change. As you confirm those changes with shots at different ranges and conditions your dope book grows and you have more information for future estimates.

[Fjold]

Are you shooting at the same location that you developed your zero at?

2 MOA is a huge shift. The difference at my local range, at 50 degrees is less than 1/2 MOA at 90 degrees.

[kcstott]

you can take any drop chart or dope you have and apply it to a new range doing exactly as you said.

but 2 moa shift is huge and it would require a 200 fps increase in velocity. just plugging numbers in my BC

if you can wait till the temp drops again and reconfirm zero for you environment then you will know nothing shifted. But I doubt it.

[drew4392]

Air density (elevation above sea level, temp, humidity, etc.) does come into play but, powder temperature also comes into play. Were you using a powder know to be very temperature sensitive? I'm assuming that you're using the exact same loads you were using before, right?

Yes, exact loads every time. I always shoot Federal gold medal match 175gr. New factory, not reloads.

This is why you keep a book. Zero and DOPE are different. DOPE is actually "Data on Previous Engagement". Your Zero is going to change often due to temperature, elevation, light, etc and your DOPE book keeps records of these changes so that when you get to a new range you can estimate what your zero is at this new range and this current condition. You don't rezero the scope, you estimate the change and dial the change, confirm the change is valid, write it in your book and transpose all your range notes to this confirmed change. As you confirm those changes with shots at different ranges and conditions your dope book grows and you have more information for future estimates.

So, this is what I was asking. I am wondering how to apply my DOPE in a new environment that might require a new zero. It seems like 2 full MOA is a bit much for the scenario I described earlier. I will re-check all equipment and if my zero hold slipped (happened before with this vortex razor, but they fixed it). To your point, I shouldn't rezero the scope, I should estimate the change and dial the change, and continue recording DOPE.

It seems like what I suggested is correct, in that if my environment is different and my zero is off, I should hold that difference on the reticle and apply my same elevation as found in my DOPE. If there are differences, I continue recording DOPE for that session.

Am I understanding this correctly?

Are you shooting at the same location that you developed your zero at?

2 MOA is a huge shift. The difference at my local range, at 50 degrees is less than 1/2 MOA at 90 degrees.

Yes, same place each time. Yep, that is what I am hearing more and more of. 2 MOA is huge. Thanks for confirming. I definitely need to check equipment. So frustrating

you can take any drop chart or dope you have and apply it to a new range doing exactly as you said.

but 2 moa shift is huge and it would require a 200 fps increase in velocity. just plugging numbers in my BC

if you can wait till the temp drops again and reconfirm zero for you environment then you will know nothing shifted. But I doubt it.

This. Yep, gotta be it. MOA shift is too big. Ugh. Gotta do a little investigating.



Everyone, thank you very much!! Really, this is helpful and I appreciate it.

[milotrain]

It seems like what I suggested is correct, in that if my environment is different and my zero is off, I should hold that difference on the reticle and apply my same elevation as found in my DOPE. If there are differences, I continue recording DOPE for that session.

Am I understanding this correctly?

Yes. I don't shoot a scope so it's a bit different for me but I have a "mechanical zero" on my sights where they are totally bottomed out. I know from my book that my general come-ups are (in 1/4 clicks):

9 clicks for 100 yards
16 clicks for 200 yards
26 clicks for 300 yards
58 - 64 clicks for 600 yards (light dependent and range specific, this is about 2MOA difference but that's my biggest spread)

I know that Coalinga's 600 yard zero is down from Pendleton's 600 yard zero, and BRRC's 600 yard zero is somewhere in between depending on season. I suspect this is due to Coalinga being high and dry and Pendleton being right next to the ocean (low and wet), BRRC can be very hot and dry or very cold and wet.

[drew4392]

Went to the range today, and all is well.

My zero was off by 3 MOA. No idea how that happened. The inner turret shows a number which the zero is held at. That number is the same as last time, so it didn't slip.

Weird.

Lesson learned: take 5-10 min and re-zero, when needed.

Still scratching my head as to why it was 2 inches off from colder weather. That requires a significant velocity change. The only thing I can think of is my last cleaning did it. I shot about 200 rounds my first day of this training earlier in the year, and my elevation kept dropping and requiring more aggressive adjustment. We cleaned my barrell, and it went back to normal. Maybe that was it, although there weren't that many rounds through it this time around. Who knows...

Thanks, all!

[kcstott]

Just remember your zero is not fixed. It floats. Every time you shoot in a new environment you need to confirm zero shift. Every new batch of ammo you need to confirm zero shift, every time you change anything in your rifle or gear or personal habits you need to confirm zero.

Your zero is not fixed. It never has been and never will be until 5 gigawatt lasers are put on the commercial market.

[JMP]

Went to the range today, and all is well.

My zero was off by 3 MOA. No idea how that happened. The inner turret shows a number which the zero is held at. That number is the same as last time, so it didn't slip.

Weird.

Make sure you are shooting similar bore conditions. A cold bore will vary from a hot one. Also, make sure the mount is secure (torqued so the scope cannot move) and tight with the rail. 3" at 100 yards is quite a shift. You may want to have someone check out the scope. The Razors should be good, but it may need to go back to Vortex. If you can, have someone with a well established load that shoots very tight groups put it on his rifle to make sure the scope and/or mount is functioning properly.

I'd think that FGMM wouldn't be that temp sensitive since everyone seems to use it. If you think it is temperature, the next time you go into a much lower temperature, let the rounds sit in your chamber for about a minute or so between shots to warm up the cartridge. While environmental conditions make a significant impact, the variance grows with distance, it sounds like it is too much for 100 yards, which should be very little for a 308.