I don't think so. Very possibly if it was just a few over a substantial period of time. But if the volume is anywhere near even half of the 600, I think he's on thin ice. What looks like a decent summary here:
18 U.S. Code Chapter 44 §922 opens with: "It shall be unlawful for any person except a licensed importer, licensed manufacturer, or licensed dealer, to engage in the business of importing, manufacturing, or dealing in firearms..."The Gun Control Act of 1968 (GCA) permits an unlicensed individual to make a firearm for personal use, but not for sale or distribution. However, the law does not explicitly preclude an unlicensed person from later selling, giving away, or otherwise transferring a homemade firearm to another person as long as it was originally intended for personal use....
The key in determining whether a homemade gun can be legally sold or transferred rests with knowing the intent of the maker when the gun was first created. Relevant factors in determining the transferor's intent may include: the length of time between the creation of the firearm and its transfer; the specific reason for the sale or transfer; and whether the maker of the firearm frequently sells or transfers homemade firearms.
There are exceptions noted in §925 but none of those relate to manufacture for personal use or limited sale. I gather from the info from the NOLO link that there is some leeway, but if you've made and sold a couple hundred rifles or shotguns I honestly don't think it's up for what I often feel are nebulous BATF interpretations...and at least on the surface this one doesn't sound like BATF overreach. But then I've been known to be wrong and I'm certainly not a lawyer. At least I hope they were polite and respectful when they served the warrant, searched the property, and confiscated guns.