Screw head in top middle of Type 38 chamber unusual?

[Patrick Chadwick]

I'm new to firearms, and I'm wondering if you can explain "invalidates the proof".

In Germany, for instance, proofing of firearms is mandatory. Alterations (such as rechambering) to pressure-bearing items such as the barrel, bolt or action body (system) render the proof invalid. The question is: what constitutes an alteration? Polishing or rebluing - hardly. But heating the bolt to red heat, for instance, in order to bend down the bolt handle for a sporter, thus possibly affecting the hardness in the lug area - that could be viewed as such an alteration. And drilling a hole in the chamber does, of course, reduce the bursting strength at that point.

So the rule-of-thumb, as far as I can ascertain, is that small holes, such as a couple of 6x32 for scope block mounting, might be OK, but any hole that goes right through into the chamber is held to have weakened the chamber sufficiently that the proof is regarded as invalid. I realize that you live in a country where proof is not mandatory, but I would personally avoid buying a firearm that had what could be a fatal weakness.