I know cooler temp means denser air, which means slower velocities. What is reasonable to expect? The last time I did some shooting it was in the 70s; now it's 20 degrees cooler. I perceived a POI drop of about 2MOA at 300m, with my chosen .223 73grFTX and 10.5" barrel, relative to my last time shooting. Does this sound like something that might be temperature-induced? I know it sounds extreme, but I don't know how wild I should expect, given the already aggressive trajectory of my setup.

To give an Idea of my trajectory, my adjustments are as follows, with the zero being at 100M:
150M: 1MOA
200M: 2MOA
250M: 4MOA
300M: 9MOA

This message has been edited. Last edited by: KSGM,

Having your muzzle velocity would allow running the data through a ballistics solver while changing the temperature variable.

But being curious, I made some assumptions and came up with this data.
According to Hornady the G1 ballistic coefficient of the 73 grain FTX bullet is 0.260. Using that information and your need for 9 MOA of elevation at 300 meters (328 yards) with a 100 meter zero, I came up with an estimated muzzle velocity of 2320 feet per second. That estimate doesn’t quite match your other drop figures, but I believe this table will still give you an idea of how much trajectory difference results from changing between 55 and 75 degrees at “standard” atmospheric pressure and humidity. I used an estimated sight height of 2.5 inches.

Drop at 55 and 75 degrees, respectively:
55°: 150 m - 1.47 MOA; 200 m - 3.58 MOA; 250 m - 6.11 MOA; 300 m - 9.12 MOA
75°: 150 m - 1.45 MOA; 200 m - 3.53 MOA; 250 m - 6.02 MOA; 300 m - 8.96 MOA

As we see, the differences are miniscule just based on a 20 degree temperature change. If you have a more accurate MV value, I can run the solver with that, but based on other values I tried, I wouldn’t expect anything more significant.

So, to respond to your question, the temperature difference you mentioned should not be the cause of a 2 MOA POI shift at 300 meters.

Added:
In thinking more about your question, something does occur to me.
I listened to a recent Applied Ballistics podcast about zeroing, and one of the commentators pointed out that even a significant change of clothing that’s worn can have an effect on point of impact. That’s evidently due to how the clothing can affect the various pressures and body positions involved when shooting.

He didn’t mention any specifics and he was referring to zeroing in good weather and then wearing heavy clothing while hunting, but considering where you live does shooting in the 50s require heavier clothing than in the 70s? That may be a long shot (NPI ), but something to consider? How about any other changes in shooting positions, etc.?

No other changes. It is likely a result of my "system" showing it's ass. I am still employing two optics on the one gun; so long as the QD mount off/on is a factor, it's likely the factor. I asked in hopes that calculations similar to those you performed would prove that temperature had an effect; it's not the case though.

Is it fair to say that temperature fluctuations won't ever have a meaningful impact on POI, inside of 300M? It seems to me that a fluctuation of something like 50 degrees, in either direction relative to the 75 degree starting point, isn't going to matter, even in a somewhat precision context.

To answer your specific question it would be necessary to look at specific loads. A total 100 degree temperature swing would be a lot and beyond what would be encountered in any common situation. Higher initial velocities would be affected more due to the fact that higher velocities degrade more quickly; I also believe (without checking now) that bullets with lower ballistic coefficients would be affected more by changes in air density. But 300 meters isn’t really far enough for some factors to become important.

I have long been fascinated by ballistics questions and the best way to understand the effects of different variables is to become familiar and play around with a solver that allows entering all the various factors that affect trajectories.

A good online solver is JBM. This is for basic trajectories:
https://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi
A list of calculators available:
https://www.jbmballistics.com/...rs/calculators.shtml

Some of the inputs may be a little confusing, but explanations are provided for most. Users here can also answer many questions.

Added: As usual ( ) your question prompted a bit of checking. One set of data I have readily available is for the Federal 308 Winchester 168 grain Gold Medal Match load at sea level and no wind. At 500 yards the total drop of the bullet at 25 degrees is 82.65"; at 125 degrees the total drop is 79.93", or 2.72" difference. At 300 yards and 25° the drop is 28.31" and at 125° it’s 27.81, difference 0.5 inch. Other factors usually have much greater effects than pure air temperature. Even a crosswind can affect POI elevation.

For distances out to 300-ish yards, your temperature swing should have negligible affects on your POI. Both from air density and muzzle velocity standpoints. I vaguely recall your bringing up this question awhile ago -- likely during last winter. Sigfreund's ballistic data looks reasonable to me.

