Dave I have also run into chambers that were not reamed straight and you can typically see one land starting to engrave further up the nose than the  rest. That first land touching I feel then pushes the nose of the bullet off and the bullet being as soft as it is then yields to that inherit weakness and thus the bullet cannot fully self straighten itself back to being centered. Not much one can really do about it other than to be sure the bullet is indexed in hopes of consistently duplicating the error form the chamber not being cut central to the bore and the lands would no longer be concentric. 

JLouis

I have always had better accuracy with a none tapered bullet but each rifle and chamber typically has an individual preference only of their own liking.

JLouis

So to corral this back and ask for a bit of clarity or more wisdom--

Hypothetical barrel (all arbitrary set numbers for discussion only)
with
bore .300, groove, .310

If a THROAT was .3105 and one inch long with no taper (basically a freebore) that terminated into a very slight tapered leade into the rifling (allowing the ogive to kiss the bore centering it), freebore having parallel sides, using a parallel sided bullet of .311 diameter (more than a friction fit) and a bullet of one inch in length plus its nose length

Keep up with this as I describe

Neck is 330 diameter (chamber neck) and walls are about .018 in total (.312 plus .018 neck walls) offering .0015 plus in neck relief.

Basically a cartridge in a chamber that the case mouth is the end of the neck chamber region, then a freebore starts for one inch that is .0005 SMALLER than the bullet absolute parallel sided diameter, having the nose of the bullet enter and kiss the rifling with a 1-2 degree taper..... like a plunger in a syringe, all parallel, all more than slightly interference fit and all WATER TIGHT due to bands of the bullet being fully married to the smooth parallel freebore walls (freebore is a bad word for the section but its easy to discern freebore from leade for sake of conversation)

Bullet would be engraved and held perfectly in line with the true bore and would self center, the alignment would be really limited to the casting, not the chamber....

This is NOT an ideal condition (if it were possible) Im assuming. 

It seems the taper in the bullet and throat are all designed (as descried by many in this thread) to offer a three point centering system positioning the nose in the bore and engraving it so it can't move, the rear end is named into a position held concentric with the BORE and SEALED. NO SLUMP POSSIBLE.

Would it be bad if the THROAT as I described above was NOT tapered conventional but was parallel sided and .0005 smaller than the bullet and devoid of rifling (absolute gas seal)??
I understand it may not be POSSIBLE to seat a 1 inch bullet in a steel tube with .0005 engraving on ALL bands at same time (basically a Lymah H&I die sizing effort).
If it were changed to .0002 (if tolerances can be held as such), allowing fairly easy seating, is there another caveat?

Lastly at what point does the bullet become a bore obstruction and does a bottle neck case (like 30-06, etc) cause a greater concern?? Ringing of barrel and chamber??
trying to wrap head around concepts.

Considering the thousand or so different combinations that have worked for one fellow or the other, the Schultz solution is, in all probability, the "optimal" throat for breech seating. 
I like the contraction, BS, there is a bit of it here, no doubt.

If the freebore is larger than than baseband diameter and there are many of those I have seen to date. It then serves no useful purpose in helping to guide the back of the bullet into becoming central to the bore while the nose is trying to accomplish the same. When the bullet is being seated it is also being rotated by the lands at the rate of the barrels twist causing the base of the bullet to rotate off center until it contacts the wall of the oversized freebore that then restricts it from moving off further and at that point it is no longer centered nor are thier any hopes of getting it re-centered. Point being most rely on the nose to to do all of the central to the bore guiding work and never give much thought about the back having to also assist in doing the same task. As Dave pointed out having the seating plug square is indeed important but while the bullet starts to rotate on the plug it will also wonder off in the direction of rotation. If the freebore is not of the proper size to restrain any such movement to help guide it any hopes of the bullet being seated central is then greatly deminished. That is also why I feel a cylindrical bullet shoots better the body of the bullet is aways in contact with the freebore as it moves in. Plus it provides other benefits as well that are not related to the seating process.
Does it really matter who knows but if does not exist or is eliminated I can honestly say nope it don't matter. 

JLouis

I also use cylindrical freebore in my chamber designs, as much as 0.200", to help align the bullet with the bore. My non-tapered bullet's shank diameter closely matches the freebore's diameter and my bullets are a slip-fit in the case as I do not want the case to influence the bullet's alignment. I also cam the bullet into the lead about 0.030" when I close the block on my highwall. I get good accuracy and consistency with this setup.

I like my throat wet with a little good Bourbon when I do My BSing.

40 Rod