Hard Dope and Rezeroing

[ReligiousShooter]

Suppose I am at my home range. I zero my rifle at 100 yards and get hard dope at my home range.

I go to Spaniard's match and find that I am shooting .75 MOA high at 100 yards. What should I do? Should I just dial down or should I reset my turrets to zero with the .75 MOA adjustment?

Suppose I don't zero my turrets and just dial .75 MOA down. Would I be right to think that I can use my hard dope from home, but I would just need to dial .75 MOA down from my hard dope to get a hit? For example say my hard dope for a given distance and conditions is 6 MOA up at my home range. So at West End I could dial 6 up - .75 down => 5.75 up to get the hit. Would this be correct?

If I rezero my gun while at Spaniard's match, what happens to my hard dope from my home range? Using the above example, since I rezeroed, is my hard dope of 6 MOA going to be good at West End as well?

[Seth8541]

Personally, if I was out at the range gathering hard dope and not in a match, I would not rezero my rifle. Id just dial the .75 down to hit and record all the dopes. Now, if I was shooting a match I would rezero, record the adjustments made, and when I got back to my home range I would adjust it back and confirm.

According to the ballistic charts for my rifle, the diffference in dopes from sea level @ 50 degrees to 4500' @ 90 degrees is 0, the same environmental changes @ 500 yards is exactly .5 moa, and @ 1000 yards is approx 2.75 moa. Back in the Corps we called this "calc in place". We would take all the data from our zero and the new data from the current shot to be made, run all the formulas for altitude, temperature, b.p., angle,etc using a calculator and subtract or add the MOA adjustments to make a first round hit at a given range.Now everyone just uses some sort of ballistics software. I use the "Shooter Ballistics" program for android. I would invest in somesort of ballistics program if I were you. All my data is set for shooting in Needles, Ca @ 1215' ASL 28.63 hg, 75.3 degrees, I then went to the matches in Vegas and Prescott where the conditions are drastically different.Using this program got me right on the $$$. Great tool.

[Seth8541]

If the condition are significant enough to change your dope at close range, than just think about how much it'll change you dope @ long range. Think about it like this, you are experiencing a wind of let's say...10mph which will require a wind doping of .75moa @ 100 yards (just an example). Will this windage dope of .75 still be satisfactory for a 1000 yard hit? No, it will require an addition of windage to hit under the same conditions. Your elevation adjustment is not any different. Hope that makes scence.

[brian01tj]

Id be more concerned as to why you had the shift in zero. I dont believe any atmospheric conditions would have enough of a ballistic effect to effect your dope that much. If you truly had a shift in zero, then yes, I would re-zero the knobs as well. Dont take your first "cold shooter" shot as the actual zero for the rifle either.

[Seth8541]

+1

I just assumed he was using that .75 as an example.

[AR15barrels]

Suppose I am at my home range. I zero my rifle at 100 yards and get hard dope at my home range.

I go to Spaniard's match and find that I am shooting .75 MOA high at 100 yards. What should I do? Should I just dial down or should I reset my turrets to zero with the .75 MOA adjustment?

Suppose I don't zero my turrets and just dial .75 MOA down. Would I be right to think that I can use my hard dope from home, but I would just need to dial .75 MOA down from my hard dope to get a hit? For example say my hard dope for a given distance and conditions is 6 MOA up at my home range. So at West End I could dial 6 up - .75 down => 5.75 up to get the hit. Would this be correct?

If I rezero my gun while at Spaniard's match, what happens to my hard dope from my home range? Using the above example, since I rezeroed, is my hard dope of 6 MOA going to be good at West End as well?

Let's say your home range is pala and you got dope there on a cool day.
Actual elevation is 500ft and density altitude would be around 800ft.
Then let's say you go to WEGC on a warm day.
Actual altitude is around 1400ft and density altitude will be around 3000ft.
That's a significant enough difference that your dope gathered at pala is not going to be accurate, especially if your longrange hard dope did not take the uphill firing angle into account.
When I shot a 308, I had once figured out and proven to myself that a 1000ft density altitude shift made a difference of 1 moa per 1000yds.
So at 500yds, there was 1/2 moa dope change per 1000ft DA change and 1/4 moa change at 250 yds etc...

Following that logic, the change in DA alone does not explain the 3/8 moa zero shift I see between different ranges.
That's why I started the "places we shoot" thread which shows the direction of fire for each range.
As corny as it is, I believe Coriolis comes into effect to screw up our zero when we drastically change direction of fire.
This is more evident as a windage shift with a north to south or south to north change than with an east to west or west to east change.
When changing east to west or west to east, you will get more elevation change than windage change.

