PRS Talk Exact zero

[anotherwannabe]

Woke up last night wondering how much it really matters if your zero is off by a bit for PRS style shooting.
Couldn't get rid of the itch so wrote a little program to figure it out using basic geometry.

You can compute a relative target size by looking at the % coverage of two overlapping circles.
First circle is target size (lets use 1moa).
Second is the 6-sigma group size for your gun (lets also use 1 moa, because your 3 shot .5 gun probably more like a 1 moa for 999/1000 shots).

Note that it doesn't matter if your zero is .1 high, right, left, or low.
The relative target size is all the same as long as you measure straight from the center of target to center of your group.

Above example is 1moa target, 1moa gun and being off by .1 moa on your zero is about 6.5% smaller effective target.

If you plug in the numbers for a 2MOA target (more typical PRS) and 1MOA gun, your zero can be off quite (0.5 MOA) before you see any degradation in effective target size.

However if you have a 0.5MOA target for a 1MOA gun on a KYL rack, your effective target gets a lot smaller than 0.5 MOA really fast if your zero is off.
Even with a dead nuts wind call, your zero better be inside .1moa

So for 95% of PRS shots, being off .1 or even .2 mils on zero is totally fine and barely hurts your hit probability.
If you're hunting those last few points on a KYL after hitting all the others, then it suddenly matters a lot.

Makes me wonder why guys are ever trying to "tweak a zero" on the zero check unless they've dropped the gun on the way to the match.

 

[southerngolfer]

This makes a lot of sense and I’ve always wondered how much an absolute zero is needed to shoot effectively. Thank u

 

[Lowlight]

Did you scale it,

As your zero being off will throw your effective drop off, I don't see where you compounded the error for distance,

So while the group size may appear to cover the target, did you calculate in being off elevation wise that will grow with the angle.

if you do a true range zero calculation, being off .1 mils gives you about a 140 yard zero, so now you have to map in all the variables that go with your dope.

In other words, look at the group as a circle, but then look at where the center of the circle will fall and how of the plate is no longer available from that perspective too

We see guys mess up their zeros when they reset their turrets, .1 or .2 off, then when they dope the rifle out, their drop no longer lines up correctly and the error grows at distances

 

[Sheldon N]

We see guys mess up their zeros when they reset their turrets, .1 or .2 off, then when they dope the rifle out, their drop no longer lines up correctly and the error grows at distances

I'm having a hard time following the idea that the error grows at distance.

I'm imagining this.... I grab my rifle which has a perfect zero, dial 0.3 left and shoot with center aim and ballistic solutions based off my 100 yard zero/dope. I'm going to hit 0.3 left at 100 yards, 0.3 left at 500 yards, and 0.3 left at 1000 yards. I would think the same if I dial in a 0.3 offset in the vertical axis.

My ballistic solutions have not changed, my dope has not changed, only issue is the crosshairs are in the wrong spot which would be a linear offset.

 

[anotherwannabe]

I could be wrong, so double check my thinking... but since this is all unit-less math in terms of MOA (could have done it in mils too) - the effective target size vs group size ratio is all that matters since it scales with the distance.

I was thinking about it at 100 yards, but...

if you had your dope dead nuts (regardless of zero being off) and no wind, the math should hold for a 1 MOA target at 200, 300, ... yards on out.

Conversely, if your zero is dead on at 100 yards, but at 500 yards your dope is incorrect (.2 mil low, call that .6MOA), thats the same effect as having a non-perfect zero and you're only shooting at about 66% of a 1 MOA target because your dope is off.

Simplistic geometry calculation eventually should yield to monte carlo uncertainty and distributions (are you within 2mph of actual wind, etc) for a better answers, but AFAIK all of the Litz books and WEZ software calculations I've seen always assume a perfect zero.

 

[Lowlight]

But you are no longer using the center of the plate

this is the difference between reality on the line, vs trying to crunch numbers

As well Group Size is not linear, a 1 MOA group at 100 does not mean a 1 MOA group at 200, 300, it's usually a lot bigger. If you are a 1 MOA shooter at 100 yards, I guarantee your group at 300 will be bigger than 3 MOA

But you stick to your linear math, I will play the angles

 

[seansmd]

I think this should hold true at distance in a pure math standpoint, and is how I understood how to use other ammo profiles by capturing zero offset, and adding that in to my ballistic calculator.

