Hi MD,
Man this is one those question's, with no definitive answers. The answer is yes, but as to why. There are so many variables that it's difficult too say, the when's, why's, and what for's. Literally volumes could be written on this subject, and likely has. Powder, bullet, and throat length and diameter, all play a part, not to mention other aspects.
As a rule, in my practices, I like to seat the bullets as within .005-.010" off the lands. Some bullets are way off though, Barnes are the exception, and accuracy is my goal. Also I use a bullet/powder combination that gives me the highest possible load density (full case), without major compression. This, I find gives me the least amount of extreme spread in velocity, aiding accuracy. Top end speed is secondary in my opinion, tight averages on speed is foremost.
But for an example, you take a .308 with a 180gr. seated to
sammi specs. you get a certain velocity. Then say in a single shot with a long throat. You can seat the bullet out, and your case capacity increases. If you do not change the charge weight velocity will drop. But if you increase the charge weight, back to acceptable pressures. Your speed will increase. This is an extreme example, but you get the idea.
Generally, in a standard chamber if you take a given bullet/charge combination. The deeper you seat the bullet, the more pressure is developed, resulting in higher velocities. And the opposite is also true.