So what I always do when I go out of town is to figure out the elevation and direction of fire of the range I will be going to and I stop somewhere with simiar conditions and re-zero my rifle.

[DirtRacer151]

I get a consistent 1/10mil windage shift when firing north/south. Have never fired west/east and collected data but im now a firm believer that Randall is right.

[DirtRacer151]

Just spent some time playing with coriollis in shooter and it shows me the same .1 windage shift that I normally see at 100yds when switching between n/s s/n direction.

[brian01tj]

Let's say your home range is pala and you got dope there on a cool day.
Actual elevation is 500ft and density altitude would be around 800ft.
Then let's say you go to WEGC on a warm day.
Actual altitude is around 1400ft and density altitude will be around 3000ft.
That's a significant enough difference that your dope gathered at pala is not going to be accurate, especially if your longrange hard dope did not take the uphill firing angle into account.
When I shot a 308, I had once figured out and proven to myself that a 1000ft density altitude shift made a difference of 1 moa per 1000yds.
So at 500yds, there was 1/2 moa dope change per 1000ft DA change and 1/4 moa change at 250 yds etc...

Following that logic, the change in DA alone does not explain the 3/8 moa zero shift I see between different ranges.
That's why I started the "places we shoot" thread which shows the direction of fire for each range.
As corny as it is, I believe Coriolis comes into effect to screw up our zero when we drastically change direction of fire.
This is more evident as a windage shift with a north to south or south to north change than with an east to west or west to east change.
When changing east to west or west to east, you will get more elevation change than windage change.

So what I always do when I go out of town is to figure out the elevation and direction of fire of the range I will be going to and I stop somewhere with simiar conditions and re-zero my rifle.

It would be interesting to see the reuslts if the phone ballistic software peeps built in this feature. The shooter program does take into account the degrees of latitude but not longitude. Combine this with the compass for the n/e/s/w bearing and it could give a corrected drift calculation. The longitude location would need to be an input as well since the closer you are to the equator the larger the effect it would have on a east/west firing direction.

I turned on the coriolis effect on shooter and it gave me .1mil windage at 100 and 1.8 at 1k. I tried several times to see if direction of fire had an impact but it doesnt change from that .1 and 1.8

[brian01tj]

Just spent some time playing with coriollis in shooter and it shows me the same .1 windage shift that I normally see at 100yds when switching between n/s s/n direction.

Correct I got the same. But, it should not give .1 if your facing east/west

[DirtRacer151]

No, latitude are the horizontal lines.. they are an angular distance, in degrees, minutes, and seconds of a point north or south of the Equator. Line That is what shooter uses for the input. Longitude is not needed.

It would be interesting to see the reuslts if the phone ballistic software peeps built in this feature. The shooter program does take into account the degrees of latitude but not longitude. Combine this with the compass for the n/e/s/w bearing and it could give a corrected drift calculation. The longitude location would need to be an input as well since the closer you are to the equator the larger the effect it would have on a east/west firing direction.

I turned on the coriolis effect on shooter and it gave me .1mil windage at 100 and 1.8 at 1k. I tried several times to see if direction of fire had an impact but it doesnt change from that .1 and 1.8

[brian01tj]

No, latitude are the horizontal lines.. they are an angular distance, in degrees, minutes, and seconds of a point north or south of the Equator. Line That is what shooter uses for the input. Longitude is not needed.

Correct. I forgot that it runs east/west but measures north/south location. I also looked closer that Shooter does have an azimuth input but it doesnt seem to change the results any.

[DirtRacer151]

Correct. I forgot that it runs east/west but measures north/south location. I also looked closer that Shooter does have an azimuth input but it doesnt seem to change the results any.

That's the direction your firing into. Hit the "get" button and spin your phone in circles. When your pointing north it should read 0 if the compass is calibrated correctly in your phone.

[brian01tj]

That's the direction your firing into. Hit the "get" button and spin your phone in circles. When your pointing north it should read 0 if the compass is calibrated correctly in your phone.

I know thats the shooting direction, but mine if giving me issues. I have a couple other programs that use the compass and they are working fine. Shooter is telling me random degrees

[DirtRacer151]

I know thats the shooting direction, but mine if giving me issues. I have a couple other programs that use the compass and they are working fine. Shooter is telling me random degrees

Stop pointing it at your belt buckle.... ;)

[buffybuster]

If you're 100yd Zero is off by 3/4MOA, then either you did not have a good 100yd ZERO from your home range or where you shooting isn't 100yds, as that would be a very significant zero shift. I would definitely NOT rezero my knobs but make the field correction (in my case I would hold off) and spot my hit on the next target. If that shift remains consistent, then I would run with that correction.