Your 2 MOA change in POI is likely a combination of:
- Regularly changing optics.
- Regularly changing ammo.
- Ammo which may not exhibit the most repeatable accuracy.
- DOPE which may not be all that repeatable.
- Shooting fundamentals that are not all that repeatable.

I recommend first eliminating the variables of ammo and optics, and working on shooting fundamentals. Develop repeatable DOPE at all distances -- not on just one day's shooting but repeatable days, weeks, and months apart. Once you have a solid accuracy baseline with one optic and 73 grain ammo, then apply the variables of swapping ammo & optics.

quote:

vaguely recall your bringing up this question awhile ago

You are correct; it was indeed last winter, and it was concerning my AR10. Like this time, the perceived POI change was certainly not due to an ambient temperature difference.

The ammo has been consistent, insofar as I clean the barrel after shooting FMJ. I pull a snake with bristles through twice, and then a bristleless snake once or twice.

I hope the Hornady 73gr Critical Defense FTX is more repeatable than some of my weird results would indicate.

My DOPE does need gone through again (and again). I have been meaning to do that.

My fundamentals are something that I would like to think I have under enough control to not see shifts of 2MOA at 300M. However, I am not too proud to consider that they may be a factor.

My switching back-and-forth between the Eotech and scope is something I am sure is starting to annoy those who may try to help me, here on SF. I do concede it is unorthodox, and I am starting to wonder if it is indeed viable. Any significant shifts are always in the way of elevation, not windage, which seems curious.

One problem has been that I bounce around so much between shooting disciplines. I get in training where I can fit it in; it's sporadic. I never have a whole day, or even a morning or afternoon, to dedicate to one thing, and I have to choose which discipline is advantageous at the time. I need to be flexible, or not practice at all.

I think I will leave the optic alone, and the ammo, for the foreseeable future. Thanks for everyone's help and motivation.

You regularly discuss the desire for increased accuracy. Repeating your current shooting methods over and over will likely continue to produce similar results -- variations in POI & inconsistent accuracy. You must isolate the variables to determine the parts of the chain which are weakest.

Your Noveske barrel is a given part of the equation at this time. I don't own a Noveske, but have shot a few and been around those who use them in competition. Assuming it wasn't made by a noob on Monday with a hangover, it should easily shoot under 1 MOA at 100 yards with the ammo it likes.

If you have an optic with 8-10 power on the top end, and you use targets that provide easy aim points, shooting under 1 MOA at 100 yards is easily attainable. Optics with 5-6 power on the top end make it more difficult, but it can be done.

I have assumed that the Hornady 73FTX is fairly accurate ammo. But I've never known anyone who shoots it. Time to try something else. With barrel twist rates of 1:7 to 1:8, there are a few factory loads that just flat out shoot well. With a competent shooter at the trigger, if the barrel/ammo doesn't produce bugholes at 100 yards, the barrel is better used as a tomato stake. The loads are FGMM 69, FGMM 77, Hornady 75 Black HPBT, and Hornady 55 Vmax. For twist rates of 1:9, use FGMM 69 and Hornady 55 Vmax. With quality barrels, these loads are capable of 3/8" to 5/8" groups at 100 yards. Day after day, month after month. With just OK barrels, 3/4" to 7/8" groups should still occur.

Shooting accurately isn't something most shooters turn on at will. It takes dedication, focused training, and often professional instruction. The fundamentals of marksmanship are simple in concept, but are often difficult to implement.

Go prone with a bipod. Or get on a bench with a few sandbags. Get off the gun between shots. Shoot dot drills. Shoot with 1 of the 4 loads I list above -- don't alternate among loads. Mount your scope and don't futz with it. Don't change anything after a day of shooting. Come back days later, shoot again, then compare results with the same setup.

quote:

Don't change anything after a day of shooting. Come back days later, shoot again, then compare results with the same setup.

This is what I shall do. For now, I will continue to use the 73gr FTX, as I am moderately invested. I think I'll do two rows of five dots; one shot per dot, coming off the gun each time. I'll use the results from the top row to make any adjustments, and use the bottom row as a confirmation. I'll then repeat the drill another day, with the top row being a 100M confirmation, and the bottom being shot from 300M, using my existing DOPE.

Alright team: I haven't taken my scope off, and I haven't shot anything but the 73gr FTX. Observe the POI shifts across two different days below...

https://photos.app.goo.gl/ZzPTmn6qGKidYcte6

The hits marked with a 2 are from 200M today; the hits marked with an X are from 200M on Wednesday. I did make an additional 2MOA adjustment, compared to Wednesday.

