POI rises, velocity doesn't. I'm baffled.

[DownhillFromHere]

Two rifles: a CZ 455 .22 and a Tikka T3x Varmint .223. Both in MDT chassis. CZ zeroed at 50, Tikka at 100. Strelok Pro has been giving me excellent trajectory curves / settings for whatever range.

Until recently. Over the last few weeks, summer has definitely arrived in central North Carolina. I am now seeing POIs for both rifles at longer ranges (100-300 for .22, 400+ for .223) rises as much as 3-4 MOA over a 3-hour range session, and the Strelok-produced trajectories don't match the bullets' flight after 75 yards for .22 and 250-300 for the .223.

I thought maybe ammo was getting heat-soaked over time, rising from 74 degrees (A/C in house, car) to ambient (90-95) with concomitant velocity increase (all gear and ammo stay in the shade throughout the session). So I checked that today. I left .22 and .223 ammo in the car overnight to heat soak (garage stays hot in the North Carolina heat and humidity), also left some .22 and .223 ammo in a cooler surrounded by bottles of tap water in the house overnight. Thus, I had "hot" ammo and "cool" ammo at the range. Compared velocities via LabRadar. No significant difference; certainly not enough to explain the rising POIs at longer ranges.

Bottom line is, I am seeing impact points for two different rifles rise as much as 3-4 MOA and Strelok-predicted trajectories disintegrate as the day's heat increases, but muzzle velocity goes up minimally if at all. Zero remains good.

This is a relatively recent phenomenon - just since it got southern-summer-hot. Can the sun heating the ground (sparse grass dying on hardpan) to well over 100 degrees account for what I'm seeing? Any thoughts on how to deal with it?

 

[ShtrRdy]

Aside from the ammo temp causing velocity change, another thing would be higher air temps will cause less dense air. Less dense air will cause your impacts to be higher. Higher humidity will also cause less dense air, but to a lesser extent.

 

[m1match]

Although Strelok Pro should have compensated for the temperature if you had adjusted the temp input as they changed. I have shot recently in 90 + deg heat with a lot of mirage and in calmer wind conditions rising mirage can make the target appear to be higher but I'm not sure mirage could cause a 3-4 MOA change at 300 yds, I've never seen that.

 

[Deleted member 10043]

Does it slowly rise over time? You will need to repeat this phenomenon in the field. Also, some screen shots of your Strelok home page and other settings would be helpful.

 

[DownhillFromHere]

Aside from the ammo temp causing velocity change, another thing would be higher air temps will cause less dense air. Less dense air will cause your impacts to be higher. Higher humidity will also cause less dense air, but to a lesser extent.

Agreed. But the confounding part is I verified velocity change was minimal and humidity change was also minimal (55% at the nearest weather station; I don't have a Kestrel - yet). Temperature rise during my session was about 13-15 degrees.

I had a PM from another forum member who brought up mirage as a potential cause. At this point, that makes most sense. I'm shooting from a 36"-high bench and the smaller steel plates at the longer ranges are only a foot to two feet off the ground. As the ground heats up, I would think mirage effect at that very low height would be intensified, and could potentially explain why my dope holds true at closer ranges.

Ehhh... but not so much for the .22. Yesterday, I verified my zero at 50 yards and put five rounds straight on a 1" swinger at 100 and then three rounds onto a 4" plate at 175. Two hours later, I was hitting 1-2" high at 100.

I also made sure I didn't leave rounds heating up in a warm/hot chamber....

 

[DownhillFromHere]

Although Strelok Pro should have compensated for the temperature if you had adjusted the temp input as they changed. I have shot recently in 90 + deg heat with a lot of mirage and in calmer wind conditions rising mirage can make the target appear to be higher but I'm not sure mirage could cause a 3-4 MOA change at 300 yds, I've never seen that.

The 3-4 MOA change at 300 is with .22 - .223 is more on the order of a minute at that range. But mirage at 100 yards doesn't seem like it would have much if any effect at all.

I don't know how to adjust Strelok for temperature only - I only see velocity-temperature pairing, and, as I said, velocity change is minimal. I'll look at it some more.

 

[DownhillFromHere]

Does it slowly rise over time? You will need to repeat this phenomenon in the field. Also, some screen shots of your Strelok home page and other settings would be helpful.

Yes, it seems to be a gradual rise.

Any specific Strelok settings you want to see? I've taken some notes; will try to get some tabular comparison between Strelok settings and what was actually observed. I'm using Wolf Match Extra ammo, which has an entry in the Strelok cartridge database (I correct velocity; I observed 1059fps@57 degrees, 1080@88 degrees, same ammo lot).

I agree that the phenomenon needs to be repeated. Hoping to get some degree of handle on this before a 30-June match with targets out to 300.

 

[Gunfighter14e2]

Simple, barrels were either never stress relieved correctly, or the more common issue, barrel fit to reciever. Once a barrel warms up it starts moving an if there is even a minor Issue with fitment the target starts to reflect that fact. Changing loads will not solve that issue, cooler range temps will help but the problem will never go away until properly corrected. You see this even in factory heavy barrels as well. There is a reason smiths suggest truing the recievers an refitting the factory barrels properly. Most factory tubes make better tomato stakes.

 

[Deleted member 10043]

What ammo for .223

 

[DownhillFromHere]

What ammo for .223

69 grain SMK @ 2816fps (Defender Ammunition - new brass). This is a new lot; the five zero verification rounds at 100 yards made one symmetrical ragged hole... but over the course of 24 rounds fired over a span of 34 minutes, I saw an ES of 148fps and SD of 45. Ouch.

