These are super simple guns to work on.
You can also buy this O-ring kit below.
RN-312 Bumper below


Hilti RN312 O-ring Kit + Bumper
Hilti RN312 O-ring Kit + Bumper
For the Hilti RN312
Complete O-ring kit
Bottom Bumper #163
Also available on eBay
Hilti RN312 Bumper
Hilti RN312 Bumper
Aftermarket bumper
for the Hilti RN312 nailer

hilti-rn312