Jack not being a bench shooter am not quite following what the issue is.  Seems to me if the Rifle's sights are aligned and hammer fall to ignition to bullet leaving the barrel is consistent it's all you can ask the rifle to do. 

If the issue is does the rifle stay aligned hammer down to hammer cocked I doubt if any gun can stay aligned in a free standing bench rest while the action is being cycled, Bolt, Auto or Single shot

While the action could flex It's more likely the movement is in the rest than anything else.

Going back to my small bore prone days it was essential to re-set after every shot no matter how small the movement after the rifle was fired. If re-set and aligned properly I could shoot smaller groups prone with a sling than I could from a rest. Reason is the rest allowed movement, so did the prone with sling, however re-setting every shot prone was more consistent than my bench shooting & lack of bench Skills.

Here is your quote

"If the return to rest is perfectly consistent from shot-to-shot, the POA shift will have no effect on POI.  However, if the return to rest is not consistent, then vertical stringing is likely to occur." 

All sorts of things going on when the rifle is fired, if they go on the same way every shot it's a non issue. Or so it seems to me

Boats

Is the lower tang actually flexing, or is the lower tang rotating slightly relative to the frame?

I might add some record setting scores as well as record setting groups are being shot with the CPA through bolt system so those of you who own one don't let this conversation crawl into your head. I would recommend you taking a look at your butt stock bedding to insure that it is correct and to then head to the range to break some some of those records.

Boats,

How's the Falling Block Works different than the Ballard or Highwall?

Bruce

I have 2  cpa rifles.  I dont use tang mounted sights.  So I tested both with an indicator to se if they flex,  mounted a last word on the barrel sight mount ,   walla..   1 moves the other dont..  the older is much harder to cock, the newer on is very light..  one reason to use barrel mounts for sights, eh..   it may cause vertical strings, eh..

I'll  do it again, as time permits, maybe tue pm, wasn't but a few tho,  it was the older one with a verys stiff spring, that has to be set strong topop the caps, more to come.....

This is progress report - of sorts.

I removed the original mainspring and compared it against several other CPA mainsprings.  The original spring  was a bit longer, a bit wider, a bit thicker, and quite a bit more arched than the comparison springs.  In other words, the original was very much "stiffer" than all the others.

This inspection confirmed the operating characteristics of the original spring.  For those unfamiliar with the 44 1/2 action, there is a set screw in the lower tang that can be used to add pre-load to the mainspring.  With the original spring installed and the set screw completely backed-out, it took two hands to pull the hammer into full-cock.  The hammer fell with sufficient force to fully puncture large pistol primers and would leave very deep craters in rifle primers. 

I replaced the original spring with one of the spares.  With the replacement spring installed and the set screw completely backed out, the hammer required very little effort to go into full-cock but did not have sufficient energy to fire a primer.

It required about 3/4 turn on the set screw to provide enough hammer force to consistently fire primers.  This spring setting did not puncture pistol primers.

So far, so good.  Or should I say "close, but no cigar."

Even with the lighter spring installed the POA still shifed upward when the hammer was cocked.  The shift was  less pronounced, but still sufficient to be troubling. 

And sadly, the rifle continues to print two really nice groups displaced vertically just about the same amount as the POA shift caused by cocking the hammer.

As I've said before, there may be no cause and effect relationship between the mainspring-related POA shift and the vertical grouping.  But, then again, there might be - so I'm going to mess with this a bit more. 

By the way, changing the mainspring did cure another pesky problem with this action but that's another story for another time. 

JackHughs