[ReligiousShooter]

I see .5-.75 MOA shift at 100 yards when going from different locations.

I zero at my home range... go to a different location and see (sometimes) a .5-.75 MOA shift... go back to my home range and to confirm zero... and I find that I am zeroed at my home range (Piru).

If I am reading Randall correctly he says he can see a 3/8 MOA difference from location to location. That's not too far off from my .5-.75 MOA observation.

You can see a .5-.75 shift at 100 yards on the same location. At the 2010 TPRC at the 100 yard stage a lot of people (including me) experienced this. We were shooting at 100 yards and all of a sudden the POI changed and we started to hit higher. To some people it was as little as a "click". To me it was .75 MOA.

[brian01tj]

Im sure a 3/8 shift is what Randal meant but thats probably on the far side of his shifts. Im still thinking your shift, which is double that, doesn't seem right. Does it stay there for the entire shooting session?

Any chance you're using a wood stock? They can be sensitive to temperature fluctuations (so Ive heard).

[Seth8541]

How r you getting your ranges? LRF or are you taking there word for it? You sure its in yds and not meters or vice versa? Just a thought.

[buffybuster]

Let's say your home range is pala and you got dope there on a cool day.
Actual elevation is 500ft and density altitude would be around 800ft.
Then let's say you go to WEGC on a warm day.
Actual altitude is around 1400ft and density altitude will be around 3000ft.
That's a significant enough difference that your dope gathered at pala is not going to be accurate, especially if your longrange hard dope did not take the uphill firing angle into account.
When I shot a 308, I had once figured out and proven to myself that a 1000ft density altitude shift made a difference of 1 moa per 1000yds.
So at 500yds, there was 1/2 moa dope change per 1000ft DA change and 1/4 moa change at 250 yds etc...

Following that logic, the change in DA alone does not explain the 3/8 moa zero shift I see between different ranges.
That's why I started the "places we shoot" thread which shows the direction of fire for each range.
As corny as it is, I believe Coriolis comes into effect to screw up our zero when we drastically change direction of fire.
This is more evident as a windage shift with a north to south or south to north change than with an east to west or west to east change.
When changing east to west or west to east, you will get more elevation change than windage change.

So what I always do when I go out of town is to figure out the elevation and direction of fire of the range I will be going to and I stop somewhere with simiar conditions and re-zero my rifle.

Randall,

You got me thinking..... Coriolis Effect will have NO effect on your 100yd ZERO.

Calculations:

Given:
Circumference of Earth: 24901.55 miles (I rounded this off to 24,000miles as I didn't want to keep typing that into the calculator)
Milradians of Circle: 6283 mils
One rotation of Earth is 24hrs ~ Angular Velocity at Equator ~ 1,000mph
Let assume Worst Case: At the Equator, shooting directly North or South (This will give the most deflection)

Mils/Hrs
6283 mils/24hrs = 261.7917mils/hr
261.7917mils/hr/60min/hr = 4.363194mils/minute
4.36319mils/minute/60sec/minute = 0.07272 mils/second

Time of (bullet) Flight: .308Win 175SMK @ 2500fps = 0.124sec

Total impact of Earth Rotation at 100yds:

(0.07272mils/sec)(0.124sec) = 0.009mils shift at 100yds due to Coriolis Effect (worst case) about 1/32"

If you're getting a 3/4moa, 100yd zero shift, it isn't from Coriolis. Highly doubt it from angle (unless it's pretty extreme) or D/A (not at 100yds). Possibly Wind, one way or the other or incorrect zero distance.

[brian01tj]

Randall,

You got me thinking..... Coriolis Effect will have NO effect on your 100yd ZERO.