The previous lot of this stuff had an ES of 112 and SD of 27. Not good at all by Hide standards but the rifle loved it accuracy-wise and, from what I've read here, not atypical for factory ammo in a Tikka rifle. So I bought a case of it at a good price...

 

[Deleted member 10043]

The only thing I can add is see if it repeats itself and make sure your atmospheric settings are correct in your app. Try using density altitude and check often. Check the weather almanac/*history for that day and time for anything out of the ordinary. You have two rifles experiencing the same phenomena. You can reasonably rely on the conditions, ammo, or shooter as cuase for that day to begin with.

 

[DownhillFromHere]

The only thing I can add is see if it repeats itself and make sure your atmospheric settings are correct in your app. Try using density altitude and check often. Check the weather almanac/*history for that day and time for anything out of the ordinary. You have two rifles experiencing the same phenomena. You can reasonably rely on the conditions, ammo, or shooter as cuase for that day to begin with.

Roger that, and thanks.

By the way, I disagree with your sig line. It was Abe Lincoln who advised caution with internet quotes.

 

[CharlieNC]

So the 223 changes 1 mil (3.5moa) at 400yd during a given shooting session? My 223 drops 1.7 mil at that distance and the ballistic calculation only varies .1 mil due to daily temp increase. Something is very wrong to cause 1 mil change in this situation. I'm shooting in your area and have not seen anything to cause such a major deviation. Since you are seeing the same effect on two rifles I would suspect a shooter induced issue, been there done that too. Try another day, bet it doesn't repeat.

 

[DownhillFromHere]

Eureka. ID-10-T error.

For the .22 - I forgot that the distances from the benches to the berms is not "exact." More precisely, bench->berm distances are reasonably close, but the targets are IN FRONT of the berms. Ten feet isn't much to a 6mm or larger bullet inside 500-600 yards, but at 300 yards it's a MOA or so to a standard-velocity .22.

When I factor in that berm->target offset for the .22, the world spins properly on its axis again. My 50-yard zero was/is correct because I ranged it with a LRF. I knew the target stands / steel were 2-10 yards short of the "advertised" even-yardage marks (100, 200, 300, 400, etc.) but I simply didn't factor it in. I have only recently started getting serious about shooting past 100 yards with a .22 - say, from about the time the weather got hot.

For the .223 - Well, I shot 25 rounds today and they did what they were supposed to. The only "wtf" moments were when I sent 3 rounds over the top of the 550-yard full-size silhouette while holding on a centered dot. The problem, again, was traced to ID-10-T. I had scope cranked to 25x. I had inadvertently "locked on" to my 1/2-scale silhouette at 300. When I actually shot at the right target, things went much better.

Thanks to everyone who contributed to this thread. "Shooter error" was the correct answer for this one!

 

[Subwrx300]

This is odd for sure but have to agree with @CharlieNC . Way too much shift and coincidence to have two rifles doing the same thing (nearly 50% decrease in drop!?) not to be something between the shooter, scope, parallax and/or all three.

Edited as I just noticed he found the cause... ?

 

[Greg Langelius *]

There's more to POI than the simple mechanical factors. There's also the recoil induced factors.

As the bullet accelerates, Newton's Third Law demands that the rifle accelerates opposite by a corresponding value. How this affects the supporting structure (the shooter) is a complex matter.

The short of it is that the rifle literally never points where it was aimed, ever, by the time the bullet departs on its free trajectory. This will vary by as much as the supporting structure's reaction varies over the day. If you become tired, or relax, or tense up; all these conditions affect(change) the POI.

Think in terms of muzzle flip. If the recoil drives the body so it allows the muzzle to climb, the POI will be higher. If the body responds to that recoil in a trending fashion, the result will be a trending shift in POI.

The ideal is that we do everything consistently. But we don't. We get really close, but we don't do it perfectly. This is a key component driving dispersion.

I would take the SWAG (Scientific Wild Assed Guess), and suggest that your own body's response to the effort, the environment, and things like muscle tone, hydration, and blood sugar levels will have a cumulative effect on the POI as the day progresses.

This is why we shoot differently off a rest than we do off a bipod, because the supporting implements will guide the rifle's recoil differently.

So what would I have you do? To answer that, I'd venture that the body is more supple and more relaxed as the day progresses. So what I'd suggest is that we start the day so we are more like the way we end up, more supple and relaxed. Always take that moment to settle in behind the rifle, oxygenate, and relaaax... after achieving NPA and Sight Picture. Then release the shot.

The point here is that there is a system, composed of rifle and ammo, shooter, and environment. They are all, each, crucial to the POI. To address inconsistencies, all factors need to enter into the consideration. All factors need to be consistent for a consistent outcome to result.

Greg

 

[SLG]

Glad you got it figured out, that stuff can be very frustrating.

I had a somewhat similar issue last Sat. Took a known verified gun, to a known verified range. A month ago, when it was slightly hotter, all my trajectories were spot on from my Kestrel. Sat., the same dope was giving me high hits from the get go. At 544 yards, my Kestrel was saying 4 mils, but I actually needed 3.7 Same deal at 643 yards, I needed .3 mil less then predicted. Temps were about 10 degrees cooler than the last time I was out there. Weird. Any thoughts?

 

[Subwrx300]

I believe the ops last post states that he figured it out. Had a momentary "miscalculation" in target he was firing at. He used the flat value for each yardage (ie. 300) for his .22LR and actual targets were a few yards in front of that (ie. 293). Big difference for a 22LR.

And he had accidentally scoped the 1/2 scale target at 300 thinking it was 500yards. Nobody's ever done that, right? ? Sometimes answers are simpler than we think.