I checked with my friend EJ, the most knowledgeable person on these matters that I know, and this is what he wrote back:"The confusion seems to come from the fact that most gun flaps today aren't really properly cut. The flap is essentially a "capelet", designed to button over and keep water from running into the upper corner on the buttoned side of the coat. If you button up a trench coat, you may notice that the part just infront of the collarbone is just one piece of fabric on top of the other, and the overlap is open on the top edge. This lets water in, and if you raise yr arms (for any reason, but for example, shouldering a rifle), it has a tendency to open up even more by separating the lower and overlapping side of the coat. The gun flap covers it. It all seems vy arcane and fussy (which it is), but it's also vy Victorian......
Storm (Gun) Flap – Assumed by many to be padding for a rifle butt, the “gun” flap is actually a protective flap to ensure water does not slip into the jacket as it runs down the shoulders.
3.因士兵举枪时,风雨易于入内,增加此挡片,名为“枪挡”实为“风雨挡”。
Storm (Gun) Flap – Assumed by many to be padding for a rifle butt, the “gun” flap is actually a protective flap to ensure water does not slip into the jacket as it runs down the shoulders.It effectively serves as a cap, keeping the wearer dry, assuming he has on headwear.We see it on the right side for men and on the left side for women as the jacket buttons up in opposite ways for the different genders.The reference to this flap being a gun flap is probably due to it being requested during WWI when officers complained about water seeping into the coats after firing their rifles.The raising of the right arm opened up and exposed the early trench coat's breast fold to the elements – not something you want in a downpour...