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Which is more useful or most used, the 2.2mm x 6.5mm or 2.2mm x 8mm partially threaded body screw?  

I suppose you can shorten an 8mm but can't lengthen a 6.5mm - but would that become tedious if the majority of manufacturers use 6.5mm as the norm? 

Would an 8mm just allow more free body movement or ... ?

Thinking of buying a quantity and don't want to end up with the least useful size