I appreciate the question from bnice.  The following rifle features may influence load performance:

Firing pin:  Small.
Barrel:  30 inch Krieger .300 x .308 1-12 twist SS, light Palma profile, threaded and firmly tightened into the action.
Headspace:  Zero, the breech block is tight against the cartridge head when the action is closed.
Buttstock:  CPA Sporting/Varmint.
Forend:  Shaped like my Palma stocks with a 4.9 inch long section bedding into reliefs cut in the rear underside of the barrel.  Two mounting screws pass through aluminum pillars, the forward section is free floated, clearance is provided between the forend and receiver.
Pressure Limit Indication:   
No loads so far have shown any significant pressure signs on the cases.  Fired primers are rounded and primer pockets remain tight.  As charge weights increase, eventually extraction becomes sticky.  I believe this is due in part to the absence of primary extraction capability as an inherent characteristic of the 44 ½ action.  This is not a criticism.  I just stay below the level where cases start sticking. 

Hello Premod70.

Thank you for your comments concerning rim thickness variation and possible binding of cases against the breach block.  I would like to present a more accurate description of headspace on my rifle in response to your input.  I selected ten used cases at random and measured the clearance to the breach block using feeler gauges when they were closed in the action.  In inches, seven afforded less than .0015 (my thinnest gauge), one measured .002 and two measured .0025.  They all produced the same feel going in and out of the action.  After trying to use a good caliper, I think I would need to build a custom gauge to capture better rim thickness measurements because the rims exhibit a substantial bevel on the outer bottom of the cases.  I will keep your comment concerning rim thickness variation in mind as I continue testing loads.

Most of my loads so far with cast bullets incorporated a gas check.  I judged that gas check designs would support a better chance for supersonic velocities at 300 yards.   Supersonic velocity at the target is required for our ShotMarker electronic target system to locate the bullets, and 300 yards is the shortest distance we regularly practice our prone shooting.  I would like to use this rifle for highpower practice.

My best estimates of operating pressures come from QuickLoad which predicts a maximum of around 30,000 psi for the 123 grain PPU jacketed SP and 13.5 grains of N110 listed earlier in this thread.  The best cast bullet loads so far were loaded with SR 4759 which is not a listed powder in my copy of QuickLoad so pressure estimates become more challenging.  I did enter a load of 5744 in QuickLoad that duplicated the velocity of the 4759 load listed in this thread resulting in a max predicted pressure of around 20,000 psi.

Both of the loads listed earlier shoot and extract without any sticking in my rifle.

Hello Premod70.

Good comment concerning sling tension variations.  I applied the best ideas I could come up with by using cast lugs and pillars for the forend installation, but the sling is still attached to something directly attached to the barrel.  I will put more thought into isolating sling tension effects.

The No. 4 Lee-Enfield is another rifle with two piece stock that may have accuracy issues due to sling pressure.  One approach is to use a two point sling attachment with the second point of attachment at the rear of the forend.  I suppose this has been done for so long that many top competitors from Commonwealth countries even use two point slings on their fullbore (Palma) rifles. 

I attached a picture of my No. 4 with a two point sling setup.  Maybe I will try a two point sling on my CPA rifle.