Thread: What's the Deal?

At 2644fps I doubt if the G7 bc will be working out at .299. Also whats your scope?

Is the G7 BC velocity dependant? My scope is a Sightron SIII 3.5-10 Mil/Mil

Originally Posted by L.R

At 2644fps I doubt if the G7 bc will be working out at .299. Also whats your scope?

You have to drop it to below .200G7 (no way it's that low, considering that they're averaged over velocity range to account for change and G7 drag model changes very little over a large range of velocities ie, the whole point of using G7 instead of G1) to get the .8mil difference

Originally Posted by Normie

Is the G7 BC velocity dependant? My scope is a Sightron SIII 3.5-10 Mil/Mil

Much much less so than G1, hence why it is better

Originally Posted by gimp

What zero range for the rifle? And what zero range entered in the app? Because I'm waiting for a plane in Singapore and have had 3 very large black coffees I have had a play with JBM.

With 2644fps and 100m actual rifle zero/100m zero entered in program the 500m elevation is 3.8mil, 650m is 5.9mil (ie the ~corrections that you required to hit)

With 2644fps and 100m actual rifle zero/200m zero entered in program the 500m elevation is 3.1mil, 650m is 5.2mil (ie the 500m ~correction that did not hit, we had no information supplied about misses at 650m)

2500fps is super slow for that powder/charge/bullet/case.

A theory: your rifle is zeroed at 100 meters but the program was set for a 200 meter zero. I can believe an easily overlooked data input error, they happen regularly.


Edit: I think I accidentally did all that with JBM set to G6 not G7 but that just kinda proves my point of how easy it is to miss one little input in a program, doubly so outdoors on a phone etc

Thanks for the help. Seems like some kind of input error then. Might ask Leathel what the zero range he entered was. Or maybe it is set for POI 1.5" at 100 for one of his rifles?

Zero is definitely 100m, angle of the shot was 5 degrees uphill for both shots. Correction was needed at 500m but not at 650m data was on. Shot was at a clay bank and splash was visible for corrections/misses. Final shot at 650m was a rock about .2 mil wide and 1 high. POI was at the left of the rock but elevation was spot on, wind would have pushed it I guess (light breeze from 5 o'clock).

I know Chronys can be off but I wouldn't have thought by that much.

Data at 650m was only on after you had fudged the velocity though, right?

It is hard to draw exact conclusions from data when the impact points are not well measurable, ie on not steel or paper, it can lead to wrong assumptions hence my comment that bullets can lie. Keeping a log is key as you can refer back to it.

Points:

I have a 22" barrelled .260 belonging to forum superstar "The Kid", previously known as Gordy, sitting in my safe, along with a box of Lapua brass, a 4kg tub of 2209 and a whole shitload of 6.5mm 140gr Amax bullets and I have done a fair bit of research on exactly how to put all these things together. None of it leads me to believe that 42gr of 2209 would push a 140gr at 2500fps in a .260 unless you actually have a 7mm08 and no one bothered to tell you. That to me likely rules out chronograph error.

The potential variation in BC simply will not have that much effect - I don't have Litz' book in front of me here at this Changi airport coffee shop (I do have an extra-large Americano) but the 139gr Scenar G7 varies only .016 from 3,000 to 1,500 fps, the Amax is a very similar form factor and the variation from Litz' average with a 2644fps mv will be much smaller than even that, it will not cause any noticeable change in elevation.

Scope calibration error seems unlikely to me due to two factors: One, the magnitude of the error - it's 20% of your adjustment at 500m. That would mean the scope is incredibly badly calibrated. Two, it's a constant error - .7mil ish at 500m and 650m. If the scope adjustments are incorrectly calibrated, the error would increase with increasing amount of adjustment.

It can't be angle: if the angle was actually steeper than 5° you'd need less adjustment, not more.

