If you pick the TH before you load the powder, all you are removing is any powder residue that is sticking to the sides of the TH.
The purpose of picking the vent( TH) AFTER loading the main charge, AND Seating the PRB, is to open a small channel into the main charge, so that the heat generated by the burning priming powder outside the barrel, can enter and ignite more than one or two granules right next to or in the TH. Those outer granules can take on moisture, and may just fail to ignite. You have a greater chance of getting the main charge to ignite, with a hole into the powder that gives you access to MORE granules of powder.
It helps if your TH is either coned on the inside, or has a coned TH liner in the barrel. I have the Chamber's White lightning TH liner in my .50 cal. rifle, as well as in my 20 ga. fowler. Because of the cone on the inside, powder remains close to the outer edge of the TH, even after I pick a hole into the main charge.
MY "vent pick" is hand make. I taper the steel pick to a "point" which I then file down a bit. The taper provides enough elongated steel to go into the TH and go across the width of the bore. I then file two opposite sides of the rounded "Point", to make a Paddle-like Nose to the pick. This allows me to run the pick into the main charge, give it a quarter twist, back and forth, and with draw it, leaving a hole, by pushing some of the powder away from the pick.
I can't remember the last misfire I have had due to the main charge failing to ignite. ( I had one a few years ago when I was stretching the "life" of an old flint too far, and it simply could no longer produce sparks.) :thumbsup:
I am loading 2Fg powder LOOSE in my rifle, and that gives me "room" in the bore to move that main charge out of the way.
If you are loading 3Fg powder, it will compact more than my 2Fg. That gives you less space to move powder after you seat the PRB. In such cases, I found that putting the vent pick into the barrel, before loading 3Fg powder and the PRB is the best way to create that hole in the powder charge. You want a pick that is small in diameter than the TH, so that air can still escape past the pick and out the TH as you seat the PRB on the powder charge. That escaping air will force "fines"( smaller granules and chips of powder than 3Fg) out and into the TH liner, giving you better ignition. :hmm: :thumbsup: