So far your handle and head shaves look good.
I suggest that you clean that contact ring on that inner tail cap where it meets the handle ring when the tail is screwed tight. It’s a critical ground contact point so I sanded mine and keep it clean.
Making the battery contact (I think you’re calling it a bump gun) is the trickiest part of this mod and will take some time because of the limited thread range that is within O-ring seal.
It’s SO important to hear your batteries begin to rattle with the tail unscrewed two-rotations from tight. If it’s beyond two rotations to rattle, then you risk losing o-ring seal when cutting power under water. What that really means is you will not be able to cut power in water. If power cuts too soon, then your spring won’t be tight enough when powered on and it will constantly change modes with the slightest bump. Pick good batteries because they will be matched to the battery contact insert you will make unless all your batteries are EXACTLY the same length.
That toilet bolt sure is long! I like the big flat head on it but you will only need, say, a 1/4-inch of total bolt length from the back of the head when combined with that brass back-plate I see next to it that you will solder it to. And even then you will be grinding it down as you fine-tune it.
And remember, something has to hold it very tight against the inner aluminum.
You might want to cut the bolt off before soldering it to that plate. It might be easier that way. I only had to pull about 1/16 off of the 1/4-inch brass screw I started with so my TOTAL battery contact height was about 1/4 –inch.
Either way, if you look at my third photo in post 223, you can see how far my batteries extend beyond the tail ring without compression. You can also see that it’s different for each of my lights!! So each light ended up with a different contact height. One battery contact is 0.26-inch and the other is 0.31-inch.
Since you know the end of the handle has to meet that bare metal inner tail cap ring. That will give you a starting point for total contact height. You just have to push on the batteries to see how far they’ll press into the handle and use that to figure how much room you have.
Or, you can practice by stacking more and more pennies in the tail cap until you finally can’t screw it down with batteries loaded. Then remove about three-pennies worth and the remaining stack of pennies should be pretty close to the contact height you want. PLEASE, PLEASE REMEMBER: TWO ROTATIONS OUT TO POWER OFF or two rotations to battery rattle.
If you have zero room as you suggest, it makes no sense since you’ve had batteries in there with a spring already. Even if you have odd-ball batteries, I don’t see how you ever loaded batteries when the spring was in if you can’t load them now. I edited it so it might help to re-read the tail cap mod section. Remember, total height of the battery contact will only be about 1/4 –inch with most 26650’s