The hits marked with a 3 are from 300M today; the hits marked with an O are from 300M on Wednesday. Same 10 MOA dialed both days.

Scope dismounting and ammo switching variables have been eliminated. Barrel bore and silencer mount interface was cleaned prior to Wednesday's shooting. I even got a trigger! The trigger is something that changed between the two days, but that seems a very unlikely suspect.

I am beginning to consider silencer mounting inconsistencies or scope tracking problems. My shooting on both days was certainly good enough to make this phenomenon measurable. Hell, my shooting from 200M today was pretty darn good, IMO, if it had impacted about 4MOA lower!

Help me out, guys! Of course I intend to eliminate more variables, and shoot more. The first thing being leaving the silencer mounted between sessions.

I shared the fresh target from Wednesday in the 10-shot group thread, along with another zero confirmation baseline target. One thing I forgot to apply in today's session was breaking position between shots. What say the likes of fritz, sigfreund, and Indianaboy?

I wish I could be of more help about this question because It’s something that I struggle with, and it’s very frustrating. In my case, though, I do believe I’ve identified the cause: inconsistency in my shooting position but that then settles in after a shot or two.

More than once I’ve had a first shot go very high after getting into the prone then followed by a nice, tight group with subsequent shots. And it usually doesn’t matter if the subsequent shots are fired after breaking position for the first series; that in particular is a mystery, but all I can conclude is that after the first shot, getting properly positioned for the next, even after a break, is easier and more consistent in the same session. The last time that happened I had deliberately not cleaned the bore following a previous session, and on the other hand, there’s nothing about the rifle and/or ammunition that would explain a first out-of-group hit. I’ve had many successful groups starting with cold, clean bores with the same rifles and ammo. In addition, the problem almost always occurs in the prone position that I usually fire first, and then I don’t see it when shooting kneeling supported with a tripod. That’s even true when I only shoot from the kneeling supported position in a session.

I believe that breaking position or making equipment modifications between shots in a string is a useful exercise for what it tells us about those variables, but to determine the capabilities of the rifle, ammunition, and—to a degree—the shooter’s skill, I agree with the advice by fritz in the other thread to shoot some groups after getting into as secure and comfortable position as possible, and then not changing anything. If the first shot goes awry, note the fact, but don’t stop there.

As for the effect of the suppressor, do the same with and without it on the rifle. A quality unit and mounting system should prevent any loss of precision (although the POI can be expected to shift).

Regarding scope tracking errors, they are hardly unknown, especially with less than top tier scopes. I believe that the best way of checking the actual tracking values (e.g., do ten 0.1 ₥ clicks actually equal exactly 1.0 ₥ point of impact shift?) is to view an accurately calibrated chart while running through a series of significant adjustments. Unfortunately, that requires that the scope be absolutely steady with respect to the chart. More scopes are probably commonly evaluated using live fire in the “tall target test” or a “box drill.” Live fire testing, however, requires a high level of precision to be valid and useful. If our groups measure a couple of MOA at 100 yards, it’s going to be pretty difficult to decide if the tracking is accurate to less than 0.25 MOA. The same is true if our test involves firing a large number of rounds quickly: Is it the scope or is it a hot barrel?

And if it’s determined, for example, that 21 (nominal) 0.1 ₥ clicks are necessary to move the POI by 2.0 mils, that can be compensated for if it’s consistent. Some ballistic solvers even allow that variable to be entered for automatic inclusion in the calculations.

Confirming that the tracking remains accurate in use is harder, but if it’s suspected there is a problem it’s time to go back to a test again.

https://photos.app.goo.gl/v7hFupkHra7xX5rQ7

I managed to squeeze in another five shots from 300M after work. Light was waning, and there was a bit of haze between me and the target, but I still managed a decent performance (that damn first shot, though!).

I know it's getting a bit messy, but I wanted to show it all on the same target. Previous hits are covered by pasters, but I noted their approximate locations with the date/time they were performed. As you can see, this evening had me hitting right where I should have; I dialed the same 10MOA.

I left the silencer attached, and I'll try to knock out another five shots tomorrow. If those hit the same (where they ought to), I'll remove and reinstall the silencer, and shoot again.

I did break position between each shot this evening.

This message has been edited. Last edited by: KSGM,

quote:

Originally posted by KSGM:
I still managed a decent performance ....

Indeed you did.

https://photos.app.goo.gl/XMPeckh31hBra5t56

Apparently the silencer being the cause of deviation can be ruled out.

