I think you have it one there Ron. Shell holes, bomb craters, call them what you will have an infinite amount of shape possibilities depending on how they were created. Just thinking it over, I would say that the round ones would be a vertical (or as near as) trajectory with a delayed fuse so that the projectile went off under the surface blowing straight up. If it came in very shallow and still went off underground, I would think it would have the elongation trench where it came in and then a round hole, similar to a stylised comet style.I guess modellers could argue the toss all day about the shape of 'holes.' The trajectory of the shell, the amount and type of explosive, type of ground ect. would all make for a different shaped depression wouldn't it?
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