Something I just found,,I have 1 frame and 2 brrls,The forearm fit really nice on 1 but put alot pf pressure on the stud of other. I didn't want to mess with the good fit for the 1st so I got another forearm for the 2nd barrel.(dedicated forearms)
Now, I'm doing the set screw/o-ring fit. On the first barrel I could turn a 1/4" long set screw all the way down, back it off 1/2 turn and got the fit I wanted. A drop of blue lock tight on the bottom during the fit and all is good. As with other barrels I've floated, I like the paper strip to slide freely with no resistance.
When fitting the 2nd forearm too the second barrel I discovered the factory inlet (mortise) for the stud screw mount in the wood forearm is a full 1/10 of an inch deeper! That's alot! The o-ring is NOT being contacted or compressed at all.
Whilst I was goofing around with it, I backed the set screw out a couple turns and put the forearm on and got that "wobble" you mentioned. Hmmm? That's a Bad thing, don't want that.
So, I'm off to the hardware store to get a large neoprene washer with a hole the size of the OD of the stud post and around 1/10 thick. I'll cut it to fit the mortise in the forearm and e-poxy the darn thing, then I'll do the set screw/o-ring fit like the other. I thought I might cut a piece of wood to fit but I think the washer will work just fine.
For what it's worth, the 1st mortice is .10 deep and the second is .195-.200 deep. It's interesting that there is such a BIG differance in the mortice depth, gotta love these things, sure does keep ya thinking
Your question, "Now comes my questions, finally. I know it says in the FAQs that the screw should bottom-out when the rubber o-ring is half-way compressed. How do I know when that really is?"
Well all I do is make o-ring contact then squeeze the forearm with my hand wraped around the barrel, turn the mounting screw in and see if there is a little more squeeze left, ya kinda gotta go by feel and looking. I want that o-ring to compress just some. Another thought, I expect that o-ring to need too be replaced every so often. I need one in there that's still giving some resistance and not set like a washer in a faucet that needs repair.