I would assume if you dont have perfect zero, or don't compensate for it in your solver, and then TRUE your MV to compensate for it in the vertical, this will get all F'ed up?

 

[anotherwannabe]

Right - the whole point of the above is to say how much of the plate are you "looking at" because you're not centered on it.
That shouldn't change regardless of distance.

Totally agree that even if you can shoot .5 groups all day at 100, even in perfect conditions you won't repeat that at 1000. People use bench rest national winners/records as an example that groups don't get bigger at distance, but those are actually anomalous results by definition, not the actual average over 1000 shots.

To account for groups growing at distance in above math you just plug in a different ratio of what the group size would be some fixed fixed plate. So at 1000 yards your 1MOA gun is now a 2MOA gun on the same 1MOA plate and getting your zero right matters more. If there was some "rule" for how much group sizes grow from 100->1000 you could plot this data as some fancy 3d surface, which is what you're thinking about (I think).

 

[anotherwannabe]

I think this should hold true at distance in a pure math standpoint, and is how I understood how to use other ammo profiles by capturing zero offset, and adding that in to my ballistic calculator.

Heh - my app solver has this feature too and in theory it could/should work, but its so f'ing complicated to know if you've got it setup/entered right that I've never even tried to use it. 1 gun, 1 ammo, 1 entry in the solver!

 

[Lowlight]

It changes in reality, I guarantee it.

error accumulate and knowing where the exact center is, is not always obvious

you are changing how much and where the plate is being covered,

It's sample size as well, where is your center, most call a hit in the middle of the plate good, others might see it being part of the group but which part, center, left of center, below center.

Then the angles take over and the plates actually get smaller and smaller, it;s why we use watermarks religiously on plates and our truing targets are vertically much smaller than normal to fix this exact problem, especially noted on steel.

At distance the angle of error will grow, so the plate is getting smaller in two planes not just the one, if you are off a .2 you will miss this plate so being off .1 with your zero and being off .1 with your dope means a miss.

 

[anotherwannabe]

Lowlight - agree that dope error and zero error compounds, no question there.

Your first picture is literally the example of how to calculate the effective target size for two circles. The relative size of the circles and their offsets from perfect center are the only things that matter. The error from your zero being off doesn't grow, its fixed like sheldon mentions above. Shoot a laser at a target that has no drop, if its off .1 mil at 100 its off .1mil at 1000.

We don't shoot lasers, we shoot unreliably grouping bullets, add in miscalculations on dope and your life gets worse without a doubt.
In my experience if your dope is off at 500, then unless you've done some crazy reverse compensating BC and MV changes in a solver, your dope will generally be off yet more at 750, horrid at 1000, etc.

So my conclusion is that once your zero is "good enough" truing your dope is far more important to spend time on. I don't think thats at-odds with getting the best trued waterline/dope you can, which is also much harder due to environmental variance.

 

[Lowlight]

You are assuming everything is linear, it's not

the errors often grow, you are assuming the .1 error is dead-on consistent, and most will dope with that error included and that error will shift. So now instead of using 2.8 at 500 yards, you are using 3.0, which compounds off the center plate shrinking the plate size further.

zeros will change too at a distance so you cannot assume it's center every time, are you trying to say you never shot at distance and things have never been "off", we see off all the time.

First off, a shooter managing 1MOA at 100 is not a "good shooter' he is gonna have more than one error. The academic exercise may look good to you on paper, but the reality is not there. It throws everything off.

As I noted, a .1 Mil offset can change your exact zero from 100 yards to 140, try changing your solver to see this vs continuing to tell it you have a 100 yard zero, you don't anymore.

If you are off .25" at 100 yards, at 500 yards you will be technically speaking 1.25" low, so you are gonna need more dope at 500, providing you maintain that .25 offset. Odds are that offset will move a bit. But combine the 1.25" with a 1 MOA group size and you might be 2"+ front point of aim meaning, you will change the dope to line up at 500 vs what might be needed to dial to center.

