It wouldn't make much sense to build a 18650-long device and use it with 18500 batteries, would it?
Uni-tube designs sacrifice flexibility and instead put an emphasis on performance and robustness, so they are basically meant to be used with one battery type only.
The 18500 Lab is the next logical step up from the 16340 Guppy. 18500 batteries are suitable for sub-ohm vaping and provide very good capacity.
The future might see more uni-tubes, but we have no immediate plans to go for them, as our resources have their limits. There are lots of other interesting projects in the pipeline which have waited for way too long![]()
I get that it's going to be an 18500 tube, and rather than tack on an extension, I'll switch to 18500's... But - and this would be personal preference, and I get that my tastes obviously differ - I'd prefer a unitube which could take an 18650 natively, and be able to take an 18500 as well if desired.
Guess it really depends on what the extension will look like to bump up to 18500. Loving the guppy-bayou combo, btw.