* Sees FY 2015 earnings of $10.80-11.20/shr vs est $11.28

* Expects weakness at defense, aerospace business

* Shares fall as much as 7.4 percent (Adds CEO comment, background; updates shares)

May 15 (Reuters) - Ammunition maker Alliant Techsystems Inc forecast full-year 2015 earnings largely below analysts' average estimate due to weakness in its aerospace and defense businesses, sending its shares down as much as 7.4 percent.

Alliant, which was spun off from Honeywell International in 1990 and is commonly known as ATK, has been hurt by cuts in U.S. government spending in the defense sector.

The world's largest ammunition maker also said it would record a restructuring charge in the current quarter.

"I don't think there's any significant growth that we anticipate coming out of aerospace (this year)," Chief Executive Mark DeYoung said on a post-earnings conference call.

The company's aerospace business has not yet fully recovered from the loss of two major U.S. government contracts in 2010 and 2011.

The aerospace business, which makes rocket motor systems for human and cargo launch vehicles, recorded a 12 percent drop in operating profit for the fourth quarter ended March 31.

The company is trying to increase its focus on its aerospace and defense businesses. It said last month it would merge with Orbital Sciences Corp, after spinning off its sporting gun business.

ATK's aerospace and defense businesses account for about two-thirds of its revenue.

The company's defense business is expected to be hit by lower demand for medium caliber ammunition at its Lake City Army Ammunition plant partly due to higher competition, DeYoung said.

ATK forecast earnings of $10.80-$11.20 per share for the year ending March 31, 2015.

Analysts on average were expecting earnings of $11.28 per share, according to Thomson Reuters I/B/E/S.

The company said it would record a $10 million, or 19 cents per share, restructuring charge in the first quarter.

Total revenue rose about 17 percent to $1.35 billion in the fourth quarter.

Net income attributable to ATK rose 32 percent to $96.0 million, or $2.90 per share. Excluding items, the company earned $2.59 a share.

Analysts had expected earnings of $2.41 per share on revenue $1.32 billion.

ATK's shares were down 6.74 percent at $137.93 on Thursday afternoon on the New York Stock Exchange. They gained about 80 percent in the year to Wednesday's close, compared with a 34 percent rise in the Dow Jones U.S. Aerospace & Defense index .

(Reporting by Ankit Ajmera in Bangalore; Editing by Savio D'Souza and Simon Jennings)