Something

unsigned int i;
int prob;

prob = 0;
for (i = 0;i < MAX_SCHEME_WEAPONS;i++)
{
	prob += wainfo.scheme->weapons[i].crate;
}

if (prob > 0)
{
	prob = seed % prob;

	for (i = 0;i < MAX_SCHEME_WEAPONS;i++)
	{
		prob -= wainfo.scheme->weapons[i].crate;
		if (prob < 0)
		{
			*result = i;
			return 1;
		}
	}
}

return 0;