Quote:
Originally Posted by MFC
Seiger and Patrick,
I'm not a reloader, so this may seem like a stupid question. The original rounds that measure .307, which is fired through a bore that measures .309, makes sense to me. As the bullet is forced into the rifling of the barrel, the metal that is displaced by the lands is forced into the grooves, forming a tight fit. If the bullet starts out the same or larger than the bore, the displaced metal can only make the bullet longer. Is this what is happening, or am I totaly off base? I really don't know, but it seemed odd to me to load a bullet larger than the bore. It obviously works, but wouldn't it also increase chamber pressure?
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Hi:
Actually, the metal displaced by the rifling moves toward the rear of the bullet and ends up in the casing, for a split second, before it is blown into the bore following the bullet out the end of the barrel. If you have a high-power rifle, examine a casing, after it is fired, and you will note, on the inside of the neck, a mirror image of the rifling pattern of your rifle. This is caused by the displaced material moving along the inside of the neck until the bullet exits the casing. The first time you see this you ask yourself how the inside of the neck ended up with the rifling pattern on it.
For accuracy, with FMJ ammo, bullet diameter should equal groove diameter; for lead bullets, 0.01 to 0.02 larger than groove is the old rule of thumb.
The goal is to "seal" the bullet to the grooves, to avoid "blow by" gas that can actually cut into the rifling over time and accelerate barrel ware.
Sieger