Need help, Sudden elevation drop at 300

[gitira6]

Maybe you guys can ease my mind a little. Went shooting today with my dad at 300 yards, and I'm shooting a Rem. Sendero sf2 in 7mm RM. I have the gun dialed in with a good load that shoots 1/2 moa or better with Berger 180 vld and H1000. The last time we went out I was dead on with elevation and about .5 inch to the right of the bull and shot a 3/4 inch group. It was about 65 to 70 Far. with no wind and I left it there. Today it was 37 degrees Far. with a very slight wind. The group landed almost 3 inches down from the bull, a little right. I dialed in 6 clicks up and I was back up there, about an inch 1/2 group. I'm wondering if it is the scope, its a Nikon Buckmasters 6-18 that I've had for years, I know its not the best scope, but could it be the tracking, since i'm resetting for 200 yard zero when I leave the range. Or could it be that the temp. was 30 degrees cooler? I have not shot much longer range in the cold. Loads were exactly the same, And I shot some from the last time with the same p.o.i. I may bite the bullet and get a better scope, maybe a vortex. Any input would greatly be appreciated. Thanks, Mike

 

[406precision]

Although its possible some of it could be scope issues.

I would lean towards barometric pressure and temp variance if it were me. We usually try and do our load work in hunting conditions if at all possible and verify drops at average high and low temps like 35 and 70 if that makes sense. you'll definitely see a difference in point of impact.

I don't know if you keep a log book or not, but if your shooting lon range you should keep one for every rifle that way you can record the temp, baro pressure, elevation, and wind for each shooting session and the results. It is really helpful to look back after a few trips to the range and compare notes and make adjustments based on an average of your data collection. It is also very helpful to look back at where each shot landed and to use that to correct your form and or drop charts..

 

[gitira6]

Although its possible some of it could be scope issues.

I would lean towards barometric pressure and temp variance if it were me. We usually try and do our load work in hunting conditions if at all possible and verify drops at average high and low temps like 35 and 70 if that makes sense. you'll definitely see a difference in point of impact.

I don't know if you keep a log book or not, but if your shooting lon range you should keep one for every rifle that way you can record the temp, baro pressure, elevation, and wind for each shooting session and the results. It is really helpful to look back after a few trips to the range and compare notes and make adjustments based on an average of your data collection. It is also very helpful to look back at where each shot landed and to use that to correct your form and or drop charts..

Yes, that does make alot of sense.

 

[Michael Eichele]

Atmospheric conditions will change a 300 yard zero very little. Much less than 3" for your situation anyway. Parallax, lighting and or mirrage get my vote. It takes pretty severe temp/pressure changes for you poi to change by 1 MOA at 300 yards. 30-40 degrees isn't going to cause what you're seeing. Unless your ammo is hyper sensitive to temp swings causing a harmonic distortion, exterior ballistics in and of themselves are not the problem here. Since your last session and this sessions accuracy (group sizes) were very good, I doubt there is a load/barrel hypersensitivity issue here either. Caulk it up to lighting or parallax or both. It's easy to underestimate their contributions to this game.

M

 

[406precision]

Atmospheric conditions will change a 300 yard zero very little. Much less than 3" for your situation anyway. Parallax, lighting and or mirrage get my vote. It takes pretty severe temp/pressure changes for you poi to change by 1 MOA at 300 yards. 30-40 degrees isn't going to cause what you're seeing. Unless your ammo is hyper sensitive to temp swings causing a harmonic distortion, exterior ballistics in and of themselves are not the problem here. Since your last session and this sessions accuracy (group sizes) were very good, I doubt there is a load/barrel hypersensitivity issue here either. Caulk it up to lighting or parallax or both. It's easy to underestimate their contributions to this game.

M

That is pretty sound advise I have seen some strange things happen do to an inversion affecting poa...but Michael is right your only looking at .25-.30 change in poa based on temp alone. I somehow read the post and then disregarded the 3" low and fixiated on the 3/4 -1 1/2 group at 300 which could be directly related to atmospheric conditions my bad!

 

[gitira6]

Good info., I appreciate the responses.

 

[gitira6]

As far as lighting, it was a little weird today at the range, bright sun out to 250, then dark and shadowed at 300. My dad even commented on it when we got there. I did notice mirage when shooting too, just wasn't as crisp. I don't know, me and dad have shot alot over the years, but I'm no expert at this long range stuff by any means. There's alot of variables involved with this, I guess that's what keeps it interesting.

