AMT load chart

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    The common way of viewing a bullet's travel is to think of it as moving in a straight line.  With repeated shots, you see a group size.  This is normally figured statistically as a probable deviation from theoretical point of impact.  The math is repeated in various ballistics books, but if there's any mention of *why* all the bullets don't go into the same hole, it's attributed to either the wind or the shooter.

 Back in 1909 Franklin Mann wrote a book called "The Bullet's Flight from Powder to Target."  It's out of copyright and you can download it here:  http://books.google.com/books/about/The_bullet_s_flight_from_powder_to_targe.html?id=QdQqAAAAYAAJ  

 Mann was what they used to call a "gun crank" back in the old days.  He experimented with all sorts of stuff and documented it in detail.

 He mostly used a naked action bolted to a giant concrete test stand.  In one set of experiments he set up tissue-paper screens at intervals between the muzzle and backstop and demonstrated, via the positions of the holes in the screens, how a bullet actually travels after it leaves the muzzle.  He repeated the tests many times with different calibers, bullets, and loads.

 What Mann found (1) was that a bullet doesn't travel in a straight line.  The spin imparted by the rifling stabilizes the bullet be comparison to a smoothbore, but the spin in turn causes a spiral trajectory.  Think of a big conical spring.  This is the conical shape that defines the impact area.

 Let's say you put together a new rifle.  You bore-sight it with your laser, take it to the range, and it prints about 4 inches down at 4 o'clock.  You twiddle the scope to move the group into the bullseye.  Later, you try different ammunition, and your group moves an inch up and over at 2 o'clock.

 The probable reason is that the new ammunition has a different muzzle velocity or the bullets have a different ballistic coefficient.  The load may be just as accurate - that is, the group size is the same - but it's printing on a different part of the target because the conical path of the bullet is a bit tighter or looser, so it's hitting the target at a different position on the path.  

 Any change in the velocity of the bullet means it will hit the target in a different position.

 Mann's book has pictures of the tissue-paper screens.  I'd kinda-sorta known about the conical path, but seeing how the bullet walked   around the screens brought it all home.

 Other than weighing your bullets and making sure none are damaged, they're pretty much a given.  The primer, powder, and neck tension are your big influences on velocity.  That's why serious target shooters eliminate as much variation as they can.  Any change in velocity means the group will get bigger.

 Yes, yes, I'm trying to make a point.  Hold your horses.

 Another thing I'd known was that for the same cartridge, some loads are more accurate than others.  You start with one load, then work up or down a bit at a time, then you change powders and do it all over again.  The guys with the little notebooks at the range, that's what they're doing, keeping track of the group size for each different load. Eventually you find what the gun likes.  

 I've kept those little notebooks myself.  But I've never had a chance to play with a chronograph, and I'd never come across any chronograph data showing variation among the same string; just between different loads.

 There's a perfect example of such a table in Bruce Stark's Automag book.  On page 111, it reproduces a chart of Bob B.'s trip to the range on May 7, 1971.  He tested XP005 with 47 different loads.  The book doesn't say how many shots were fired for each load, but the table lists average velocity (which is common) and the spread of velocities, which is not.

 Oh, my.

 Bob tested H-110, 1020, 630-P, 2400, 4227, WC-295, and Unique.  I've used all of them in one cartridge or another, except for 1020, which I'm not familiar with.  However, it's the most interesting one in the table.  You've probably read about powders that didn't like light bullets or reduced charges; this must surely be a good (bad?) example.  A 180 grain bullet over 19 grains of 1020 had a 430 fps velocity spread.  That batch probably printed like a shotgun.  Bear in mind this is for the SAME load in each case.  On the other hand, a 240 grain bullet over the same 19 grains gave the tightest spread on the table - 4 fps.  Meaning it has a chance of being the most accurate load.  There are other important factors in accuracy, but if you can't get the velocity spread under control, you're hosed before you start.

 I could probably find more data like that on the internet, now that I know what to look for.  If you have a copy of the book it's worth flipping to page 111 and examining the table even if you're not a reloader.

note 1 - it was known before Mann, but as far as I can tell he was the first to actually *show* it in print for ordinary schmucks to see