Grassholm Island was used as a target for bombing practice by the United States Air Force during the Second World War, leaving small bomb craters and shrapnel across the surface (Driver 2007, 87; Hague 1972). The sites of these craters, some with metal fragments, have been recorded in the past by Douglas Hague. During a field visit on 14th October 2016 the nose cone of a projectile was recorded still embedded in the ground surface at the NGR of the record. The lack of small fragments around it suggested to military historian Mark Kahn that it may have been a Rocket Projectile but a practice round, rather than a fragmentation round. Numerous other small craters or likely craters can still be seen on the eastern side of the islet.

T. Driver, RCAHMW, 17/10/2016

References:

Driver, T. 2007. Pembrokeshire Historic Landscapes from the Air. RCAHMW.
Hague, D.B. 1972. Grassholm. Archaeology in Wales 22, 40-1.