Okay so I've realised why it's happening.
The button doesn't have an insulator. It relies entirely on the battery's wrap (the overlap on the negative side) to prevent the negative from making contact with the button and subsequently the mech tube which prevents the circuit from completing.
So, the connection/autofire was A - intermittent. And, B - usually after I knock or put the mech down and it hits the table with a bit of force.
As mentioned I like to keep my batteries freshly wrapped. Turns out, this contributed to the problem. You see, aftermarket wraps are somewhat thinner than original wraps, at least the ones I've got are. Also, the wraps need to be fitted so that a significant portion of the negative end of the battery is covered. Apparently, I wrapped them a little short of the ideal position.
The combination of the (give or take) 1-2mm
Short negative wrap overlap. Combined with the thinner wraps is what caused the issue.
Because the wraps didn't cover "enough" of the negative end of the cell, it meant that the cells point of electrical contact was notably closer to the button, albeit not much, but enough to be a factor.
As mentioned before, the wraps are not much thinner but notably thinner and again, enough to be a factor. Which meant the battery had ever so slight play inside the tube.
If one of those "issues" happened without the other, there would be no problem. But because both of the "issues" were combined what would happen when the mech would take a knock, a bump, a little momentum from basically any kind movement , the battery would move (due to play caused by thinner wraps) when the battery moved within the tube it would push itself closer to the wall of the tube. Since the contact break relies entirely on the wrap, the insufficient coverage of the wrap on the negative end of the cell combined with the battery play, caused the battery to shift far enough for the exposed negative part of the cell to make contact with the button wall, completing the circuit and causing the device to fire.
I hope this makes sense. I feel like I over complicated the explanation in way that makes me seem pretentious but after typing all that I can't be bothered to re-type.
My solution was:
Replace the spring with a stiff spring that has more distance and resistance.
Rewrap the battery with a thicker wrap.
Ensure that the new wrap has adequate protection for the negative end of the battery.
Create and fit an insulator for extra protection.
I will remove the insulator and spray paint the inner side of the button (except for the contact pin) to achieve the same insulation result as this will mean less maintenance when changing batteries.
I'd like to give a seriously sincere thanks to everyone who contributed to this thread. Initially I had no idea where to look for the issue and a lot of the information shared not only guided me but made me realise that there is clearly a logical way to diagnose the problem.
Apologies for not posting photos, as mentioned, I get very little free time.
I won't post photos now since the issue is resolved but for the sake of science the mech in question is a SMPL
Sand the surface a little and use enamel paint, other than that.well done glad you got it figured out.
