'Cocker barrels flat out have an excessive amount of material in both the length and OD of the shank.
Impulse threads:
Major - 0.917"
Shank OD - 0.820"
Length - 0.869"
Cocker threads:
Major - 0.930"
Shank OD - 0.865"
Length - 1.250"
The length is the killer. And there's still enough material on the Impulse threads for a freak insert style barrel.
Edit: and that's all apart from the advantages of a coarser pitch and quick-disassembly of the double lead.