Irfan Naseer, 31, and Irfan Khalid, 27, travelled to Pakistan for terrorist training before returning to the UK in July last year, jurors at Woolwich Crown Court were told.
In one conversation, Naseer was heard agreeing that the July 7 attacks had not done enough damage because there were no nails in the bombs.
The two men, along with Ashik Ali, also 27, are accused of being "central figures" in the alleged extremist plot.
Ali told police in interview that the plan had involved him wearing a suicide vest as well as carrying a gun, the jury heard.
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