We have tested several bullets to a very fine resolution on b.c.
My point is that the same gun at different pressures and velocity will shoot the same bullets from the same lot at significantly different b.c's
The 30 cal. 175 is an example where the same gun fired the same bullets at b.c.'s as low as .472 and as high as .518
Bullets often vary .018 when fired at near the same velocity. Just look at Brent's picture of his 43 software. The reasons for the variation are several. It is easier to understand if you break the components of drag down into several parts. Frontal drag, rear drag, and frictional drag. Frontal drag exists because of work or energy needed to push an object with frontal area through a fluid, and the higher pressures that result on the Ogive of a bullet.
Just the variation in the size of the Meplat from lot to lot of the Sierra 300 gr. Mk and even from bullet to bullet much less gun to gun will make nailing this number down with confidence very difficult, and the true answer will change with every load in every gun in every dissimilar wind condition.
Having said this, I do think that when you get a load you really like, it is very usefull to note the actual b.c. with that load in that gun. Keep in mind that this number will change as the bullet flies, some of this is due to small changes in angle of attack, but I believe most of the change is a result of the boat tails design and how it interacts with he atmosphere at different velocities. Rear drag is mostly made up of vacuum, and I have noticed trends over the years that boat tails of a certain design cause a bullets b.c. to go down as the bullet loses velocity, and other designs to influence a bullets b.c. to go up as the bullet slows.
I can't remember where at I posted the pic S1 refers to but if you havn't seen it I can post it here in a bit. I'm uploading some pics to ImageStation of our bowlingball cannon shoot and it's taking its time doing it, I'm stuck on the 56k dialup still and this part bites. [img]images/icons/grin.gif[/img]
I think the BC is usually over .760 at the range most shoot, many times it has been right up to the high .790s. Might have been warmer then? I do know I've never tested it that it fell below .760 though, at least the average anyway. Most were quite a bit higher than that though.
I remember some time back Darryl saying he figured them to be running about .797 or therabouts, and Warren telling him they found something different at Yuma during some tests he did, but he wouldn't say what it was. [img]images/icons/confused.gif[/img] [img]images/icons/confused.gif[/img] The first batch I tested were almost exactly identical to those of Darryls findings. I never thought too much about it after that. I have never tested his rfle at 1000 yards for the moa needed so this doesn't answer the question I know your really looking for. I've wondered the same thing before starting out with my Ultra.
Len, keep one thing in mind I've often taken for granted when backing in for the BC, that is multiply the total moa needed by the actual fraction/decimal of MOA the persons scope is really calibrated at.
Did that make sense?
Some are really .95 and some are 1.12, or whatever it is, it will skew the BC bigtime either way.
Either way, I think the ones that shoot 300s at 1000 yards should be able to tell both, at least I hope. [img]images/icons/smile.gif[/img]
Thanks, all. My reason for caring to know is that a new rifle I ordered from Dave Tooley in 338 Yogi will be throated for that bullet. I hate to make a major equipment commitment if the bullet BC is not really as high as it's supposed to be.
You will NOT be disapointed about that part Len. I will tell you that when throating for it my brother ran into a problem! The bullets measured .3089", yes almost .309". The problem was the throater was .3080" and they had to open the throat a little at a time then use sandpaper on a deal they use for polishing the chamber, I can't remember what he called it, but it opened it up just enough to keep from pulling the bullets. It was a SLOW, VERY SLOW process they termed... A PAIN IN THE ASS!
I think this was the first one Dave Caboth had throated for the 300gr SMK, but I think Dave Tooley probably has quite the handle on this with all his experience with them.
What OAL will it be throated for? The 300s eat up alot of case capacity at mag fed OALs unless you have a long one.