Many thanks Schuetzendave for your fine reply and tutorial.  I don't profess to have the ultimate experience in bench shooting skills, but I keep working at it.  I purposely made a bench stock for the CPA, and it does help over the schuetzen style.  I do very much like the CPA rifle.
Your advice on the both the bullet alloy, and powder charge are well taken, and I will try both starting this week.
I use Paul's plugged case as my breech seater, but with a harder alloy that may prove to be problematic.  A seater where the bullet is directely held by the case, and then inserted into the barrel I would think is the best of all worlds to eliminate concentricity of seating errors.  That I will have to look into more closely.
I have tried using cork, foam, and PVC (all .062" thk) as wads, but only at the case mouth to keep from dumping powder into the action. 
My findings are that they all work for the purpose intended, and didn't bring much to the accuracy equation.  Except for the PVC, which may afford protection of the bullet base at higher velocities.  The other two disentigrate upon firing.  I use no filler of any kind.
As far as lube, I find that it is very important for consistant accuracy, and that SPG and Improved Emmerets are both good in that department.  Alox and others did not fare well at all.
I'll keep you posted with results of alloy and powder charges.

Rap

A lot of people on this board know more about tweaking a load than I do.  However just to lay out my methods

The first thing to do is get the bullets seating depth right. I push one in then out with a cleaning rod looking with a glass at the rifling engraved marks.  I want the bullet seated into the lands with it's base well supported and ahead of the case. I take pains with this process and do it more than once getting it right. You can coat the bullets nose with black marker to get a better read on whats touching and whats not.

Proper bullet seating is why we breech seat and the key item in your experements.  I don't adjust bullet dept to suit powder burn, the bullets proper seating comes first.

When it comes to powder charges.  There is an optimal charge for every bullet and chamber. The larger the chamber the more powder it takes smaller less.  However I look for that charge by adjusting with my powder measure not changing the bullets seating depth.  When I find it it's only a baseline load.

For example I shoot my 32/40 using 30 clicks on my Harrels charged with 4227.  I may on match day drop down or bump up a few clicks.  The measure setting only gives volume. When it's hot or cold there is a difference in burn rate that may require more or less. Different lots of powder may have different burn rates too.  There are people who think wind may be something worth changing powder volume for too. I don't but would not argue with them on the point there could be something to it.

16.5 grs weight may look like the magic charge one day and not be the right one another. Be flexable. 

While group size is the best indicator looking at the paper on your target is good too. Are the holes round or do they show signs of tipping. If so you need more powder.  If it's possable to recover fired bullets they are very good indicators of how the bullet is supported when fired.  It's not easy to do but Slug Gun shooters for years have used fired bullets as one of the key things in working up loads. 

Actualy I am very suspicious of group sizes. There are so many hold and recoil influnces that can throw them off. Unless the bench methods are tried and true short run group sizes don't tell you as much as one would think.

Seat your bullet properly and then play with powder

Boats