Precipitation had me on steel rather than paper. My first shot inspired some optimism, as it hit the 6" plate. I said "Awwwww yeah, I'm gonna smack these other four too". But no; I fell victim to both the shift and the first shot phenomenon! What the heck! I am at my wits end.

Those four shots after the first are inside 1MOA (of each other). I think my rifle is capable of accuracy, and I am capable (to a point) of consistency. However, these first shots, and the overall POI deviations are maddening.

I did break position between all shots. Any help from the guys is appreciated!

*It's also worth noting that I did experience a lateral POI shift today. The majority four-shot group was slightly left, as well as low.

This message has been edited. Last edited by: KSGM,

This is a thought based on something that occurred to me only just now:

Try getting down to and up from position and dry firing several times in succession before the first live round, and with emphasis on firm contact with the rifle as if you had fired. If you do that I’ll be very interested in your results because as I’ve said, I sometimes experience the same first shot high anomaly. Now that I’ve thought of it I’ll be trying it myself, but between recovering from recent surgery and the weather that varies from tolerable to impossible, it will be a bit before I can do that. It would also be good to know if it helps anyone else.

quote:

getting down to and up from position and dry firing several times in succession before the first live round.

Excellent! I will! That's perfect. Thank you.

So, thanks to sigfreund's suggestion, I may be onto something, concerning the first shot problem.

In conducting the dry fire, I noticed my scope's reticle shift pretty significantly, on the first hammer drop. Subsequent hammer drops caused only a minimal "twitch" of sorts. In my researching scope tracking evaluation, I did read about something I had previously known about, but hadn't considered: tapping the turret after making an adjustment, to "set" it. I think it's inappropriate that my Leupold VX3HD should require that, but I am making obtuse elevation adjustments regularly, so perhaps it's coming into play.

After noticing this, I got five more rounds and went out to 200, so I could simultaneously re-check my DOPE at that distance. I tapped the turret after adjusting, and experienced no first shot fluke.

I'll continue to apply this method, and we'll see how it goes.

This still doesn't help with my overall POI difference on entire groups, the other day, but I'll take what I can get.

When the reticle shifted, it did so counter-intuitively, based on results I have been seeing. It shifted down. It seems, to me, that this would make my first shot lower than my others.

_______________________________________________

It's an off-day, and my wife was content to let my daughter and I do some more-extensive-than-usual shooting. That being said, I have more to report. It seems the turret tap may indeed be most (if not all) of my first shot solution. I did not experience any more large deviations from the main body of the group, since I implemented the tap. All the below shooting was conducted from 300M.

The overall group POI shift is still plaguing me somehow. I shot these two groups in succession; the only events occurring between them being a trip to the target, and dialing to zero, then back to 10MOA. The result here had me seriously considering some kind of tracking issue. The first group had one just over the top of the gong, three above the plate, and one on it (not a great group). The second group had all five on the gong, low and left of the plate (not a bad group).

https://photos.app.goo.gl/fr74XCxxah7z9EVZ8

After that result, I opted to repeat the same conditions, to see if I would get another result that would reinforce the tracking hypothesis. I did not. I shot a (for me) very nice first five-shot group (marked with 1), and then an average second group (marked with 2). There was little/no difference in POI between these two, but the overall POI shift is observed in both these, and the previous two, groups. The results of this second pair of five-shot groups would have me, and most likely anyone else, considering dialing 9MOA instead of 10, but 10MOA was confirmed and used successfully over the past week.

https://photos.app.goo.gl/3jeAFuHLn9rdZ24GA

I am considering requesting that you guys suggest a from-square-one process, to square-away both me and my rifle. It seems I need to do something fundamentally different.

This message has been edited. Last edited by: KSGM,

Thanks for the update. It’s always good to know how other shooters are doing with these sorts of problems.

I have experienced POI shifts between groups with high tier scopes and therefore I keep going back to the probability that slight differences in shooting position or pressure on the gun is the cause.

That said, if you’re having to tap the scope to get the reticle to settle into its normal position which is a thing with some scopes, I’d definitely contact Leupold and ask them to check it, and especially if you believe you may be having any other tracking issues. I tap red dot type sights because very often they don’t seem to track very consistently and I figure the least it might accomplish is to scare off the scope gremlins. But that shouldn’t be necessary with your sight.

Although Leupold is a good line, based on their prices the VX3HD series may not always get the full attention during manufacture and perhaps something really is going on with the sight.

