This article is an excerpt from our Sefer
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6. A thin Shofar: 
If the Shofar was sanded down, whether on its inside or outside, until it became very thin similar to the thinness of a scab, nevertheless it remains valid.
 The reason: Now although this Shofar is not the same structure as it was when it grew on the head of the ram nevertheless since it follows the same form as it was originally when it grew on the head of the ram therefore it is considered “the way of growth”. The verse only excludes a Shofar that was turned inside out as in such a case one changed the Shofar from the form of its original growth on the head of the ram. [ibid]