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Hmmm another little trick you can use is increasing the zero range...
Think about it this way...
If you had a "100 yard zero" and I asked you how much you would hold over to shoot a target at 140-150 yards? You would say none because it's a a pretty flat shooting caliber.
However punch this into the calc and it will change things considerably.
It is also better than fiddling with the BC and MV as it shifts the predicted trajectory rather than changing its curve.
:)
We had trouble with a long range rigs impact when getting out past 400 yards. We looked at all possible explanations as to what may be causing the inconsistency between the ballistic readings and the actual impact point.
We then changed to a different crony and discovered 170 fps difference in the velocity readings which made a huge difference to the dialling and subsequent impact point.
Also wonder what distance is the optimal distance from the muzzle as the bullet will take a bit to reach max speed. Chronograph instructions seem to be quite flexible in directions about the distance from the muzzle.
Another interesting observation when firing factory ammo through 2 different 300wm's was the large variation in velocity between the barrel lengths. One rifle with a 24 inch barrel was achieving velocity of 3225 fps with 150 grain projectiles and the other with a 27.5 inch barrel was getting up to 3400 fps.
Quote:
Originally Posted by yogi
3400 fps ? What factory ammo was it!!?
I emailed the Crony people a couple of years ago about the distance from the muzzle
they told me 15 feet
Hornady American whitetail. On the box it said 3250 fps. Out of my Tikka it was doing 3225 and up to 3400 fps from my mates rifle.
Seemed mighty fast but then velocity from my rifle was fairly close to what was suggested on the box.
It shot very well consistently hitting a 12 inch steel plate at 600 yards.
Hi Kimjon,
It will be interesting to see if anyone posts their actual trajectory and also ideas for getting it right.
I've used the Shooter app and a program on CD which came with Litz book Applied Balliastics 2nd Ed. These give the same results. I think Shooter uses Litz method but now that Litz has his own app maybe they will diverge.
In simulation terminology, you "train" your model with a preliminary set of data (Data on previous engagement). In the case of ballistic models, this training is adjustment to the BC & MV till you've got a good fit with your own personal dope. Then you try it out it with new "test data" to see how good the predictive value is. Then you can go out and apply the model and shoot deer with it. Sounds a bit mathematical but its common sense really.
I only ever used a chrony result as a starting point for velocity and adjusted inputs manually till a number of 2820 (860m/s) matched the trajectory I'd observed.
I've estimated the G7 BC of my 140 gr BT bullet from Litz published figures for Nosler BT in 120 and 150 gr. As suggested above a different BC might bend the trajectory differently further out and fit third points better. But, I decided that for a start, trying to adjust two input parameters at once would just be a way to getting lost. I'll try out a few scenarios later but don't have time tonight.
Here are my principles for collecting dope:
- Check each distance with a rangefinder and write it down.
- Get the tightest group you can. You might not want to ever shoot 1 MOA targets but if you can tune your trajectory better than that its very helpful. A 10" group at 500 yd i great for hitting gongs or deer but not much help for predicting trajectory at 800 yards.
- It goes without saying you need to use the same shooting technique especially rests, bipods and positions at all ranges.
- Always establish a very exact 100m zero before you start a session. This needs to be with 5 shots at least. Small errors like 2-3 cm here will leverage out to significant uncertainties about velocity and BC at longer ranges. Preferably finish with another 100m group to verify there has been no zero change during the session. Again, 2-3 cm might not seem much but when did it move - was it before or after the 500m group ? This is also where it becomes difficult if you have progressive barrel heating or fouling issues. Shooting a 5 shot group is one thing but establishing dope is another level of challenge.
- You require absolute control of your sights. They must dial the point of impact up and down just what you expect. It doesn't have to be exact 1/4 moa or IPHY but you need to have verified and proved it for yourself with the whole system, shots on paper.
- Use one load and stick with it. Load developments and trajectory doping are incompatible.
- Record key ballistic variables at each session, specially temperature and altitude. I once did some testing at 400 and 600m ranges at 1100m ASL and later when adjusting my nominal MV put those figures back into shooter to see which value gave the observed trajectory at altitude. If at all possible, shoot 100m and two longer ranges at the same session, on the same day.
- Use paper. Gongs and even electronic targets are too temporary. You need to take the paper home and apply rulers to it a few days later when you are calm again. The results always look better and more comprehensible after a good night's sleep.
Here are pics of a recent check at "short range", before the Ruahine Biathlon.
First target is re zeroing at Taupo NZDA range on the way down. Altitude 400m 15 C and sunny. Virtually no wind, from 6 oclock. I couldn't resist spending a couple of leisurely hours there. A calm good shooting day in the hand is worth two in the bush.
http://i812.photobucket.com/albums/z...20Bnchrest.jpg
The bottom group of 5 is the last. Then I packed up all my gear and was off to the Taihape branch range. I didn't want to muck to muck around on the 100m range, just set up and shoot once. The land owner kindly showed me to the "400" target frame and back to the giants' bench. It was now 7 degrees C and raining lightly. Altitude was 750m.
For 356 m I dialled up just past my 350m value glued to the rifle butt, using 1.9 mRad. It was too cold and wet to get out the phone for the app.
http://i812.photobucket.com/albums/z...20Bnchrest.jpg
Wind about 2-3 m/s apparently from 11 o'clock so I had made no allowance, but after seeing the paper perhaps it was corkscrewing around ...
I also wonder now whether there could have been some lift from the wind as the firing point is at the top of a slope (a bit like some places at Hangawera) and the first 100m could have been subject to perhaps a quarter value wind updraft.
Seeing this result and having had some higher than expected groups in the past, I decided to allow about 0.1 less than my chart, which served me well the next day when I hit 9/10 gongs at 290m (skite I know but it feels so good to hear that thwak ! that I just have to tell you about it). I wasn't the top score but equal to Phillipo, who only beat me by running faster.
So, my trajectory was good enough out to 350m anyway.
Mate, what an awesome reply. I really appreciate the effort you've gone too.
Kim