A goodly portion of the firearms sold in the United States are not proof-tested at all. It isn't hard to understand why. The ATF cares nothing about proof-testing, nor does Customs, nor does the United States Government, with a few exceptions such as NATO EPVAT standards, which affect only NATO chamberings in military use.

Guns fail prior to actual firing.

“Barrels are  deemed to have failed prior to testing if they show marks of any sort inside. Should this happen the maker is sent away, tail between legs, and advised to 'get it better' and to return again another day when they have done so.”

“Once the test has been completed the superintendent will again examine the barrel to look for deformation, bulges, scratches, gouges, cracks, etc. The action and barrel fit will also be checked to ensure that they are still in contact with each other. If they are not this is known as being ‘off the face’ and the gun will be rejected. Once all tests have been completed successfully the barrels can be proof marked accordingly. Should the barrel fail this test for any reason, the maker is given the chance to rectify the problem.”

“If your barrels have English proof marks it doesn’t necessarily mean that they have been made in England. For example, the SAAMI standard used in the USA only deems it necessary to proof a percentage of a batch rather than every gun, so any guns arriving in the UK from US shores which have been proofed to SAAMI standard have to be reproofed and it is not uncommon for European barrels to be reproofed with a UK proof mark.”

Too often, guns made in the United States are not proofed at all. Why should they be? It costs time and money, and no one monitors it. Without proper inspection prior to proof-firing, and thorough inspection after proof-firing, firing a single shot through a barrel is meaningless. Damage caused by an over-pressure load is not detected if you don't bother to look at it. Who does? No micro-cracks or inclusions can ever be found if they aren't looked for in the first place.

Firestone may have claimed that their tires were properly made and tested, Johnson & Johnson may have claimed that their baby powder was safe, Ford may have claimed that the Pinto was properly tested and safe, but self-testing is not a grand idea. The self-accounting of Enron, General Electric, Bernie Madoff, Lehman Brothers, Worldcom, and so forth did not end well. VW tested its own diesel engines. In 2015, company engineers installed a special type of software in 11 million of its diesel-powered cars to detect when cars were being tested for emissions and change their results. The Volkswagen vehicles’ actual nitrogen oxide emissions stood 40 times higher than U.S. legal standards allowed. When, belatedly U.S. regulators discovered the “Diesel-gate” scheme, Volkswagen had to recall approximately 480,000 vehicles and fork over $30 billion in fines and penalties. Self-testing is not legitimate testing, much less "proofing" that can be relied upon. 

There is no such luck for the American gun owner. If you have a CIP proofed firearm, that is the only civilian independently administered proof-firing and inspection that there is. For everything else, you need to rely on a trustworthy, highly reputable gun manufacturer. CIP citizens don't need to rely on or guess about anything, for the guns are independently proofed under color of law before they can be sold.

This unseemly situation presents two areas of great concern. Unethical gun manufacturers, from Turkey and elsewhere, who put profits over safety and quality can ship whatever untested. spot tested, improperly or poorly tested fodder into the United States as they please: and they do just that.

It shocks the conscience that voluntary proof-testing, even if done, may be only one shot of a percentage of guns, with no proper inspection either before or after. For the American gun industry, it is a massively huge embarrassment, for the guns sold every day to U.S. citizens are so insufficiently tested, so potentially dangerous, that they cannot be legally sold in the same form to any CIP nations including Chile, Slovakia, France, Hungary, Germany, Italy, and so on, on the basis of safety. It is so very bad, that it is loudly illegal to do so. I would like to see the American gun industry highly respected on the world stage, not relegated to the low class, dubious product category they currently occupy.

Moreover, if the American Gun Industry were capable of producing firearms deemed legal and safe in Germany, France, Italy, Spain, and so forth . . . American guns would be far more competitive in these several markets when not encumbered by the additional time, double-shipping, and costs associated with CIP proof-testing just to make them saleable throughout Europe as shipped. As it is now, we are quite sadly not.