Firstly, why not get the full length g36 handguard?
My recommendations for additional upgrades:
1) Intellect 9.6v 1400Mah (fits under both types of handguards, the full g36 handguard requires some "finessing" to get it in)
2) Systema Magnum torque up motor (expensive, but bad-ass)
3) Prometheus/Systema helical torque-up gears
4) Type 0 cylinder if you're considering going with a full g36
5) Upgraded piston head (I prefer metal, but I think these are a bit slower than polycarbonate heads)
6) Upgraded cylinder head (Again, I prefer metal, but the real quality that matters is a double O-ring)
7) 110 Spring
8) King Arms Sector gear clip (these are godly on g36s, never had a single misfire after using one of these)
9) Metal Spring guide
10) Prometheus hard hop-up rubber (if you want it to shoot over 350fps)
This set up is fairly ideal for an outdoor gun shooting at ~400 FPS, just keep the stock spring if you want a lower FPS gun. Also with the 9.6v it gets a really good ROF with consistent torque on each shot
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