Calculations:

Given:
Circumference of Earth: 24901.55 miles (I rounded this off to 24,000miles as I didn't want to keep typing that into the calculator)
Milradians of Circle: 6283 mils
One rotation of Earth is 24hrs ~ Angular Velocity at Equator ~ 1,000mph
Let assume Worst Case: At the Equator, shooting directly North or South (This will give the most deflection)

Mils/Hrs
6283 mils/24hrs = 261.7917mils/hr
261.7917mils/hr/60min/hr = 4.363194mils/minute
4.36319mils/minute/60sec/minute = 0.07272 mils/second

Time of (bullet) Flight: .308Win 175SMK @ 2500fps = 0.124sec

Total impact of Earth Rotation at 100yds:

(0.07272mils/sec)(0.124sec) = 0.009mils shift at 100yds due to Coriolis Effect (worst case) about 1/32"

If you're getting a 3/4moa, 100yd zero shift, it isn't from Coriolis. Highly doubt it from angle (unless it's pretty extreme) or D/A (not at 100yds). Possibly Wind, one way or the other or incorrect zero distance.

Thats exactly what I was saying in my previous post....lol jk. Nice explanation. But I would have assumed that the coriolis effect would causse a difference of more than .09 mils at 1000 though.

[buffybuster]

Thats exactly what I was saying in my previous post....lol jk. Nice explanation. But I would have assumed that the coriolis effect would causse a difference of more than .09 mils at 1000 though.

I'm only talking about 100yds. At 1000yds, the effect would be considerably more . It would be about .12mils...... one or two clicks at 1000yds. Unless you're shooting in a 1000yd tunnel, it would be "white noise" to the wind adjustment. Though that is just about the size of the X-Ring on a 1000yd F-Class target..... so I suppose it's something for shooters to think about.

If you're shooting artillery with a TOF of ~6secs, the Coriolis does become a major factor.

[264Charlie]

Say you fire at the first round at a new range and notice it's .75 off. Have you ever firing 20 or 30 rounds without adjusting? Are they are .75 off too or does the zero "fix" after X rounds? I know this would not be an option in a match but it might help you resolve the issue.

You might also want to check your scope to see if it has a backlash issue.

[ReligiousShooter]

Don't really remember. POI change due to heat is a possibility. I essentially have factory guns with factory barrels.

I rezeroed my .243 and my .308 at Piru today. I'll try to see what it does at Angeles tommorow.

[brian01tj]

Don't really remember. POI change due to heat is a possibility. I essentially have factory guns with factory barrels.

I rezeroed my .243 and my .308 at Piru today. I'll try to see what it does at Angeles tommorow.

How many rounds did you shoot after zeroing today to verify that you had an absolutely solid zero? Tomorrow you need to shoot 20+ rounds to check it.

[ReligiousShooter]

With the .308, I shot four rounds at four different dots. Then after adjustments I did three shots to confirm. Another 3 slow on another dot. Then another 3 fast on another circle to confirm.

I was having problems with the .243 at the last CAPRC match. I shot 3 and didn't hit paper. Looked at the rifle and saw the rear screw looked like it backed out. So I tightened it up and shot 3 and 3 to get to a zero. Shot 3, then 3, then 3, then did the final adjustment. Then I shot 6 to confirm.

After shooting the .308, it dawned on me that shooting a .243 regularly will make you a pussy.

[Seth8541]

LOL

In all seriousness though, back in the Marine Corps we never worried about the coriolus effect until shooting elr. This may have been due to the fact that our scopes were in .5moa increments in which case making an adjustment of .12 mils would be insignifacant when concidering the other factors ie wind, moving target, heart beating its way out of your rib cage etc. Not to say you should not be aware of it and when to take it into account, but its kind of on the same plane as humidity. When all other factors are considered, it makes very little difference in impact. The .5-.75 moa impact is more likely to be caused by incorrect range, scope/rifle components not remaining torqued, inconsistencies in the shooters position, rifle cleaning methods and #of rds fired between cleanings, shooters perception of the target is different causing a difference in his POA, incorrect NPA, ammunition inconsistancies, etc. I liked what Charlie asked about if it was all the rounds fired impacted @ the same .5-.75 change or if they eventually calmed down and fell back into the original zero position. If so this could be contributed to cleaning methods possibly. Just some thoughts to concider.

[buffybuster]

In all seriousness though, back in the corps we never worried about the coriolus effect until shooting elr. This may have been due to the fact that our scopes were in .5moa increments in which case making an adjustment of .12 mils would be insignificant when concidering the other factors ie wind, moving target, heart beating its way out of your rib cage etc. Not to say you should not be aware of it and when to take it into account, but its kind of on the same plane as humidity. When all other factors are considered, it makes very little difference in impact. JMHO

+1 This....

[Gnzrme]

I frequently have shifts in my zero when going from range to range, but it's no more than a 1/2 MOA. I also have a rifle stock that has a lot of custom holes in it and its not bedded, so sometimes I have changes in my zero/POI that I chalk up to that....