None! Try it out as is before you invest in upgrades. They come stock with a 6.03 ASG Brass barrel and Maple Leaf Bucking.

If you're still looking for more, or use heavy BBs, a new nub would be a great place to start!

Any major brand of BBs feed fine. I personally recommend premium brands like BLS, Elite Force, and HPA, but if you're looking to save a bit of money, other high quality brands like EMG or Warhead Industries are great options.

.25s or higher for indoor

.32s or higher for outdoor

The stock nub is the first thing that should be upgraded for heavier BBs. There are many good options, but the Maple Leaf Omega is most highly used. The 4UAD Omega style nub is another great option.

The stock Maple Leaf Buckings are good, but there are other great options:

Maple Leaf MR

4UAD 4UANTUM

G&G Green (For Cold Weather)

The stock 6.03 barrels are great, but if you do want an upgrade, PDI, Prommy, EDGI, and ZCI are all great options!

The stock hopup is good, but you can add the Silent Industries Metal arm for heavier BBs.

Maxx and Silent Industries both made good aftermarket Hopup options.

The Wolverine Phoenix can be tricky to setup and adjust, but the CNC silver hopup is great!

Adjust your dwell, it is very simple to do and one of the most overlooked functions of MTWs (or any HPA engine for that matter).

How to Set the Dwell on Spartan Electronics