• HVAC

Compromise of Bullets; Rifled Barrels, Minie Ball, Twist Rate, Weight & Ideal Bullet

Nothing seems to be straight forward. In engineering there are a long string of compromise in machine design. This applies to ballistic projectiles; the bullet. Originally a ball shaped projectile, it has changed much over the last couple of centuries. In fact, a case could be made that the round ball, made of lead, ruled the battlefields and the hunt for over 6 centuries. The ball is made of pure lead or a lead alloy. Antimony hardens lead and is a primary additive creating a ‘hard cast’ alloy.

Rifled Barrels

The ball is still with us in the form of shot and buck in shotgun shells. The incorporation of rifling into the barrel was a game changer. The old smoothbore muskets could only be depended on to hit out to about 40 yards. Rifling extended this to about 300 yards. A musketeer can reload at a three to four rate faster than a rifle and was therefore good for mass fire but only at ranges making it difficult to reload before engagement, hence the militaries dependency on the bayonet. The musket hunter was under severe range restrictions, but the slower rate of fire was not as critical in hunting with a rifle.

Minie Ball

The patched ball ruled rifle ammo until the introduction of the Minié ball changed this. For the first time the projectile could get heavier without increasing the bore. The hollow base elongated cylindrical bullet also allowed for speedier reloading. As such the round nosed cylindrical bullet was the rule during the black powder era.

Ideal Bullet

The ideal bullet would resemble the shape of a needle tapered at both ends, but this is not practical. First a few terms:
• Base this is the back of the bullet and can be concave, flat or tapered.
• Boat Tail, a tapered base used to make the bullet more areo-dynamic.
• Bearing Surface: This is the maximum diameter of the bullet with the side being parallel to the axis of the projectile. This is the area in contact with the bore and is ‘cut’ by the rifling. The larger (longer) the area of the bearing surface generally the more precise and accurate bullet. Friction robs velocity and kinetic energy converted to heat as the bullet makes its passage down the barrel.
• Heel is where the base curves into the side of the bullet.
• Ogive is the curved nose of the bullet tapering to the point.
• Meplat is the very tip of the bullet. Flat or hollow point.
• Shoulder of the bullet is where the ogive meets the bearing surface.
• Tip is the front of the bullet if pointed. Any flat areas, rounded areas or hollow points is the meplat.
• Spitzer anglicized version of the German word ‘Spitzgeschoß’.

Twist Rate & Bullet Weight

Twist rate is determined by the length of bullet and caliber ratio. A 100 grain .30 caliber bullet will stabilize at a slower twist rate than a 200-grain bullet. The longer the bullet for the caliber the faster the twist rate needs to be, also influenced by the muzzle velocity. Given a fixed twist rate, the higher the velocity will result in a faster spin rate. As bullet weight goes up (the bullet gets longer) the lower the velocity (takes more energy to start and push a heavier bullet for caliber than a light one). A 150-grain bullet will have a higher velocity than a 220-grain bullet given the same caliber.

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