The evening has been spent exercising some new software that uses the Modified Point Mass (MPM) exterior ballistics equations. For those of you not up on this techie stuff, these are about the best equations, for predicting the exterior ballistics of spin stabilized, small-arms projectiles, available to the civilian population.
One of tonight's exercises was to investigate the effects of
headwinds and tailwinds on our longrange slugs over the 1,000-yard range shot
in Creedmoor competition. For those of you that shared in these "special"
effects at the AZ Creedmoor WinterNationals during Sunday's 1,000-yard relays
you know all too well how "special" these effects are (tongue firmly
platted in cheek.)
During our turn in the pits there was the periodic whoosh of bullets missing the target high as the tail wind picked up and then the ever popular "dirt shower" when the fish-tailing tailwind backed off. Moans and groans were had and heard by all in the pits and on the firing line.
Over the course of the past few years these headwind-tailwind effects have been a topic of discussion with other shooters. While spotting/coaching special attention has been paid to try and better understand and quantify these effects on our bullets' trajectories.
Last spring during the California Creedmoor Spring Classic a few observations by fellow shooters started this crank thinking about how caliber, MV and bullet BC affects the headwind-tailwind effects on our bullets. The situation that precipitated this though process were a few comments from Klaus Schattleitner and Rick Moritz who were shooting next to me all weekend. The comments that were most interesting was that I was "shooting through" conditions that they held-up in due to their match experience suggesting that the wind effects would be more than they cared to dope. We had a fish- tailing headwind off and on for most of the weekend so I believe it was the up and down effects of that headwind component pushing their bullets up and down that they were trying to deal with by waiting on the condition to go back to their "standard" condition for the string of fire.
Klaus and Rick were shooting 45-90's. Klaus' load had a 525-grain bullet launched to 1,250 fps and Rick's load was about the same only his MV was 1,260 fps. The BC of the 525-grain bullet they used is about 0.430. I was shooting a 12-twist 38-70 launching a 373-grain bullet to 1,425 fps with 72 grains of Swiss 1.5. The BC of my bullet is about 0.630. It was evident to all three of us that the headwind was not hurting my shot placement as much as it was hurting theirs.
The issue of reducing the difficult to deal with effects of headwinds and tailwinds on bullet trajectory has been a subject of interest since starting into the domain of BPCR LR match shooting. These effects are noticeable in BPCR silhouette, especially at the ram line, but what Creedmoor shooters have to deal with at the 1,000-yard line make the ups and downs at the ram line seem trivial. Research has been ongoing with an equation found that deals with these effects but it is almost 100 years old. It was developed by the British Royal Artillery ballisticians. It holds that the effects of headwinds and tailwinds can be estimated by the formula:
Bullet Impact Change = 10
x Wind x (Distance in 100's of yards)^2
By simple inspection of the equation one can see that pumping up the MV and shooting the highest BC bullet possible out of your rifle are the two ways to reduce headwind and tailwind effects on elevation at long range.
The Modified Point Mass equations allow one to experiment with these variables or plug in the ones that define your load and see what the estimate is for headwind and tailwind effects. That is just what was done this evening with the new software. Results track with match and range-test experience. Certainly more work needs to be done to better understand these effects, but some progress is being made.
The MPM software produced some interesting results when Klaus' load was plugged into the software and compared to the load I used during last spring's Creedmoor match. Using a BC of 0.430 for his bullet with a MV of 1,250 fps a 10 MPH head wind is estimated to push his bullet down about 31" at 1,000 yards compared to a no wind condition. In contrast the 38-70 bullet was only being pushed down about 11" from the same headwind condition. The 38-70 bullet weighed 373-grains and was launched at 1,425 fps.
These computer simulation results track quite well with what we have been seeing on the firing line and in the pits. Much more research and experimentation remain to be done before these effects are satisfactorily digested.
One last comment before pushing the "send" button.
A number of LR shooters are still convinced that "heavy and slow"
is the way to go.
Every shooter I know that is following that philosophy faces a daunting task at 1,000 yards. Their loads shoot fine at 800 yards but I have yet to see a good score shot by anyone following the "heavy and slow" regime at 1,000 yards. They have been told that crosswind affects are reduced by shooting slow, but the headwind and tailwind effects along with the vertical dispersion caused by random velocity variation flat kills their 1,000-yard scores.
28 Mar, 2005