Always change.
If you hit the bottom of the vial, the end of the pin can 'barb'...put that into yourself and when you are withdrawing you could risk snagging muscle fibre...ouch!
This is a small risk, but a risk all the same.
I know I'm still a 'newbie' but have been here for ages.
yep, the best way to develop a strong punch is to practice punching.
I've trained with some 'monsters' in my time some with strong punches, others not so strong...I've also trained with very short and slim Karate-ka and they could put a hole in someone.
All in the technique.
As far as...