Questions about zero consistency.

[WVshooter]

1: Is it common to need to make small zero adjustments when shooting your rifle in different environmental conditions?

I have a 20" Rem 5r .308 and zeroed it with FGMM 175 back in June when I was visiting family in WV. conditions were mid 90's temp, 80%+ humidity and 1300 ft elevation. Zero at the time was a perfect center 5 shot group of about .5"
This morning I started shooting and had a group about .2 mil high and about .1 mil right. Same ammo and equipment, different weather and location. Today 62F, 52% humidity, and about 600ft elevation. Anyway it was a consistent shift. I checked the torque of everything and all was good. Made a small adjustment and everything was good.

Based on the first response I got I suppose I should preface this by saying that I am not assuming that the weather affected the bullets flight because I know that reasonably large changes in atmospherics don't make much difference in bullet flight until longer ranges, certainly not at 100 yards. I'm questioning whether or not it's reasonable to expect a slight shift in zero between shooting in hot summer conditions and cool fall conditions.

I'm assuming the answer is no but I can't think of what else might have caused it. I have the 5R which is fairly consistent .5" to .75" gun with match ammo. Seekins rail bedded and installed by SAC. Seekins rings torqued to Seekins specs. SS 5-20, and action sits in a Manners MCS T2 and action screws are torqued properly. Both sessions shooting from the prone with same bipod, bags, etc.

 

[SWRichmond]

Go this this web site: JBM - Calculations - Trajectory (Drift)

pick your bullet, give it a reasonable starting velocity, and then scroll down to the bottom and click "calculate". The program assumes a 100 yard zero.

Now, change the temperature only by 25 degrees or so, and see what happens to your come-ups at 300, 600, 1000 yards.

 

[WVshooter]

Questions about zero and accuracy consistency.

Thanks but that's not really what I was asking about. Im not under the impression that the temp change would affect the bullet flight at that range but questioning whether the gun and scope at a resting temp of about 60F would be expected to show any deviation from being zeroed when the equipment was at a resting temp of about 95F.

Basically my zero was off and I'm wondering if that would be expected under the conditions I described or if I should look somewhere else.

 

[roggom]

The elevation would effect the trajectory at longer distance, 700+ yards.

There are many variables that can effect your zero which your initial post did not have enough info to invalidate.

Bench or prone, uphill/downhill. Could be a change in your NPA, many other variables.

Also you are talking a 30 degree temperature shift, 30 degree ambient temp. Not sure if you were in the shade or had the sun beating down on the ammo.

In short, cooler temp, less velocity, may be enough to effect your zero.

 

[hk dave]

Yep I second what roggom said... 30 degree temp change could affect your ammo and cause different pressure, speed, amongst other things.

Perhaps it could be parallax coupled with mirage?

 

[WVshooter]

Questions about zero consistency.

It would be hard to address all the variables I suppose. I'm going to take a day course at a long range shooting facility this Wednesday so hopefully I can address this question there.

I unfortunately don't have enough opportunities to get on this gun as often as I'd like so I wasn't sure if anyone else expected to fine tune their gun any occasionally in different seasons or if it was supposed to pretty much be set it and forget it.

 

[DALE '45]

WV Shooter,
We have run into the same thing while prairie dogging in South Dakota. We have a 300 yard target in camp where we zero our rifles along with a 600, 930, and 1170 yard gongs. Using JBM calculations in 5 yard increments everything is good. Getting out in the town during the course of the day we have to change our turrets several times. We do most of our shooting in the 450 to 900 yard range so the change is very noticeable. What works for us is after a good go at a well lasered range we change the turret to that setting. What is causing the change we do not understand but it it there in all of our rifles. Dale

 

[MoBoost]

.5" gun shot .5" high ... What's the dilemma?

 

[WVshooter]

.5" gun shot .5" high ... What's the dilemma?

Math apparently

 

[brwn bggr]

I would ask your round count...
A low count may suggest that your rifle settled...
My GAP built spr required .5 moa adjustment at about round 60(guessing w/out checking log)...

 

[Elite_KG]

Temperature/atmospheric variations affect more than the bullet flight and cartridge velocity. Now I'm not saying this is the case here, but the metal in the rail, rings, and barrel all undergo expansion and contraction with increases and decreases in temperature. Different metals and different allows expand differing increments per degree. This can cause added or reduced stress in the mounted setup in the case of optics; as for the barrel, harmonics will differ as well based on shift in internal stresses in the barrel.
Please keep in mind, before someone goes blowing this out if proportion, these changes in dimensions and stress are very small, however if they all work together it can lead to minute changes in POI.
I don't know if this is the case for you or for Dale, but these are facts of metallurgy. Thought it may be good info to ponder when you are chasing the phantoms as we all often do.

KG

 

[WVshooter]

345 rounds as of today. Bore snake after every session cleaned with brush and solvents last time at around 150. I'll get 100-150 rounds through it Wednesday so I imagine I'll have it all worked out.

 

[HodgdonExtreme]

Can't say I understand the physics behind it, but I routinely see a zero shift (on the order of ~.1 in any given direction) when shooting in different locations.

Same goes for my shooting partner.

 

[WVshooter]

Good point. The lot hadn't occurred to me. I've got stacks of FGMM stored up and looking at lot numbers of the boxes of empties from before today vs. what I was shooting today it's likely I was shooting a different lot today. I'll need to pay more attention to that as another variable and make sure I take all the same lot with me to the course.
May not explain it but one less variable.

 

[jashmore1234]

Simple answer is yes. Simple solution is to always rezero after travel or major change in environmental conditions or to shoot so often that you learn your rifles personality and can compensate for changes.

 

[Drifter™]

Can't say I understand the physics behind it, but I routinely see a zero shift (on the order of ~.1 in any given direction) when shooting in different locations.

Same goes for my shooting partner.

Same for me.

I believe different targets in different lighting can have a slight effect on perceived zero. For example, shooting a black dot on white paper in the shade might show a click (or two) difference versus a bright orange glow dot in the sun. The rifle and ammo perform the same, but perhaps our eyes see the target differently and it affects the hold.

One other thing that comes to mind is variation of cant from one session to the next. An anti-cant level might help.

 

[doc76251]

345 rounds as of today. Bore snake after every session cleaned with brush and solvents last time at around 150. I'll get 100-150 rounds through it Wednesday so I imagine I'll have it all worked out.

This is your answer. You cannot expect to have a consistent CBS after you clean a rifle. Once it is dirty, leave it that way. If it jumps off zero then you have other issues. Your ammo lot numbers may have an effect also as noted.

Cheers,

Doc

 

[WVshooter]

Thanks but considering it's been 195 rounds since I cleaned the barrel last and it's my zero that shifted and not my CBS I don't think a clean bore is the issue.


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[Greg Langelius *]

I would take the same ballistic calculator and change the zero distance to 300yd, then feed it different altitude and temp values while keeping track of the 100yd Trajectory value. You may see some variance, and if so, I'm guessing it will somewhat mimic your results. This is a mind experiment, no need to actually fire this test.

Essentially, any zero at a given distance, like 100yd, is also a zero at other distances, only the impacts will be higher or lower, but pretty consistently so (i.e., if I'm on at 100yd, I'll be 5" low at 300, etc.). To see how the ambient conditions affect this, do the above. The logic may appear fuzzy, but I'm guessing some value will emerge from the process.

Greg