Allow me to help. As you know, I'm the guy behind Beaufort Ink and the Mistral suite of pen kits. I've addressed this issue on Facebook and I'm happy to address it here. In fact I will just copy and paste what I wrote there. Suffice to say, that the wobble you are referring to is no more or less than you experience with a Dayacom, or any other well designed pen kit that uses acme threads on the cap.
This is what I wrote:
Despite comments to that contrary, every pen out there will wobble to a certain extent. If it doesn't, it's not going to last very long. Try a Dayacom jr, or a Majestic and prove it for yourself. For that matter, screw a faceplate onto your lathe spindle and tell me there is no wobble. It's a mistake to confuse quality with design - they are not the same thing.
To prevent the plating on the connector being worn off by the cap thread, the cap thread is nylon, and it would be easy to eliminate any wobble completely, simply by manufacturing the thread insert to be tighter. However, the Mistral is designed to stand the test of time, and if the thread insert was tight and free of wobble, it is then subject to wear and abrasion, meaning that the insert will simply just wear out over time. A looser fit more or less eliminates the risk of abrasion, but the pay off is a bit of wobble. The important thing with any thread though, is that it locates properly on the shoulder so that it doesn't wobble when the cap is fully fitted, and it doesn't come undone by accident in your pocket, which in both cases, the Mistral is designed to achieve.
The Mistral cap has a double lead thread, which means that you can screw on the cap from either of two starting positions. If it was a triple start or a quadruple start, nobody would be saying it looks course because there would appear to be more threads on it. But because the thread, by design, starts in only two places, it appears that the pitch is courser because there seem to be less threads on it. In fact, fully open to fully closed is two and a half turns, which for a fountain pen kit is a particularly fine thread - designed that way to emulate the classic marques of yesteryear, for which pen collectors pay a fortune. Take a Baron with a standard metric thread, which closes in about one and a quarter turns - now that's course, but it looks fine because its a quadruple start. The Mistral thread is double lead by design, and is not a coarse thread.
There are literally thousands Mistrals out there now, and I can honestly say we have never had any feed back from makers that their customers think there is too much wobble in the cap thread. The usual feedback is how nice, smooth and easy it is to fit the cap, and how well it stays closed. Having said that, every maker must judge for themselves, and I can fully understand if it's not to everyone's taste. But as I say, please do not confuse quality with design and specification. The Mistral is designed the way it is BECAUSE it is high quality, not because there has been any corner cutting through poor quality. In twenty or thirty years time, both the plating and the threads will still be going strong on any Mistral you make today.
I hope that helps.