We would suggest your First Blaster Modification to be a spring upgrade.
A stock blaster shoots around 85-90fps.
A simple K26 Spring upgrade will take your Rival Blaster to around 135fps.
This will feel like a HUGE improvement to your accuracy and effective distance.
After a spring upgrade, maybe look at some cosmetic Mods to personalize your blaster.
What was your First Blaster Mod?
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An easy mod to do if you have limited resources (or have to wait around for shipping!) is to lube and tighten up the seals on your blaster. Just working with what you have and squeezing more efficiency out of it.