Air gaging blanks for button rifling

I just bought a Federal air gage without the gage probes. I understand I need a master ring gage for each size needed and the special probes or gages.
A customer who brought an air gage for us to use making his parts paid to have a probe made, I assume I can make my probes following the fashion they are made after.

The button rifling blanks need a .3465" bore I think after experimentation, and fortunately they are ,380 ACP pistol barrel blanks. I was told bores should be ,348" but the rifling is not high enough at that size, however, it might be my measurements too, in error.

Also, I'm told the bore needs to decrease ever so slightly from breech to near the muzzle, though no numbers were given, I suspect tenths are the correct values.
Watching high speed film of shooting I can see the need, as a considerable amount of gas blows out seemingly long before the bullet arrives. Not fatal for a low pressure pistol barrel but likely a cause of gas erosion for high powered rifle barrels.

In my earlier button rifling thread a member mentioned air gaging, and I've come to believe that it is the best way to map the inconsistencies inside the blank before rifling.

I'd be very interested in how gaging is done by commercial barrel rifling.
Thanks,
parts

This is a good way to measure bores. Extensions are available that will allow you to measure the entire length of a pistol blank. You can also purchase probes that will allow you to measure the grooves in a finished barrel.

Products - Solutions for gauging - Measurement of diameter and length - Split Ball probe - DIATEST Hermann Koltgen GmbH

you do not want the bore to have any type of taper prior to rifling.

Click to expand...

The comment was based on something said in "Weapons Guild", and was aimed at a comment I'd made about final lapping. The point being that when lapping the post-rifling process barrels feel is needed, because the barrel needs to be ever so slightly tighter at the muzzle. A lot of those kinds of comments seem more important to rifle barrels to me, rather than pistol barrels. Maybe for match pistol barrels.

I had suggested using a shaper to lap the bore, and the respondent wanted to make the case for hand lapping.

Got my ebay Federal Dimensionair today, it looks brand new and has a siamese output for two probes, I guess for using one or, alternatively, the other after the second op.
But it doesn't work like the one we have now. Do you send them back to Federal or is this something any instrument repair shop can fix?
The top has two knobs, a big one at the back top and smaller at the front of the top. I understand the big one is used to get the needle adjusted between the two tick marks, and then the probe is inserted into a master gage and adjusted with the smaller knob until it reads zero. On mine the big knob doesn't seem to work. Maybe I should take the back off and see if it;s anything obvious? We have the same model here a customer brought for QC of his parts, very close tolerance burnished bores in copper parts, so we are familiar with using them.