Thats a really difficult task you have set yourself, the pellet must be behind the card and only just clipping it whilst moving away. I was thinking along the lines of a gas driven 'gun' like the more expensive air rifles or a mini crossbow which should both be more repeatable and if you used steel  pellets for the gun you could eat the grapes.
		 
		
		
		
		
		
		
		
		
	
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