If the rifle is zeroed correctly (and ~.7mil is quite off-zero, so you would probably notice) that leaves as far as I can see:
-shooter error (including possible different POI due to different rest surface or similar)
-input error to ballistic program
-compounding effects of minor errors
-incorrectly reset turrets after zeroing
-mechanical error with rifle such as action screws or suppressor loosening, but that usually results in a random pattern

Yes, muzzle vo was fudged (2500fps) at 650m to get the correction I used. I was shooting prone off bipod and rear bean bag. Pressure on bipod might not have been consistent,1st Variable. I know baro was fudged/guessed that day, 2nd variable (probably not out enough to make such a big error though?). Turrets were not reset after zero so adjustments were added on to this (.5 mil up from previous zero) pretty sure I counted correctly though.

I think i need to shoot it again and do some more logging.

Originally Posted by gimp

You have to drop it to below .200G7 (no way it's that low, considering that they're averaged over velocity range to account for change and G7 drag model changes very little over a large range of velocities ie, the whole point of using G7 instead of G1) to get the .8mil difference

I agree that the G7 wont be that low, however it is something to look out for if you are running bullets slower or faster than their "usual" speeds. G7 is velocity dependent although not as bad as G1.

Looking back across your data Normie it most likely is a velocity error like others have said. Your load is what i use in a 6.5x47 so will be rather slow out of the much bigger 260 case.
You are welcome to borrow my Oehler if you like to check you velocity again.

Thanks LR. I might take you up on that. I started getting pressure above 42gr for some reason? Only slight ejector marks and a little cratering though. I might have to do a bit more work with the 2209 but is fine for a hunting load for now. I've looked over my measured shots again and of 42gr and one is way higher than the others at 2694fps so removing that I get an average of 2634fps with an ES of 20fps.

If that load is doing 2500fps as the numbers would suggest its a bit of a lost cause really. Id try another power I think. If you'd like to grab the crony just let me know.

I was shooting 42gr of 2209 with 140Amax out of a 20" 6.5x47 too. I found pressure too high and have backed off to 40.9gr (for better accuracy as well, bonus). Previous velocity was 2810fps average via my Shooting Chrony f1 (oh no!) and that velocity was accurate for centre hits to 910meters (As far as I ever bothered to shoot it) so my crappy chrony can't be too far off. I haven't had a chance to chrony the new load but I'm making an educated guess of between 2740-2760fps.

My .260 research for whenever I get around to loading it has indicated that a max load can be anywhere from 41-44gr of 2209 (H4350, it's the same thing) and a 139/140gr will get anywhere from 2700-2850fps in a 22-24 inch barrel, depending on the brass and the chamber and the barrel so 2644fps for 42gr of 2209 sounds reasonable to me. Loads seem to be quite inconsistent but H4350/2209 seems to be universally 'the powder' for a 140 in .260.


Similar to 65x47 actually - some people can't get over 40gr of 2209 with a 140gr without hitting pressure, I was at 42gr and only getting real pressure on 30°+ days when I left my ammo lying in the sun - sticky bolt, ejector mark and 1-2% pierced primers. Normal weather, no signs at all.

Originally Posted by Normie

Thanks LR. I might take you up on that. I started getting pressure above 42gr for some reason? Only slight ejector marks and a little cratering though. I might have to do a bit more work with the 2209 but is fine for a hunting load for now. I've looked over my measured shots again and of 42gr and one is way higher than the others at 2694fps so removing that I get an average of 2634fps with an ES of 20fps.

Is it a custom or factory setup Normie?
Is there much diff in length between a new case and a once fired one?



Sent from my iPhone using Tapatalk. So please forgive my sausage fingers!!!

My scales are shit really and walk a little but I do re-zero them before each drop of powder. They are always between .2gr out which should be normal for digi scales?? They are Smart Reloader's basic digital model so maybe my weights are out. Would explain the SD being 20fps.

Hey Pete I did a ladder with Hybrid 100V too. Got good results of 150fps more with no pressure signs. Results in Reloading area if you're interested.

Yes Gimp all of that sounds reasonable. Looks like Normies 42gr load may be a bit slower than 2644 tho unless something else is drastically wrong.
The load I use now in my 6.5x47 is only 41gr of H4350 and 130 VLDs, its slow at 2777fps but you can't argue with the accuracy even at 1k.

R93 it's a customish rifle. Howa action trued with MAB 1:8 22" PTG reamer for SAAMI chamber spec .298NK .088 FB I think Abe set headspace at 1 thou or 2. Will measure fired case length and get back to you.