Weapons are of two kinds.

One is called Sastra ,another Astra.

While Sastras denote conventional weapons like Bow and Arrow,Mace(Gatha)`,Swords of different type…, Astra denote weapons that are energized by Mantras.

Some of these Astras do not  any supportive conventional weapon like Bow and Arrow.

Mere uttering of the words will do.

Another type is the one used frequently in Indian legends are the Astras that are used with a bow and arrow simultaneously reciting the Mantra ascribed to the Arrow.

These weapons are to be learnt from a Guru.

The Bow, Dhanus and the Arrows are to be worshiped daily.

Only then they will be effective.

Important  fact is that the user must be spiritually inclined and should have attained the status of an effective Sadhaka.

When a Sadhaka meditates and raises his Kundalini (Brahmastra in this context), it derives energy from his base chakra (Mooladhara) and propels upwards.
Then, it penetrates through 5 other chakras deriving energies from each of them at every stage.
Finally it hits the target : Crown Chakra (Sahasrara) and explodes there with a brilliance.
That explosion annihilates all “Illusion(Maaya)“, and leaves the sadhaka with the debris called as “Aham Brahmasmi (I’m Brahma)” feeling.

Otherwise the Astra will not be effective.

It is true that even well-developed Sadhakas turn bad morally, like Ravana.

The Astra will still be effective  because of the spiritual efforts involved in obtaining the Astra.

Brahmastra is an Ultimate weapon.

It is energized by chanting Gayatri Mantra by chanting it in a certain way.

Brahmastra leaves destruction all around, including the environment.

How to chant the Brahmastra Mantra, I have explained in one of my earlier posts .

How the Mantras affect weather, bring rainfall, produce heat, change thoughts in human minds around us, control animals and birds are explained thus,

phantam vahnisamayuktam vyoma halasamanvitam |
mesadvayam dantayutam halahalamatah param || 34-5 ||

ghanadyam vayupurvam ca dantayuktamathantimam |
sarasam carksaparyayam bhantam bhrgumatahh param || 34-6 ||

ambaram vayusamyuktamarimardanamapyatah |
pradIptamatha vaktavyam paramam ca padam tatah || 34-7 ||

tatte pade prayoktavye gayatryaA madhyamam tatahH |
padatrayam prayoktavyametad brahmastramIritam || 34-8 ||

“It contains air, fire and cosmic poison, two goat-like fangs, full of poison, weighty, emits air, contains mercury, fiery, sparkling, sky is filled with air, enemy killing greatly radiant and it is projected with three hymn, Gayatri at its centre, it is known as brahmastra”(Ahirbudhnya Samhita of the Pancharatra Agama volume1)

The counter Astra to Brahmastra is  called the BrahmaSirōnāmāstra, which was never used in war, as it had power equivalent to (Brahmāstra),representing 4 heads of Brahma and in Mahabharata war, it was known only to Dronacharya, Aswatthama, Arjuna and Karna.

http://booksfact.com/mantra-sastra/brahmastra-nuclear-weapon-energized-gayatri-mantra.html