I'm also used to the design mechanism of weapons from field catalogue. I already designed some cool weapons with it.
BUT I also understand the said issues. It is a good idea but should have needed a bit more polishing. About all these additional rules ... well yeah it IS said that they are all meant to be optional. But a lot of them seem like not fully thought through.
Quickdraw could have been just an Initiative modifier for example.
The recoil mechanic seems a bit useless because we already have bulky and very bulky. Nontheless I like the idea, because handling a weapon in full auto is more a thing of training, not of pure strength (even though it helps to have some muscles

). But the implementation is ... discussable.
I really like the penetration mechanic because that is how armor penetration works. Having a better penetration means less damage to the person when the bullet comes through.
I have to agree with the people answering my post that all this is more like a suggestion (or like written in the rules optional). As the GM (or referee) of my group I tend to use not all of them and some I have modified a bit.
One last thing is the topic of reengineering weapons from standard rules. You can rebuild them with the rules, but you don't get the complete same characteristics. Take the Autopistol for example. With the design set of field catalogue you can build a semi-automatic handgun with medium handgun calibre (3D-3), 15 Rounds mag and range 10m. It is a bit more expensive but as referee you can easily adjust that. The weight is 1.16kg, also easy to adjust. So the thing is: you got Recoil 1 and Penetration -1 what results in Lo-Pen 2.
Lets look at this:
Recoil 1, yeah ... no! Controlling a 9mm Semi-Automatic Pistol does not require to be a trained shooter (Gun Combat 1). So just ignore it (your right as a referee), or give your Autopistol a Recoil reduction which would result in a modified Damage of 3D-4. Not too unrealistic if you ask me if we take a look at same level armor.
Lo-Pen 2: Yes! (In my opinion) Why: Again we look at the armors. We are talking about a 9mm Pistol. The lower TL Flak Jacket (so more or less like a common kevlar vest in the real world) has protection +3. If you shoot with a 9mm Pistol at a moderate Kevlar Vest the Vest will stop the Bullet in a lot of cases. With protection +3 and 3D-3 damage it goes through the most time though a rolled 6 with 3D is a really bad result. With Lo-Pen 2 you need at least a rolled 9 what is still not a good result with 3D. Yes of course it is a game. Usually we don't play with hit zones and the vest just protects your torso. And getting 2 or 3 damage after armor could also just be a bruise and not a penetrated vest. Long story short: in my opinion Lo-Pen 2 for 9mm Pistols isn't that implausible.
Another example could be the submachinegun, but I will make this short: Since I play traveller (what is way longer than I'm active in this forum

) I was bothering with the fact that the submachinegun does the same damage as an assault rilfe and not the same damage than a Pistol.
In the end this is the problem. That you got weapons out of the design rules that are not totally fitting between the standard ones, as long as you don't just bend the rules a bit in the end. I understand that and I agree with that. It would have been cool if they had created a List of the standard weapons rebuild with field catalogue and added that list to the book (I did ask them for such a list btw.

). Ok they did not, maybe we will get one at some point, I don't know.
My personal solution is to code a tool to create weapons with the field catalogue rules and recreate all the standard weapons with it to see if I'm happy with the result and if not just bending it a bit. Also I'm not using all the additional rules or some we altered in a way we agree with. I always loved to create Ships, Equipment, Weapons and so on in Traveller and other Pen&Paper systems and I just want that design rules going to work!
(No surprise: I'm not a native english speaker. So sorry for some weird written parts in this long wall of text. Please forgive me!

)