Cold Bore Coke Can Challenge, Got Me Thinking ...

[wally]

I liked the idea as the first shot is likely always the most important. I couldn't try it this time out as last time out I swapped an upgraded scope on my .308 suppressed Ruger Precision Rifle -- went from a Primary Arms 4-14x44 FPP Mildot scope to a Nikon Black FX1000 6-24x50 FPP Mildot scope. Used up the last of the Privi 165gr JSPBT ammo I'd been using so I had to switch to a new ammo, 155gr Hornady Steel Match. I knew from trying different ammo when I first got my RPR that it was 4 or 5 clicks different in elevation zero but I didn't remember which direction.

So I had to cheat a bit. Took one shot at 50 yards (my zero distance) it was high by about 4 clicks, adjusted, took a second and was satisfied with the zero.


Next I fired up Nikon SpotOn app on my cell phone and setup the parameters and let it calculate the ballistic table. This took a few minutes.

It said 0.99 mil at 300 yards, and 1.98 mil at 400 yards. So I took my pseudo cold bore shot at a 12" steel plate at 300 yards, here is the result:

It was incredibly calm, I was very satisfied with the result from a center 1 mil hold.


So after a couple of minutes basking in satisfaction, I tried a smaller target, 6" steel diamond also at 300 yards with the same 1 mil hold:



Liking the result very much, I adjusted the parallax for the 400 targets and let one fly at the 5" "heart" steel plate sitting in front of an IPSC torso plate at 400 yards with the 2 mil hold suggested by "SpotOn":



Looks like my windage zero be a bit off, so I gave it one click and shot again at another 5" plate at 400 yards holding 2 mils:



Looks like I might have needed two clicks, But since every shot had been over a minute apart while I looked and futzed around, decided I'd let the last four shots fly fast as I could work the bolt at an 8" plate at 400 yards again with the 2 mil hold:



All I can say is that its almost too easy when there is no wind! Immensely satisfied, with nowhere to go but downhill,
I put the RPR up and moved on to the task of zeroing the Primary Arms scope on its new home. All in all a great day at the range! (despite the 95 degree heat).

Next time out I will do my 400 yard version of the cold bore coke can challenge at the 5" "heart" plate and see how well I do with a truly "cold" bore.


I'm shooting off a portable bench made a lot less portable since bolting on a larger U-shaped 3/4" plywood top, and using a Caldwell folding bi-pod. I'd prefer to shoot prone, but the weeds won't allow it at the moment.

 

[wally]

Got out again today, no pictures, but truly cold bore same situation at 400 yards -- first shot was about 6" high.

But so was everything else. My holds went form +1 mil at 300 and +2 mil at 400 to 0.75 mil at 300 and 1.5mil at 400 yards. At these ranges other than the change in hold there was no detectable difference between first and last shot, at least at my skill level.

I bumped the humidity 20% from 75 to 95 in Spot-On and it didn't make any difference still calling for 1.98 mil at 400 yards. Same 95 degree heat as before, for sure.

Color me puzzled. AlI can think of was I've got confused and switched lots without realizing it, but then I would hope for much less lot to lot variation, although perhaps this is considered good for factory ammo.

 

[Varminterror]

What was your density altitude on both days?

 

[wally]

I don't know, not sure how to calculate it. I was at the same physical location, temps were with in a couple of degrees of each other both days, so other than humidity what other variable goes into the "density altitude" calculation?

A barometer in the house reads 768 mm don't know how accurate it is and I was shooting in Columbus 80+ miles away. We've been under "high pressure" keeping the rain away, supposed to be moving out bringing rain tomorrow, on TV channel is saying 100% chance I've left Spot-On at the default 29.97 in of Hg.

Both days was about 95 degrees, definitely drier than normal, 75% was what I recalled for setting up Spot-On from the mornings weather report. Today its oppressively humid "feels like temps" of 103+ Which is why I didn't walk down to take any pictures.

Maybe Spot-On has some issues, changing humidity to 20% did change the holds. But is was dead on the first time. I may need to dust off the chronograph, but is really too hot outside for me to want to mess with it for the time being!

 

[Varminterror]

Density altitude is a correction for temperature, humidity, altitude, and atmospheric pressure. It can change surprisingly from one day to the next, even with a seemingly small shift in perceived conditions. I shot a match last weekend at a physical altitude of 1390, the DA was 5000. I shot a positional primer class this weekend at the same range, DA was 3200. Rained the night/morning before both. Perceived conditions were almost identical for both days, but bullets flew higher last weekend than they did this weekend.

Something seems wonky if you were .25mil difference at 300, and .5mil difference at only 400.

 

[wally]

I agree something seems wonky, but my holdover at 400 was double the holdover at 300 (50 yard zero) so I'm not all that surprised the difference at 300 was half the difference at 400. I guess that is why serious longer range shooters use portable "weather meters".

Spot-On has a button to "get weather" which then seems to want to connect to some blue-tooth device. I'll have to investigate what device it works with and how much it costs. I did by a little wind measuring device, but I don't find it very useful as the wind swirls badly on our 400 yard range, when I spray paint the targets its often left to right at the firing line, in my face at 200 )blowing the paint back at me) and right to left at 400.

Both of these days were about as calm as it ever gets.

 

[taliv]

guys, DA is just not going to be much of an issue at 400. quick check of calculator using 308, 168g smk, a change of 0 DA to 10,000 DA at 400 yards is a 1 MOA change in POI. realistically, light conditions, deviation in MV, position, parallax, etc probably are all more capable of that change than DA. and humidity is only a small part of DA

 

[hdwhit]

wally wrote:
Cold Bore Coke Can Challenge,...

