[Bart B.]

tobnpr, one way groups enlarge is muzzle velocity spread. A .308's typically going to have bout 2/10ths inch drop difference between a given bullet leaving at two velocities 50 fps apart; that's 2/10ths MOA. At 1000 yards, the difference in drop's about 20 inches; that's 2 MOA.

Another way is the small spread in BC's bullets have. Even the best match bullets are not all perfectly balanced and they'll wobble different amounts spinning as fast as they do. The more they wobble, the more drag they have. As drag increases, BC drops. So, even if 20 of them leave at the exact same speed, a 1% spread in BC causes a spread in how much the slow down and drop. It's typically not more than a couple inches at 1000 yards. If those bullets are deformed enough when fired, they'll have more spread in BC and the vertical shot stringing at long range will get bigger.

A third way is the atmosphere. It's not all the same density from muzzle to target. Thicker air slows down bullet speed more than thin air. It slowly changes as those subtle air currents move it around. It also changes a bit with air temperature. And even slight wind speeds are not the same from the line of sight to the maximum height a .308 bullet goes above it; about 11 feet above at 550 yards or so for a 1000 yard zero. On different terrains, the wind speed that high above the line of sight will be 30% to 90% faster than at the line of sight.

Groups at 100 yards open up about 10% for eacGh 100 yards further with the best stuff making them. Lesser systems will have a greater amount of group angle subtension as range increases. Best way to see it is compare the benchrest records for aggregates at 100, 200 and 300 yards. They get bigger in MOA's as range increases. Same for 600 and 1000 yards.

All the groups are about zero MOA at the muzzle, aren't they?

But there's an exception; positive compensation due to barrel whip in the vertical axis and muzzle velocity. Some barreled actions (the Brit's .303 SMLE's for example) have a lot of whip in the vertical axis. Their cordite loaded ammo has a wide muzzle velocity spread as the left on the muzzle's up swing just before it reached its peak. Bullets leaving slower left at a higher muzzle angle than faster ones that left slower. At long range, this improved accuracy. But accuracy suffered at short and mid range targets where M98 Mauser barreled actions shot that .303 ammo more accurate than at long range. No wonder the Brits did so well in long range competition when that arsenal ammo had to be used.

If long range accuracy tests are done just before sunrise when the winds are calm and it's cool enough to see bullet holes in targets, that's the closest thing to a "Houston Warehouse" for 1000 yard tests.