I think they're generally round, regardless of trajectory. Here's one from the Castlemartin range.

http://s0.geograph.org.uk/geophotos/01/99/95/1999554_65d2513b.jpg

And some others:

https://farm4.staticflickr.com/3140/3096856252_745f7397b3.jpg

http://farm5.staticflickr.com/4017/4444627276_1af8ae6f6b_o.jpg