
sich verknallen in
to fall for
(Colloquial) To suddenly and intensely fall in love with someone. It describes a strong, often youthful crush.
Er hat sich total in die neue Mitschülerin verknallt.
He has totally fallen for the new girl in his class.
to fall for / to shoot off
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to fall for
(Colloquial) To suddenly and intensely fall in love with someone. It describes a strong, often youthful crush.
Er hat sich total in die neue Mitschülerin verknallt.
He has totally fallen for the new girl in his class.

to shoot off
(Rare) To shoot off all available ammunition.
Die Soldaten hatten im Gefecht ihre ganze Munition verknallt.
The soldiers had shot off all their ammunition in the battle.
This verb is reflexive and always needs the preposition 'in' with the accusative. So the structure is: 'sich in jemanden verknallen' (to fall for someone).
Ich glaube, ich habe mich in meine Kollegin verknallt.
I think I've fallen for my colleague.
'Sich verknallen' (to fall for someone) is very colloquial. You use it in informal conversations with friends, but not in formal situations.
Meine kleine Schwester hat sich im Urlaub total verknallt.
My little sister totally fell for someone while on vacation.
The word contains the stem 'knallen' (to make a loud bang). This emphasizes the sudden and intense nature of falling in love, as if you were struck by lightning.
Als er sie zum ersten Mal sah, hat es bei ihm sofort geknallt und er war verknallt.
When he saw her for the first time, it hit him immediately and he was smitten.
to miss / use up / fade
Both verschießen and verknallen can mean using up all the available ammunition by shooting. Verschießen is the usual choice for this meaning. Verknallen has the same sense of total consumption in this context, but it is rarely used that way.

Der Soldat verknallte während des Angriffs seine gesamte Munition.
The soldier shot off all his ammunition during the attack.
Der Soldat verschoss während des Angriffs seine gesamte Munition.
The soldier used up all his ammunition during the attack.