Portable weapons used for big-game hunting include firearms, bow and arrow, spears, atlatls, and other longer-range weapons, as well as close-range, handheld weapons like hammers, axes, knives and other bladed weapons. By far the most common weapon used by hunters is the firearm, and among firearms the most common weapon is the long-barreled rifle. Other firearms, such as shotguns and handguns, are also used in big-game hunting. Large-caliber ammunition is considered to be most effective in taking down large game effectively and humanely. Big-game hunting ethics require a clean, humane kill, and most hunters work diligently toward this end.[5] Advances in ammunition and the guns to match have made longer-range kills of big game possible with margins of error considered tolerable. Some common calibers and types of ammunition for big-game hunting include .30-30 Winchester, .30-06 Springfield, .300 Winchester Magnum, and .358 Winchester. The calibers and types of ammunition, and the firearms to shoot them, are numerous, and the science of ballistics is continuously improving to allow hunting in a tremendous variety of situations. Bullet weight and shape, cartridge size, powder load and type, and virtually every other variable of firearms ammunition is continuously changing. Bow and arrow hunting is popular and effective among skilled hunters for big game. There is a variety of types of bows available, including long bows, short bows, recurve bows, compound bows and crossbows, all made of various materials. Arrows are also made from various materials including wood, fiberglass, carbon fiber and others. Arrowheads have different configurations and materials as well. Primitive hunting using spears, atlatls and other weapons is a skill popular among hunters seeking greater challenge and knowledge than more conventional weaponry.