I will admit that some points Dr. Drew made, got me thinking that I should be taking more measurements to complete the picture. Part of the reason I don't often turn the barrel around and measure in from the breech is laziness due to the manipulation of the barrel, without that extra set of hands. I have always been tuned into those 'improvements' that aren't: chamber lengthening. My preference is towards smaller frame guns, so knowing the wall at the forcing cone is perhaps more important that knowing what the wall is between midway and muzzle. The fore end area is critical to me also, because I have a permanent image in my brain a of guy's thumb tip blown off at a field trial when he let loose to much oomph for the English hammer gun he had only just passed around at show-and-tell.
Brian makes important points, and simply knowing how to properly use the Manson style gauge, and knowing the real numbers, isn't of any value, if you don't also know what numbers are OK, and those that aren't

I especially like the one at the 7 o'clock position.





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Yep, that one and the one at six o'clock could really get down into pits well. The tips at 4 & 7:30 mimic the 'ball' that appears to be on most gages. Having various options would be a nice feature built into a purpose built gage like these.

I considered using a replacement dial indicator tip, which would have required milling a flat on the rod, sufficient such the the dimension from tip-to- back of rod was less than .550”. The whole reason for making the new rod was to measure 28 gauge barrels. Damn tips are more expensive than Harbor Freight dial indicators.
As John Hosford showed in his video, aligning the indicator tip, to the ball is delicate; more so, the smaller the inner tip.
Without question, a finer tip inside the ball will more accurately measure bore pits, however I didn’t anticipate doing barrel evaluations on guns for others, as much as I wanted something for my personal use. I really don’t buy so many guns, and of questionable pitting to offset the smooth needle characteristics given with the .500” diameter ball

The tips would be the cheaper part of the endeavor b/c I'm thinking they would require modification to make them work (ie: the .55 dim you mentioned). It would take a little fine machining to make the working end of the gage work-out just right.

The tips in the photo are Starrett but that's only b/c I buy Starrett everything. There are cheaper alternatives through company's like Enco etc. Also, if one did want to stick with Starrett tips, they can be purchased individually.

My 16 ga. O frame VH has 26" barrels struck 2-15 on the flats, supposedly weighs in at just under 6# (have not personally weighed it). My best efforts indicate .028 MWT both barrels about 12" forward of the chamber's end right along the bottom rib. I use a Galazan tool, results are very repeatable. Only thing wrong with that tool is it should come with wheels and a pull handle.

A couple of thoughts. Yes, rotating the barrel to take readings around the circumference is vitally important. The GH I just sent back was more than thick enough...until I measured adjacent to the bottom rib. Readings became noticeably "thinner", dangerously so, as I got closer to the bottom rib and this was evident throughout most of the barrel length. Had I just measured one zone without considering eccentricity, I would have believed the gun was safe to use.

Pits. There really is no accurate and reliable way to measure the depth of a pit using a gauge. Pits can have minute pores, fissures, etc. All you can do is know that the pit is at least the depth you have measured, and know that it is likely deeper. Pits tend to erode more uniformly with fluid steels since there are no edges of dissimilar metals to follow, but until they are carefully honed out, one can never know.

This is one reason that British proof houses require that all pits be removed by boring or honing, then the bores polished, prior to reproofing. There is also the matter of the tendency of pits to grow with time and humidity, compounded by the fact that it is very difficult to get a pitted barrel truly clean.

I measure lots of (softer than AISI 1010 and up) pattern welded barrels and my 'tip' is brass. I can point it up as needed with use.

http://pic20.picturetrail.com:80/VOL.../406674911.jpg

I agree with Garhart that it is very difficult to a. get into pits and b. know if you are at the bottom of the pit.
Which is also why a full evaluation should include bore visualization with a rigid or fiberoptic bore scope.

Parker barrels are very consistent from the measurements I have taken on my 13 Damascus guns from 10ga to O frame 20ga. All 13 guns are .040 or more at 9 inches and all are .030 plus min. wall except one 12ga #3 frame that was returned to PB with the request "take dent out of barrel". That barrel is has a spot that measures .026. My 6 frame 10ga has a min. wall of .080 in both tubes.
I did purchase a Parker 16 ga that had been honed and had barrels with a min. of .019 and .035 at 9 inches. The .019 was to close to where I hold my left hand for comfort so I sold the gun.
My Fox guns are similar but tend to be very close to .040 at 9" and one tube measures .036 at 9" the gun is very high condition but the bores are .733. Many Fox small bores I have measured and/or owned have min. walls of .025.
I don't feel a need to have them proof tested, but go with the practice shoot shells for which the gun was designed.

Calibrating the Manson/Brownell should be done in a level plane and the 2 points that establish that plane are the anvil ( the measuring point inside the barrel) and the point where the tool is held during use. After the measurements have been taken I check the calibration again to verify that the measurements I have taken are accurate. While i take the wall thickness measurements I repeat the measurement in each position to verify accuracy. Edgar, the pictures of your work area remind me of home on intense days.