By Jonas Elmerraji

Senior Contributor

09/19/12 — 10:51 AM EDT

BALTIMORE (Stockpickr ) — Billionaire investor Carl Icahn isn’t a fan of half-measures.

When Icahn buys a stock, he intends to be an active part of the business – whether management wants him to or not. As a result, Icahn has become the poster boy for activist investors, going so far as to launch The Icahn Report. a blog that shares his thoughts on the struggles between investors and the management teams they employ.

The approach works; Icahn’s self-made wealth is estimated at $14 billion.

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His deep conviction strategy for buying stocks means that when Icahn likes a stock, he loves it. That’s all the more reason to pay attention to his buying and selling activities a regular basis. To do that, we’ll turn to his firm’s latest 13F filing.

Institutional investors with more than $100 million in assets are required to file a 13F — a form that breaks down their stock positions for public consumption. From hedge funds to mutual funds to insurance companies, any professional investors who manage more than that $100 million watermark are required to file a 13F. By comparing one quarter’s filing to another, we can see how any single fund manager is moving their portfolio around.

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Chesapeake Energy

First off is the sole new position that Icahn Associates added to its portfolio in the second quarter: Chesapeake Energy (CHK ). The investor bought more than 50 million shares of the firm during the second quarter, remarkably taking nearly a $1 billion position in the firm within a single quarter.

The decision to buy Chesapeake is significant. Not only was Icahn buying weakness from CHK’s dramatic and public shakeup earlier this year, but the purchase also represented his second time owning a major stake in the energy company. Icahn sold off his firm’s previous holdings back in 2010, when he lost confidence in the board.

Now he owns 7.6% of outstanding shares — and he wants the board to make some big changes.

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Chesapeake is mainly an unconventional natural gas firm, directing the lion’s share of its efforts on 15 million acres of shale and basin projects spread across the U.S. 18.8 trillion cubic feet equivalent of proved reserves make Chesapeake a significant player in the U.S. natgas market, and its outspoken CEO makes it a significant presence in the industry. The firm has had a “for sale” sign out in the open for the last several years, shedding noncore businesses and generating substantial cash for its trouble. As those tertiary asset sales continue, CHK should continue to drive balance sheet liquidity and income statement performance.

While CEO Aubrey McClendon has caught a lot of heat for under-disclosing related party transactions, I think it’s clear that Chesapeake needs him at the helm. He’s navigated the business through treacherous waters before, and his well participation arrangement wasn’t in and of itself inappropriate.

With shares already slammed from the “scandal,” now looks like a bargain opportunity to be a buyer, especially if Icahn and company help to shake extra profits out of the corporate structure.

This past week has been a dramatic one for Navistar (NAV ), the small-cap commercial vehicle and engine builder. While Icahn loves the company (his firm bought nearly a million shares of NAV in the second quarter), he hates its management team. To that effect, Icahn has been exchanging nasty letters with management, most notably scolding the board for making major decisions (like a CEO change) without input from major shareholders.

While the drama isn’t likely to come to an end soon, it’s clear why Icahn still likes the business.

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Navistar is a leader in the heavy vehicle business. Brands include flagship International, Monaco Coach, and its MaxxForce engine marque, giving NAV market-leading positions in 75% of its business lines. As oil prices continue to ebb and flow, the trucking industry should get some semblance of a shot in the arm, providing a boost in the parts and vehicles that roll off of NAV’s manufacturing lines. A slow embrace of nontraditional fuels, particularly electric-drive vehicles, could be a big growth avenue for the firm in the next few years as city operators look for ways to trim overhead.

While Navistar carries considerable debt, much of it is tied to a captive finance business that’s separate for all intents and purposes. With more than $750 million in cash on its balance sheet and proven cash generation ability, NAV looks like a solid bet right now.

That’s especially true now that shares have slid more than 33% this year. With the Icahn fight going strong, investors may be able to get a bargain here.

CVR Energy

On the other hand, CVR Energy (CVI ) is having a stellar year. Shares of the refiner are up more than 85% so far this year, boosted by Icahn’s public bid and a plan to take the company private. That plan got scrapped more recently when Icahn conceded that commodity prices made taking the company private unfeasible — but he’s not selling.

In fact, as recently as the second quarter, Icahn continued buying to the tune of 58.6 million shares. According to Icahn Associates’ 13F, the firm owns 82% of CVR right now.

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With going private off the table, Icahn is going to operate the business with hopes of continuing to shop the firm once refining spreads improve. Mid-cap CVR is an independent refiner with operations in Texas, Kansas, and Oklahoma, that also owns a fertilizer business. The firm’s 185,000 barrel per day capacity mean that it’s on the smaller end of the refining spectrum, but its Midwest positioning exposes the firm to a much more profitable side of refining than most supermajors have access to. That’s evident in CVR’s consistently high net profit margins.

With activist investor Carl Icahn helming the ship — and with a willing crew in the firm’s management team — CVR has the potential to extract some attractive value for shareholders in 2012. The stock’s rally means that the biggest profits have already been made, but investors seeking out energy exposure could do worse than a stable, highly-profitable refiner.

American Railcar Industries

On the much more small-cap side of the spectrum is American Railcar Industries (ARII ) another existing position that Icahn Associates added onto in the second quarter. American Railcar is a concentrated position for Icahn — his firm’s $43 million stake represents more than 55% of outstanding shares, giving him full control of the company.

As its name suggests, American railcar builds hopper and tank railcars. The firm also services railcars and provides fleet and design services. The ARII position is a sort of compliment to Navistar in that they’re both secular bets on transportation — only ARII is primed to do well if fuel prices continue to increase (rail transport is many times more efficient per ton shipped) whereas Navistar’s trucks win out if crude stagnates. As a pure play on transportation, my money’s on NAV over ARII.

While the latter is debt-neutral and has become respectably profitable in the last quarter, its income statement has also been a whole lot more volatile, impacted by much lower deal volume and long-term declines in revenues.

Take-Two Interactive Software

Carl Icahn’s last position increase in the last quarter came from shares of Take-Two Interactive Software (TTWO ), the video game developer and publisher behind game franchises like Grand Theft Auto and Borderlands. Icahn added 578,000 shares of TTWO to his firm’s position in Q2, building his stake in the gaming firm to $74.6 million.

Take Two’s status as an independent game developer and publisher gives it a lot of freedom to develop games that make more mainstream firms anxious. The Grand Theft Auto franchise is a perfect example of a title that’s hardly politically correct, but proved to be a smash commercial success for TTWO. Operationally, though, TTWO has problems. The firm’s earnings are haphazard, with hit-or-miss performance driven by success in a small handful of titles. At the same time, the firm has been slowly building a substantial debt load, tripling its borrowings in the last three years.

While cash has been growing in kind, it makes little sense to hold onto so much balance sheet liquidity when debt service costs start to materially erode already tentative earnings. That being said, the firm has a pipeline of highly touted titles that should help make the coming year an impressive one financially.

Investors looking for a more speculative way to play consumer discretionary numbers could do worse than Take-Two. But they could do better too.

To see all of Carl Icahn’s plays — including a complete list of which stocks he added or sold off — check out the Carl Icahn Portfolio on Stockpickr .

— Written by Jonas Elmerraji in Baltimore.