As I’ve mentioned before there are ways to check tracking ourselves, but if I were in your situation I’d just send it back to the factory. The worst that can happen is the expense of shipping it because if their report is, “Everything’s fine,” that might give you a little more confidence.

A number of things can cause POI shifts.

Suppressors often cause shifts upon installation, but the shift should be repeatable. A loose mount causes unusual shifts, generally downward (6 o'clock). I see this regularly if my direct-thread cans loosen up. It takes very little loosening to make a noticeable shift.

A barrel that isn't tight to the receiver will cause noticeable shifts, usually downward.

I had a Luepold Mark 4 that experienced turret lash. This line of scopes was known for mushy turret feel. For a 5-ish MOA elevation change, I would dial 5-1/4 MOA, then back off one click to 5 MOA. For 10-ish MOA elevation changes, I would dial 10-1/2 MOA, then back off 2 clicks to 10 MOA. But the error was only 1/4 or 1/2 MOA. I haven't used your line of Leupold scopes.

Bipod hop is notorious for POI errors. For many shooters the hop induces an upward and/or lateral shift. Mainly 10 o'clock through 2 o'clock. The amount of shift depends on the amount of hop.

Rear bag use is notorious for vertical POI errors. Mainly 6 or 12 o'clock. This is an area where I struggled for years. I would loosen my squeeze on the rear bag right as I shot, sending shots high. Awareness and a larger rear bag helped.

Too much pressure on the cheek weld was also an issue for me. I once had my optics height set so that I had to put a fairly firm cheek weld on the stock to have my eye in the center of the scope. I raised my scope height so that I now need just a gentle cheek weld. The old method tended to throw shots high.

Scope parallax setting can be an issue, especially if your eye isn't centered in the scope.

If the shooter's eye isn't centered in the scope, even with proper parallax setting, POI will shift. The shift can be in any direction, and may or may not be repeatable. This is one of the biggest accuracy problems for many shooters, but they generally feel it can't occur for themselves.

Trigger issues affect almost everyone. For most shooters, poor trigger control pushes shots towards 9 or 3 o'clock. It comes from not pressing straight back on the trigger. The heavier the trigger pull, the more likely the POI shift. With really heavy triggers and a mashed trigger pull, the POI shift could be downward.

Breathing can affect POI -- generally in vertical shift, both up and down. Holding your breath before and during the trigger pull can send rounds in any direction. The most repeatable method is to break the shot at the natural respiratory prize at the bottom of the breathing cycle.

Poor alignment with & behind the rifle can send rounds in any direction. This is a consistent problem for many shooters. Align the rifle to the target, align your body to the gun.

Dry fire is a shooter's friend. I learned to dry fire in the morning in my hotel room before every match. Prone, from tripod, off the bed, off a chair, bipod jammed up against a heavy piece of furniture. 30-50 pulls generally warmed me up, and I was ready to go on the first stage. If I'm training at my own range, 10 dry fires are often close enough -- although sometimes I need more. With a good dry fire, the reticle doesn't move off the POA. OK, maybe 1/4 MOA off line, but that's it. I try to dry fire until the reticle doesn't shift at all -- as if the rifle were locked in a vice. In order for a shooter to spot impacts and to spot bullet trace for close targets, a rock-steady dry fire is mandatory. A rock-steady dry fire is mandatory for precise shooting.

quote:

there are ways to check tracking ourselves

I am going to do a tracking evaluation of my own, using a Redfield sight-in target (the one with the five diamonds on a 1-inch grid). I intend to secure the scope to something like a cinder block or paver, and, using shims under the corners of said block, level the crosshair to the target grid at 100Y. I'll then do some up/down tracking assessments, and run an up, right, down, left track. I hope to be able to do that by Wednesday. If I see weirdness, it'll go in for service; if I don't I'll take a square-one approach to my system.

I thank you both, for your feedback.

I will be more critical of my rear bag influence. I use a pretty small bag, and I'd like to keep it that way. I'll work on rear bag awareness.

I am fairly certain that my body alignment is inconsistent. Not in a huge way though. I'll be more critical of myself there too.

I did my redneck tracking assessment. It actually ended up being a pretty solid setup, and the results cast doubt on my tracking theory. It was certainly nowhere near the level of scrutiny that a factory test would inflict, but I saw no identifiable deviations.

I intend to set the no-shit zero (turret locked) to 300M, on my next shooting day, and do three or four groups. Obviously, nothing about the scope is moving, so nothing scope-related should be the cause of any POI changes. We'll see what happens.