Please forgive my ignorance, but I am not familiar with the so-called "Coke Can Challenge" and what I see in the photographs appears to be steel plates (rather than Coke Cans") of various configurations shot at various distances. Can you explain?

 

[wally]

but I am not familiar with the so-called "Coke Can Challenge" and what I see in the photographs appears to be steel plates (rather than Coke Cans")

It was this thread:
https://www.thehighroad.org/index.php?threads/cold-bore-coke-can-at-100-yards.837591/

The various distances are because I'd swapped scopes the previous time out, and needed to swap ammo that time out. Once I tweaked the zero at 50 yards and "verified" the ballistic calculation at 300, figuring that in real situations the first shot will likely be the "most important" I wondered about a similar "challenge" at 400 yards, the longest distance available to me.

Wasn't a true cold bore the first time as I'd fired four shots, although with a lot of unmeasured time between them. The second time out was a true cold bor first shot. I was shocked by how high it was, but all other shots were equally high. Its possible the "zero shifted" but I've no evidence to support it at this point. My working theory at the moment is I switched lots of ammo without noticing it. I'd hope lot-to lot variations would be less than the shift I observed of 0.5 mil at 400 yards.

Adding 100 fps to the velocity Spot-On had for the Hornady Steel Match ammo changed the hold to 0.83 mil at 300 and 1.75 mil at 400 yards. Not too far off from what I'd observed. So I regard lot-to-lot variation in the ammo as the most likely explanation. So its easy to see why serious long range shooters generally are re-loaders too.

At 400 yards you are only beginning to see the effects that really make all the difference when shooting at real "long range".

 

[tarosean]

Maybe Spot-On has some issues, changing humidity to 20% did change the holds.

20% near HTown???

Columbus is only around 200ft elevation. Not exactly much. Probably need to find a better app.

 

[wally]

I changed by 20% to see what effect going from 75% to 95% humidity might have to see if that could account for my observed lower holdover the second time trying. Latter I changed to to 20% to see how much it might change the holdovers which was basically nothing, so maybe something is wrong with Spot-On with regards to humidity effects, but its calls were right the first time when set for 75% humidity.

As I said my working theory is that I unintentionally loaded my mags from a different lot of ammo. Adding 100 fps to the muzzle velocity Spot-On had in its table for Hornady Steel Match basically gave me the the holds I'd observed, so I think this is the most reasonable explanation for all my shots being high this time.

 

[wally]

I'm sticking with the unintentional change if ammo lots. Had 10 rounds left in the box I'd loaded mags from last time out.

So this time I did cold, clean bore shot at 400 yards. I wasn't expecting much as the wind was pretty variable, but by the time i got ready for the shot it got dead calm and I nailed it at 400 yards using the same 1.75 mil hold from the previous trial. (for the eagle eyed readers, the 1.5 mil in post #2 is a typo)

Here is the first round shot:


The wind picked up again so I switched to the 12 and 8 inch plates and I didn't do so well 3 for 3 on the 12" but only 3 of 5 on the 8" so no more photos, but for my last shot, the wind died down again and I nailed the 5" plate off to the side where I get no feedback on misses. Definitely need to work on my wind reads!

I think this concludes this thread but I am very satisfied with my RPR performance. I think Hornady Steel Match is pretty darn good ammo for the price ~$42/50

 

[taliv]

nice shooting! post a pic of the rifle

 

[wally]

I posted a photo when I first got it in this thread:
https://www.thehighroad.org/index.p...n-rile-308-it-can-shoot.808625/#post-10334125

I'd upgraded the scope at the start of this thread. Nothing wrong with the Primary Arms 4x14-44 FFP mildot R-grid scope and its great for the price, I wanted to try a bit more magnification so I replaced it with one of the new Nikon Black FX1000 6-24x50 FPP Mildot scopes. Haven't taken any photos of it with the new scope -- its not all that different looking, just a bit larger. The PA quickly found a home on another rifle, causing a scope "waterfall" of updates as I moved better scopes down the line to less frequently used rifles -- great excuse for more shooting of some less frequently shot old friends, got to sight them in!

 

[Bushpilot]

What was your density altitude on both days?

I don't know, not sure how to calculate it. I was at the same physical location, temps were with in a couple of degrees of each other both days, so other than humidity what other variable goes into the "density altitude" calculation?

Density altitude is a correction for temperature, humidity, altitude, and atmospheric pressure. It can change surprisingly from one day to the next, even with a seemingly small shift in perceived conditions. I shot a match last weekend at a physical altitude of 1390, the DA was 5000. I shot a positional primer class this weekend at the same range, DA was 3200. Rained the night/morning before both. Perceived conditions were almost identical for both days, but bullets flew higher last weekend than they did this weekend.

To further elaborate on this....

Density Altitude is a measurement of air density compared to a theoretical standard atmosphere. Standard atmosphere being a surface temperature of 59 degrees F at sea level with 0% humidity.

As air temperature and humidity increase, the density of the air decreases, just as if you actually climbed higher in altitude, and the Density Altitude number increases, ..... A decrease in barometric pressure decreases density altitude....

As the Density Altitude increases rifle/bullet performance also increases because the bullet is actually traveling through air that is less dense, so it doesn't slow down as fast and therefore shoots flatter...

To summarize, the hotter it is, the higher you go in elevation and the more humid it is the better your rifle will perform. The higher the barometric pressure (high pressure on the weather map), the less your rifle performs. The lower the barometric pressure (low pressure on the weather map) the better your rifle will perform.. All of these calculations together determine Density Altitude....... High temp, high physical altitude, high humidity along with low pressure can substantially increase ballistic performance... and vise versa....