I would not recommend you to use these in break barrel airguns. I've tested this principle years ago and I didn't (well I totally forgot to...) measure the overall diameter of the projectile. I designed and 3D printed a couple of .177 caliber "tubes" with PLA filament and then capped them with toilet paper on both sides and of course, prior to capping them, I've loaded them with lead shot that was roughly 2.5mm in diameter. The shell packed 13 lead 2.5mm balls and when I've gone to chamber the shell into my Magtech AR600 (.177 cal of course), it seated fine, but then when I pulled the trigger the rifle recoiled violently and the gun gave a really dampened report...
It turned out that the shell jammed itself in the barrel, it didn't go further down the bore, any more than an inch. When I latched open the barrel, a loud pop was heard and "smoke" came out of the bore.
So, if you're planning to use these, make sure you'll drive one or two down the bore with a ram rod to see whether they jam or not and whether the plastic gets deposited in the grooves of the bore, as there is almost certainly going to be some melted, or even better said "smeared" plastic deposited in there after shooting.
Use with care and observe barrel fouling. Have a good one.
