The small-caps of today will be the mega-caps of tomorrow, and that's why these stocks are popular with growth-hungry investors. Defined as companies with market caps between $300 million and $2 billion, small-cap stocks have the potential for big returns.

Here is a list of three small-cap stocks to buy in May. The first pick, Glu Mobile (NASDAQ:GLUU), is a bet on the fast-growing mobile gaming industry. The other two picks, Upwork (NASDAQ:UPWK) and American Outdoor Brands Corp. (NASDAQ:AOBC), are bets on the freelancing revolution and America's passion for firearms in times of uncertainty.

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1. Glu Mobile: Current market cap of $1.2 billion

What do you do when you are stuck at home with nothing to do? If your answer is to load up a video game on your smartphone, you aren't alone. Mobile gaming is more popular than ever, and it accounts for 60% of all gaming revenue. With the coronavirus pandemic trapping millions of people at home, the industry may post better-than-expected results in 2020. 

Glu Mobile has a strong position in the mobile gaming industry with fast-growing intellectual properties like Tap Sports Baseball, Covet Fashion, and Kim Kardashian Hollywood. These titles helped the company generate $423.3 million in bookings for the full year 2019.

Glu's management is guiding for first-quarter 2020 bookings to come in between $93 million to $95 million, which is flat from the prior-year period, and the company is projecting full-year 2020 bookings to be between $423 million to $433 million, which is a modest projection compared to 2019. The company is giving soft guidance, and lockdown-related demand for entertainment may help it beat expectations when first-quarter results are released on Thursday, May 7.

2. American Outdoor Brands: Current market cap $517 million

Love it or hate it, America has a passion for guns. The country has the highest gun ownership per capita rate in the world, and the market for firearms is expected to grow at a compound annual growth rate (CAGR) of 4.5% until 2025. The coronavirus pandemic makes this a good time to watch the industry because gun sales are soaring as Americans become increasingly uncertain about the future.

FBI background checks for firearms leaped 80.3% in March, with checks for handguns and long guns up 92% and 72% respectively. American Outdoor Brands is uniquely poised to benefit from this surge in demand because it generates around 75% of net sales from its firearms segment.

The company reported $638.3 million in net sales in 2019, and management is guiding for 2020 revenue to come in between $650 million to $660 million. While the company has a relatively slow growth profile, it boasts a compelling valuation. With a market cap of just $517 million, American Outdoor Brands trades at less than forward sales, and that leaves room for significant upside when market sentiment improves.

3. Upwork: Current market cap of $981 million

The gig economy has pushed remote work into the mainstream, and the coronavirus pandemic is speeding up this megatrend. With millions of people around the world now working from home due to government-mandated lockdowns, this is a good time to look at freelancer stocks like Upwork.

Founded in 2014, Upwork operates a global freelancing platform that enables businesses to connect with independent contractors. The company also runs a managed services platform called Upwork Enterprise that helps its clients pay and manage their contract workforces remotely. Upwork's sales jumped 20% from $223.8 million to $268.3 million in 2019. And with first-quarter earnings expected on Wednesday, May 6, investors should watch closely for a potential coronavirus-driven boost to its top line.

The bad news is that Upwork isn't profitable yet, and management will have to push for profitability before they run out of runway. The company has $48.4 million in cash on its balance sheet and burns through around $16.7 million annually, so there is plenty of time to bring that bottom line into the black before the cash runs out.