This has to be a bug. Zero guard resistance is not what it was last season, but beyond that it means you effectively cannot guard because it has zero damage resistance.
The same happened with Burst Guard, but in reverse. 100 Guard Resistance, but zero endurance. What the heck?
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It's another effect of the unexplained stat changes. Most special Grenade Launchers have 100 Blast Radius now, while some Rocket Launchers go as low as 20. There some items with 0 Reload Speed, like Sweet Business and Tommy's Matchbook. Leviathan's Breath boasts an impressive 5 Stability, 0 Handling and 0 Reload now.