To answer your question as to weather or not the "barrel blast at the window sill" could affect bullet flight, the answer is no. By the time the "blast" gets to the window sill, the bullet has already gone by. The bullet will push some air out of the barrel, BUT the "blast" comes after the bullet is already gone.

Bronko has a good point. I learned to place my front rest right under the action bolt every time and it did improve consistancy.

You said you developed this load when it was cold out. What was the temp when you developed it? If it was an extreme temp swing these cold weather loads could be creating an over pressure condition that could be outside of SAAMI specs. This could lead to pour accuracy do to a large swing in velocities. Just another thing to think about. Also if this is the case these rounds could be greatly overpressured so for safety sake re-develope this load for warmer temps.

Andrew