 

[Scrubbit]

I shoot in the Arctic at temps varying from -30 to 80 deg F, so have dealt with temp variation quite a bit, and how it affects the muzzle vel. A general rule of thumb (it has worked fairly well for my shooting) is a change of 3 fps for every degree of change. All depends on powder type etc. As previously mentioned, this alone prob wouldn't account for the entire change in POI, but may be a contributing factor.

Note: The only chrono that works reliably for the colder temps up here is the Magneto Speed. I take the LCD controller off between strings and warm it up in an inside pocket.

 

[Scrubbit]

All right, I got un-lazy and ran the numbers. The change in POI at 300m shooting a 215 Berger at 3000 fps at a temp of 70F and shooting the same bullet at 2900 fps at 40F is approx 2 in. This takes into account both the drop of approx 100 fps in muzzle vel due to the change in temp, and the increase in air density due to the colder temp. Given that that's just a rule of thumb calculation, your change in POI could be as a result of the temp change, although it's likely your site picture due to cloudy vs sunny etc could have had some impact as well.

 

[Michael Eichele]

I'm curious as to what powders you've seen 3 FPS per degree of temp changes. I cannot Phathom 90 FPS changes from 30 degrees. I'm also fortunate to live where I get to test loads in sub zero temps and compare them against loads that are 80-100 degrees. I've actually compared some as cold as -20 against 90 degrees. The results have been quite shocking. Much less than I ever would have thought. At least with RL powders and H extreme powders. Never tested ball powders that way. Double base powders have been more sensitive. Like .65 FPS per degree. To me, that's quite a bit. I'm always hearing how bad RL powders are and I still don't get it. R15, 19, 22 and 25 for me have shown to be very minimal. As good or better than the H extreme powders. R17 is a bit different. But then again, it's a true double base powder.

 

[Scrubbit]

I'm curious as to what powders you've seen 3 FPS per degree of temp changes. I cannot Phathom 90 FPS changes from 30 degrees. I'm also fortunate to live where I get to test loads in sub zero temps and compare them against loads that are 80-100 degrees. I've actually compared some as cold as -20 against 90 degrees. The results have been quite shocking. Much less than I ever would have thought. At least with RL powders and H extreme powders. Never tested ball powders that way. Double base powders have been more sensitive. Like .65 FPS per degree. To me, that's quite a bit. I'm always hearing how bad RL powders are and I still don't get it. R15, 19, 22 and 25 for me have shown to be very minimal. As good or better than the H extreme powders. R17 is a bit different. But then again, it's a true double base powder.

Interesting. The only two calibres and powders I've worked with a lot up here are Varget in 308 and H4831SC in 300 Win Mag. I've always been a little leary of my velocities as I didn't trust my chrony that much, but my results have been fairly consistent. I just rxd the Magneto Speed and am working up a load with 215 Bergers and Retumbo (and H1000 when it comes in) in my 300 Win Mag. Last year, I had to tweak my 300 WM load significantly (3-4 gr) after my winter work up period as it was too hot in higher temps. Standard hunting temp around here is about 37F, so I'll fine tune things in the spring when we cross that threshold again. It's going to be a loooong winter. Always is up here.

 

[dkhunt14]

When on the 1000 yard line the difference when a cloud comes over to full sun is usually 3 to 4 inches. Dark is low sun is high. I doubt this alone caused that much at 300. Now if you don't take the parallax out of your scope that can affect it a lot. You would be surprised how many guys on the firing line at a match don't check or know how to check for parallax. Mirage is another vertical killer. Matt

 

[LazzInc]

you all probably don't need my 2 cents worth, as you have a very informed group here, but I have seen 2-3 fps per degree of variance above or below 72 degrees F using real slow ball powders like H870, AA8700 etc ,,,,,,

in the big Lazz/Wby/RUM mags, using reloader 19, reloader 15 & VVN560, you are looking at 1 to 1.5 fps per degree, above or below 72F ,,,,,,,

the reason that 72 degrees F is important, is that is the temperature that most SAMMI pressure/velocity standards are established at ,,,, to be consistent from caliber to caliber ,,,,,