Nothing in reality, works in a straight line, trying to say a 1 MOA shooter will maintain that group average out to 1000 yards is comical at best, you have obviously never been around real shooters on an actual firing line. A 1" shooter at 100 will often shoot 2.5" at 200, 3.25" at 300, 6" at 400, etc, it's never linear.

On paper every rifle should drop at distance, when we tried the Gunsite Drop Drill in class, we had one rifle every class that refused to drop correctly and line-up. We would zero at 100 yards, move to 300 yards on paper and have the shooter give us a group at distance. We then would measure the group center from 100 and put that into a computer for some basic data. It did not always work nor did it always line up correctly.

I believe Casey on here had his 224V not really drop at 300 but still needed the average .8 mils to hit the target. But with nothing on the scope, it would hit repeatedly just below center only an inch or 2 from point of AIM. Seeing this over and over each class, we stopped the practice because reality did not line up with the math.

 

[morganlamprecht]

Makes me wonder why guys are ever trying to "tweak a zero" on the zero check unless they've dropped the gun on the way to the match.

its always interesting to see for sure

i havent checked my zero at a match since...2015??

i check it on the few days i true dope...adjust my app/kestrel as needed, and the zero stays where its at unless i physically change something on the rifle or load

i could take people to 3 different ranges in tx and shift their zero's .2 up or down based on the location of the sun alone

 

[anotherwannabe]

i could take people to 3 different ranges in tx and shift their zero's .2 up or down based on the location of the sun alone

Its really hard to convince yourself its an just an illusion when it happens and I'm certainly not immune. I like to shoot a group just to make sure there is no gross error from the truck-ride bumping along, but I've had a set of guys in my group all say their zero had shifted .2-.3 (magically in the same direction) so they adjusted it, same guys were then off all day.... go figure.

Above data was to help convince myself that a minor shifting .1 mil zero on a quick check also isn't "that important" on most PRS sized plates. So more risk than reward trying to make some last minute adjustment before a match.

 

[Lowlight]

Matches used to have a certain amount of paper, that made sure you had a good zero.

Removing the 100-yard paper completely gives you a ton of fudge room for sure, steel is super forgiving, so a zero can be "off" for lack of a better word, but your thought process behind the equation is flawed.

A left-right shift is just gonna mess up your windage, not much else, and with edge of plate holds, that is muted. It's the up - down stuff... the other thing we see with elevation is using diamond targets. We shoot a lot of diamonds, being off a tiny amount elevation wise will hurt you with a diamond

If they put paper stages in place, you'd see a lot more emphasis on knowing how close to the center that zero is.

Wouldn't take much, a paper KYL or a few dots to score, guys would be much more conscious of it.

Lastly on Snell's Law,

In Alaska, with their high moisture content in the air, we see a .5 Mil shift at 1000 yards, it lasts just about an hour, maybe a hairless, but one minute you are shooting center on a plate at 1k, the next minute you are .5 mils over the top. The sunlight and water content in the air does crazy stuff, then as fast as it happens, things go back to normal.

 

[Dthomas3523]

its always interesting to see for sure

i havent checked my zero at a match since...2015??

i check it on the few days i true dope...adjust my app/kestrel as needed, and the zero stays where its at unless i physically change something on the rifle or load

i could take people to 3 different ranges in tx and shift their zero's .2 up or down based on the location of the sun alone

I read a post of yours a few months back about leaving zero alone once dope is done prior to a match and not “tweaking” it the morning of.

I don’t think it’s a stretch to say that alone increases my scores 5 hits or more a match.

 

[seansmd]

I took his analysis to be strictly theoretical and NOT taking into account shooter or environmental shifts at short or longer ranges. Simply how much does a zero offset affect target size. I did not read into it that he was dismissing shooter error, exterior ballistics, or environmental impact to aiming. I might be reading his original post wrong.

 

[Dthomas3523]

Also, I’d agree that getting as close to perfect dope as possible is more important than having a 100% zero (or correct zero offset), *if* you have to pick one over the other.

