NONE, unless an older model then a simple upgrade kit consisting of O-rings
Strengths:
self contained unit, easy to load and change CO2
Weaknesses:
None
Review:
I have been using my sydarm as a backup for many years (serial number 137) and have had no probelms. Even stock games I have played in where it was being used as the primary marker, did not fail me. If you get your hands on one it is a keeper and you won't look back.
Conclusion:
One hell of a back-up marker or even a stand alone. Easy to load, change CO2, maintain and use
