Quoted:
I can't see anything in this pic. It could just be a scratch.
You might try a dye penetrant test. I don't recall specifics, but it is a red colored low viscosity fluid and a spray can. Wipe it on the area you want to test, wipe it clean, and then spray a white powder on it to draw any colored fluid back out, staining the white powder.
Something like this:
http://www.pegasusautoracing.com/productdetails.asp?RecID=1207
I know nothing about this specific product, but it looks like what I would be looking for.
They have a better explanation of the test than I do:
Many cracks that extend to the surface of metal parts are completely invisible to the naked eye. This is because they may not open up to a visible size except when subjected to operating stresses or high temperature. Yet, these small invisible cracks can continue to grow until they fail catastrophically. This quality dye penetrant system can make many of these small cracks visible because the low viscosity penetrating dye wicks into the tiniest of cracks. Later, when the developer is applied, the red dye bleeds out of the crack making it clearly visible to the naked eye. This handy, low cost system allows you to perform these tests yourself. Because it is designed to be used with visible light you don't need an expensive, high powered source of black light to use it. Kit includes one can each of Penetrant Dye, Cleaner/Remover, and Developer –– enough for up to 500 small scale tests.
well you could ask an NDT tech -
dye pen would be the route to go, just stop by most welding shops, like an airgas supply and ask for it. i think they cary the magnaflux brand. it will cost about $15 a can +/- there will be 2 types w or s - water or solvent , if you get the w type, you will need 2 cans , 1 being the dye, and 1 the developer, the solvent type has 3, with the 3rd being the solvent for cleaning the dye.
step 1- clean the part free from grease and oil, let it dry
step 2- spray a little dye in a dixie cup, and paint it on with a small paint brush, i say this because a little of this stuff goes a long way, and makes a hell of a mess. let it sit for 10 min
step 3 - with a dry rag, wipe off the extra dye, and recoat with dye, let it sit 10 min
step 4 - with a dry rag, clean off the extra dye, when you think that you have most of it, spray a clean rag with water or solvent, wipe it clean, this may take 2 or 3 times.
step 5 - spray a light and even coat of the delveloper, wait untill dry, if there is a crack it will show.
note, do not spray the solvent directly on the art, if it is a hairline fracture, you could wash any dye out of it.
i hope this helps.
here are a few pic of an ar bolt i did.....
the damm thing blead out so much i could'nt tell where all the cracks were, so i just wiped it clean with a dry rag, and put it under the scope, the dye will stay in the cracks and makes it easy to see