However, if you’re taking the time to dope properly, you should take the time to get the zero or zero offset g2g (unless you gather dope before each match or outing and make calculator line up).

I think the biggest reason I don’t check zero the day of (unless I screwed the barrel off or something else) is not to mind fuck myself into either a) changing zero because of lighting or me or b) thinking in the back it my head “my zero is slightly off” on every shot.

 

[Sheldon N]

interesting discussion. While I think there are lots of non-linear factors at play, I don't see zero offset as one of them. Sure your group size is going to be larger and because of that you might miss by more than your zero offset, and if your group doesn't overlap the plate shape then hit percentage is going to be worse. But ultimately the center of the group is going to be offset from a "perfect zero" group in a linear fashion with distance.

 

[JustSendit]

I always thought if your zero is off .2 at 100 then that is consistent .2 off at every target. I see students with those shifts hit exactly .2 high on targets from 5-850m. I see bad MV as an arc that grows inconsistently, or with the arc of the ballistic curve.

 

[morganlamprecht]

I always thought if your zero is off .2 at 100 then that is consistent .2 off at every target. I see students with those shifts hit exactly .2 high on targets from 5-850m. I see bad MV as an arc that grows inconsistently, or with the arc of the ballistic curve.

see the same...assuming the optic works as its supposed to....same error @ every distance, zero issue...error changes as distance changes, MV/BC issue

 

[jsimonh]

Dragging this up just because I was searching some info on zero's and found the discussion interesting. I'm personally a zero whore lol. I obsess over getting it just right to the point its ridiculous. However after I get it set, I don't check it or mess with it again unless I change the load somehow. I've seen guys change their zero the day of a match and have hell all day. I recently have been fighting with my zero on my FF load on my dasher. Here in SW Oklahoma we are hitting 40's at night and upper 80's low 90's during the day with a lot of moisture on the ground. I live in the country and have went as far as using flashlights to light up the target and shoot late at night to mitigate mirage. Finally got it confirmed yesterday morning, and ended up about two tenths on an inch right. Found this thread researching if that was really gonna matter or not.

 

[Lowlight]

This was pretty good

I wish I had more images of the 100-yard groups from students, I know I have a "bunch" but I wish I tracked one student or one single group over the 5+ groups we shoot in a class.

While the elevation will be "pretty consistent, they are never perfect, nor do they repeat in such a way that the elevation is not constantly shifting a small amount.

Normally the targets we shoot, we have a lot of wiggle room. We have shot guys at 200 & 300 yards on steel, pretty small targets, but then moved to 3" targets on paper and find the variation is pretty noticeable. It's not uncommon to have a guy hit steel with no problem, 4" & 6" plates than when moving to a 3" Paper target miss the target completely requiring a minor tweak.

Consistency is the key and consistency tends to the biggest problem with new shooters. In Version 3 of my Fundamental Eval Sheet, I had to add "Multiple Press" and Multi-Placement" when it comes to trigger control. Think of it this way, the group center of the target when shooting groups at 100 can and does "dance" around a circle for lack of a better term.

Have I seen students who are off .3 and it carries out with their dope, yes, but again we are shooting pretty forgiving plates so you can conceal a lot in there so the .3 appears consistent. For the sake of 2 MOA or bigger steel it probably is, but move to 1 MOA or less and things start to walk around a bit.

Studying the True Range Zero can be helpful to see just how big small variations can be, a 1/4" here or there throws things around a lot. The bigger the plate, the more wind you deal with, the easier it is to hide it, but it's there.

 

[Lowlight]

Just a quick scan of my images on my phone and I found this, group shooting my Valkyrie,

Following the order of images from my photo library:

Group 1

So about 7 Oclock on the 10 ring,

Group 2

I opened up a tiny bit but still roughly 7 Oclock elevation wise seems pretty good

Group 3

I see enough vertical variation on my groups that it might translate to .1 or .2 here or there.

I could easily say are they consistent, but then again, they are noticeably off.

Personally, I think it comes down to the quality of the dope, the range as presented, and then the target size. In 99% of the cases, we probably can hit a 2 MOA no problem because are looking at 1/8" variation, but I would think I am consistent enough in most cases.