When Defense Secretary Lloyd J. Austin III and other top officials assembled for an event this month at the Ronald Reagan Presidential Library, they walked into a lesson in how the high-stakes world of Pentagon lobbying is being altered by the rise of defense technology startups.

Inside, at this elite gathering near Los Angeles of senior leaders from government and the arms industry, was a rapidly growing group of participants: former Pentagon officials and military officers who have joined venture capital firms and are trying to use their connections in Washington to cash in on the potential to sell a new generation of weapons.

They represent a new path through the revolving door that has always connected the Defense Department and the military contracting business.

Retiring generals and departing top Pentagon officials once migrated regularly to the big established weapons makers like Lockheed Martin and Boeing.