Tinker, I only have one double. It prints different loads different places. And different brands of bullets regulate slightly differently. And it is more accurate with some bullets than others (it is best with GS Custom). Your statement about burn rate and barrel time makes sense, but I have not tried to correlate where the group prints on paper with burn rate of the powder, but I will start noticing it now. Note that the GS Custom bullets achieve a higher velocity with a given powder charge than WOodleigh, so they tend to regulate earlier and print a bit lower on the target.