An oligarch who helped finance a Russian gun rights activist accused of infiltrating American conservative circles has been a discreet source of funds for business ventures useful to the Russian military and security services, according to documents and interviews.

The oligarch, Konstantin Nikolaev, emerged in July as the enigmatic backer of Maria Butina, the activist charged with conspiring to use the National Rifle Association to cultivate Republicans in the United States. Mr. Nikolaev has acknowledged underwriting her gun rights advocacy in the past, but denies any involvement in a Russian influence operation and says his only dealings with his government are limited to routine business needs.

[Read a special report on Russia’s 2016 election plot.]

Though his public persona is that of a billionaire in the prosaic industries of ports and railways, a cache of 9,000 hacked emails — from the account of Alexey Beseda, whose father is a general in Russia’s Federal Security Service, the successor to the K.G.B. — reveals another side to his business activities.

Mr. Nikolaev has been an investor in a gun company run by his wife that developed a sniper rifle used by the Russian National Guard, which reports directly to President Vladimir V. Putin. He is also a major investor in a satellite imagery firm that has a license from the Federal Security Service, or F.S.B., to handle classified information.