Even though most people think that a Shofar is measured as net length, the correct way to measure a Shofar is along the curving surface on the longest side.
The Shofars for sale are really a type of trumpet. The fact, is the Shofar is one the oldest wind instruments known to man.. A Shofar is a trumpet made a hollowed-out horn from a ruminant except other than a cow. Permissible horns include those of ram, goat, antelope, mountain goat, or gazelle.
All Shofars feature bends, curves and spirals. The length as well as compactness and shape of these curves and bends renders the sound of different Shofarot distinct. For centuries Shofars have been measured by following the outside curve of each horn with a string, then measuring the string.
Shofars comes in 3 finishes: natural, half-polished, and finely polished. Which to buy is a matter of personal preference. The finish does not affect the sound or Halachic suitability of the Shofar. Natural Shofarot are neither smoothed nor polished except in the mouthpiece region. Half-polished horns are polished in part but some to most of the Shofar is left natural (this is favored by Christian worshipers. Fully polished is the traditional Shofar used in Synagogues the world over and are still preferred buy most Jewish Ba’ale Tokeah (Shofar Blowers).
The Golden Calf incident may be the reason why cow’s horns are forbidden, but the Talmud disputes this. The horn that is to be made into a Shofar is first heated then hollowed out, then depending on the religious tradition of the artisan or his customer, it is bent or twisted. Sometimes, very fancy Shofars are made by gilding or carving religious symbols; whatever the decoration, the mouthpiece must be plain. The Piah or mouthpiece is always at the narrow end. The mouthpiece a little smaller than on a French horn, but similar in shape. a Shofar is not allowed to have slides, valves, or keys, Small Shofarot (the Hebrew plural), can be 8 or 10 inches long and produce higher pitched sounds; longer Shofarot, (up to 3 feet long) are made and produce deeper, richer tones.