I got a puncture with my 650b 42mm tyres, no problem, got spare tube.
I only got the 700 x 23 type tube.
Gingerly I pumped it up in the 650b tyres, and to around 25psi to avoid any kind of issues (normally 40psi), rode 10 miles to the nearest shop and swap tube for an MTB one.
Tube can stretch and work fine, it’s running tube bigger than the tyres (says 28-32 tube on a 25 tyres) that cause headaches.
Getting a 26” tube on a 700c rim was a challenge but managed to achieve without injury or swearing (well, swearing happened when I realised I’d got the wrong tube).
Not recommended but it got me home and survived for several months after
Fun story.
I got a puncture with my 650b 42mm tyres, no problem, got spare tube.
I only got the 700 x 23 type tube.
Gingerly I pumped it up in the 650b tyres, and to around 25psi to avoid any kind of issues (normally 40psi), rode 10 miles to the nearest shop and swap tube for an MTB one.
Tube can stretch and work fine, it’s running tube bigger than the tyres (says 28-32 tube on a 25 tyres) that cause headaches.
Also I fit tube for a living.