What causes this?

[someguy2800]

http://bulletin.accurateshooter.com...on-of-bullets-why-is-too-much-spin-a-problem/

Very interesting and the only plausible explanation I've seen!

I had a laugh at some of the comments at the bottom of that page. Most of them amounted to, "It can't be true because I've never seen it." Well... I haven't seen a snow leopard either but I'll take your word that they exist.

 

[murf]

gamestalker,

one other possibility is a bad barrel crown. if it is cut crooked or is dinged up, it could cause this short-range instability.

murf

 

[Bart B.]

Barrel Heat and Bending Barrels

All commercial barrels are fairly well stress relieved. When they heat up, they expand quite equally in all directions

The barrel tenon shoulder keeps it from screwing into the receiver to much. It bears hard all the way around its contact with the receiver. Providing the receiver face is square with the receiver's tenon thread axis.

Commercial receivers are normally not trued up. Their faces are not squared up with the thread axis that the barrel fits into. Barrel tenon shoulders are typically quite square with their thread axis. So, there's one place around the face that's hardest pressed against the barrel when it's torqued in. That point is where a stress line goes out from.

As the barrel heats up, it expands and presses harder against the receiver face. The hardest point is where the barrel was already hard against it. So it gets worse and bends the barrel away from that point. Point of impact relative to the aiming point moves in the same direction. If the barrel cools down, the barrel straightens and point of impact is now where it originally was.

It's all fixed by having the receiver face squared up with the tenon threads with it held by a mandrel threaded for it that's turned between centers on a lathe. Face the receiver off all the way around, about.005" typically cleans up all of it. Then have a .005" thick shim put in between barrel and receiver so the barrel clocks back in to maintain headspace and sight hole alignments.

Few, if any barrels, walk shots when fitted to squared up receiver faces. Those that don't are poorly stress relieved to begin with. Both whippy, long and skinny barrels will shoot just as accurate to the same point of aim as they heat up as stiff, shorter and thicker ones.

Some folks want a pad of epoxy under the barrel's chamber to support it thinking that will relieve some of the pressure the barrel puts at the bottom of the receiver face as it droops from its own weight. Too many 1.2" diameter straight taper 28" barrels have been fit to receivers shooting bullets most accurate without that pad. Besides, as the barrel heats up and get several millionths inch larger in diameter at the pad area, more pressure will be applied to the pad and change how the barrel whips vertically; bullets start stringing on target that direction, too. No pads, please, if accuracy is important.

 

[Bart B.]

Bullets Maintaining Line of Fire Angle in Flight?

If bullets maintain their line of fire angle throughout their flight, wouldn't their BC increase dramatically at longer ranges as their long axis starts making greater angles to their trajectory axis?

Military large caliber projectiles maintain their long axis parallel to their trajectory for the distance to the target. They've been recovered buried deep in the earth with pointy end down at their entry angle into earth.

If bullets really do maintain their launch angle all the way to the target, how come their makers don't supply drag functions for each degree of their long axis angle is to the trajectory? And at the different velocities they'll do that at all the way to maximum range of the bullet?

PS: First shots from a barrel that foul it often have more jacket material rubbed off one side more than its opposite. If that side's to the right or left when it exits the muzzle, the centrifugal force the bullet has will make it jump towards the heavy side as it exits. Once the bore is uniformly fouled, the rest of the bullets are not unbalanced by that cause; they all shoot straight. A common problem with regular commercial and arsenal barrels; the better match grade barrels have bores dimensionally uniform and smooth enough that fouling is minimal and if there at all, it's very thin and even all over the entire bore surface and stays that way.