Didn't bother to go through pin gages to find the actual size, but a #20 drill bit (0.161") just fits.
So, the transfer port is huge--about 0.035" larger than "normal". There are a lot of ways to reduce the port size. Could've drilled it out even larger, threaded the hole and made up several set screws with various sizes of through holes. Or made interchangeable sleeves that are secured with a perpendicular setscrew. Could've done a bunch of work, but the gun's firing cycle was so harsh, and with the end result unknown, I couldn't find the motivation to spend the hours doing the machining. The fastest, easiest way to see if this idea would work would be a simple reducing sleeve bonded into place. I've done this very same fix in the past on a Haenel 303-Super.
Turned a piece of brass to 0.160" then drilled a 0.135" hole down the center.
Leaving a very thin walled sleeve.
Mixed up some quick setting JB epoxy...
and pressed the sleeve in just below flush.
I know. Not much to it.
No protrusion to catch on the edge of the base block.
While it was apart, I removed the main pivot's locking bolt.
And found a replacement socket head cap screw. Just taking it to length.
Also reduced the head's diameter by a couple thou. Here it is re-blued.
And installed. Well, the pivot bolt looks a bit ragged. Might need to turn a new one at some point.
The sleeve is a success! The noise is gone and the rifle is producing just over 28 foot pounds with H&N Baracuda Hunter X-Treme pellets(670 fps). H&N wadcutters are moving out at 740 fps. We'll call this a victory for the moment. The gun still has a pile of recoil no doubt due to the sheer weight of the huge mainspring and the heavy walled piston. Reducing the piston weight may just be on the list.