Erica Goode / New York Times News Service

A growing number of shooting enthusiasts are creating legal trusts to acquire machine guns, silencers or other items whose sale is restricted by federal law — a mechanism that bypasses the need to obtain law enforcement approval or even undergo criminal background checks.

The trusts, called gun trusts, are intended to allow the owners of the firearms to share them legally with family members and to pass them down responsibly. They have gained in popularity, gun owners say, in part because they may offer protection from future legislation intended to prohibit the possession or sale of the firearms.

But because of a loophole in federal regulations, buying restricted firearms through a trust also exempts the trust’s members from requirements that apply to individual buyers, including being fingerprinted, obtaining the approval of a chief local law enforcement officer and undergoing a background check.

Lawyers who handle the trusts and gun owners who have used them say that a majority of customers who buy restricted firearms through trusts do not do so to avoid such requirements. And most gun dealers continue to require background checks for the representative of the trust who picks up the firearm. But not all do.

Christopher Dorner, the former Los Angeles police officer who embarked on a weeklong assault on law enforcement officers this month that ended with his death on Feb. 12, said in a rambling 11,000-word manifesto that he had used a gun trust to buy silencers and a short-barreled rifle from a gun store in Nevada without a background check.

Referring to a program available from the personal finance software company Quicken, Dorner wrote, “I was able to use a trust account that I created on quicken will maker and a $10 notary charge at a mailbox etc. to obtain them legally.” Dorner was not a felon and probably would have passed a background check had he received one.

Mike Campbell, a spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives, which enforces firearms regulations, said that applications filed with the ATF for transfers of restricted firearms to trusts or corporations have more than doubled from 2008 to 2012, to more than 39,000 from about 15,000. He said the increase was largely attributable to the growth in the number of trusts.

Campbell confirmed that under current regulations, background checks were not required for the buying of restricted firearms through trusts. The agency, he added, was aware of the loophole and was reviewing changes to close it.

Lawyers who prepare gun trusts said requests for the documents had been increasing in recent months as proposals for gun legislation proliferated in state legislatures and on Capitol Hill. They said some gun owners were even creating trusts for unrestricted firearms like semi-automatic rifles and pistols, hoping to protect them from future legislation.

The cost of setting up a trust can vary from a small amount for an online form to $100 to $2,500 in lawyers’ fees, depending on location and the type of trust.

The sale and possession of silencers, fully automatic guns manufactured before 1986, and other firearms and accessories that fall under the 1934 National Firearms Act are legal in many states. But the ATF keeps a registry of the firearms and must approve their sale, a process that can take several months, and the buyer must pay a $200 tax.

David Goldman, an estate lawyer in Jacksonville who pioneered the use of gun trusts six years ago, called the notion that criminals might use the trusts to buy the firearms through a dealer “ridiculous.”

“Illegal versions of these items are not only cheaper,” he said, “but you can obtain them six months faster, and you don’t have to form a trust, which could be $500 or $1,000 depending on the level, and you don’t have to tell the ATF about it.”