OK. Show me a picture of those radiused edge adjustable parallels that work like the top slide of an auto pistol..
Not an unreasonable request, quite reasonable, rather.
Not everyone has had this class of goods in their own hands.
Here is the first one that popped up:
https://patentimages.storage.googleapis.com/61/98/d1/3a5356223b5607/US4168574.pdf
There are others. It should not be a surprise they have variations. Patents force that even when better ideas have not already done so.
Dimensions of the "Tee" and mating "Tee groove" are not the same as the slide rails of an auto pistol - there's no BARREL to be accommodated down the middle as requires splitting the Tee, optionally reversing it outside in, or inside out. Happens I have "some of each".
The basic concept, however, IS the same.
The two pieces are meant to be able to slide, one over the other, but NOT mean to separate in use and fall on the floor in two separate impact zones.
That would make the parallels too clumsy to handle, and the firearm bloody dangerous to fire! See Walther and P-38, which actually HAS been known to shed its top slide in hard firing, even though that was not in the original plan!
I brought it up because I first had Mark One Eyeball on these around fifty or sixty years ago - loooong before Google started hosting patents or personal confusers, the internet, or Google yet existed.
AND.. because the very
presence of rails or dovetails or any similar feature on the two mating parts once separated make the insertion of any intermediate spacer not already purpose-built
and fitted by the same maker a "non trivial exercise".
Any field-expedient DIY to be done-up in a shop with only average equipment, not a precision metrology shop, has to jump through a few hoops to either CLEAR those rails, grooves, dovetails - wotever by the OEM's choice - and rely on the external cage I mentioned to prevent all the now LOOSE bits falling on the floor...
ELSE has to be machined and fitted to UTILIZE those rails or other interlocking features to just as high a standard of fit and accuracy as if the factory had done and QC'ed it themselves.
I did say "non trivial", yah?
There are saner and more convenient methods for gaging larger bores than that extrapolating off this tired old wheel with DIY high heels, falsies, or stilts.
Anyone as needs pitchers of such, just ask. Or google and "images". Not hard to find those, either.