Heavy Metal Core
Spin Gear Core: Heavy Metal Core
Heaviest Neo Core available - and to the very vocal chagrin of fans, never released by Hasbro.
This makes it invaluable to combinations for which weight and RPM maintenance are important - Compacts, Weight Based Defenders, Force Smash, Traditional Upper Attack Combinations and most Driger V2 Combinations (due to their reliance on Upper Attack as a primary means of defeating opponents, as well as adding to their surprising defensive capabilities from their weight).
Can be used for Attack Types, and in general tends to be an excellent choice for the more powerful Attack Rings which benefit from Rotational Recoil control, but one must be careful that the Blade Base is fast enough not to be slowed down by the weight and distribution. As such, it is not effective on more traditional, lower power Smash Attack Customizations where a lighter SG is often beneficial, but highly effective with Attack Rings such as Square Edge which have a significant amount of Rotational Smash.
Overall, an incredibly useful part and an absolute must-have, proving there was a great reason for the community outrage over Hasbro failing to release it.