The splash radius on the 40mm Bofors round is far too small for a round its size. The MkII. 40mm HE round used by the US Army (since the Nasty's used an Army gun, interestingly) has an explosive mass of 68 grams. It was using TNT as filler.

For comparison, the 40mm HE grenade M381 used 32 grams of Composition B explosive, roughly 33% more powerful than TNT. This would mean that the effective explosive content of the grenade is about 42.5 grams of TNT.

While its probably that the M381 was given more emphasis on blast-area due to its nature and intended deployment, the 40mm HE round as used by the Bofors gun on the Nasty class PTR should have a similar area of effect due to the greater explosive mass.