When a Las Vegas police SWAT team blew the hinges off the door to Stephen Paddock’s luxury suite at the Mandalay Bay Resort and Casino on Sunday night, they found a lifeless mass murderer and a cache of weapons. What appeared to be an AR-15 rifle was among the stockpile of firearms found in the room, which wasn’t surprising.

HuffPost Illustration

Such semi-automatic weapons have been used in several mass shootings in recent years, including those in Aurora, Colorado; Newtown, Connecticut; and San Bernardino, California. Paddock used them to kill 58 people ― the deadliest mass shooting in modern U.S. history ― and wound hundreds more.

The AR-15 is appealing to gun enthusiasts, as it’s versatile, reliable and affordable. It’s also highly customizable: almost every part is replaceable.

Photos leaked to the media show at least one of weapons in Paddock’s room had been customized with a bump or slide fire stock. A bump stock is a legal modification, which allows a semi-automatic rifle to fire at a rate closer to a fully automatic one, as seen in the video below.

Authorities are still examining Paddock’s firearms to determine if he modified his weapons in other ways. While it could be several weeks before this information is made public, an AR-15 rifle’s firepower can be enhanced in just a few minutes, with minimal expense and without extensive gunsmith knowledge.

Here’s a look at some of those modifications that can make an already lethal weapon even more so:

Triggers

Upgraded triggers can be easily purchased online. Triggers that have less pounds of pressure are easier to squeeze, allowing a gunman to fire more rapidly and have a steadier hold on a weapon ― and thus make shots more accurate.

Flash Supressers

These devices, which are fitted onto the end of the barrel, help reduce the visible signature left behind when ignited gases exit the barrel.

High Capacity Magazines

While 30-round magazines are typically used by AR-15 owners, it’s possible to purchase stick magazines with 60- to 100-round capacities, as well as drum magazines, some of which can hold more than 100 bullets.

Cooling

Some have wondered why Paddock, who authorities say died of a self-inflicted gunshot wound, felt the need to bring so many weapons into the hotel. Rifles can and will overheat in rapid-fire situations; indeed, standard-issue AR-15′s tend to overheat quickly with continuous firing. However, there are ways to combat the speed in which the gun overheats. Some of those methods include swapping out the standard bolt carrier for one that can withstand repeated heat cycling, upgrading to a chrome-lined barrel and purchasing a hand-guard that can handle more intense heat and provide better ventilation. Still, the gun will eventually overheat, which Paddock may have anticipated, prompting him to bring multiple weapons.

Silencer

A silencer or sound suppressor, which reduces the noise emitted by a gun, can legally be purchased with little hassle in 42 states. While there are a few hoops to jump through, you can have one in less than a year. Silencers can also be fabricated, although doing so is illegal.

Gat Crank

Some experts who have reviewed videos taken while Paddock fired on concertgoers have speculated he used a weapon with a “gat crank.” That’s a simple bolt-on device, which allows the attached firearm to be fired by turning the crank. As seen in the video below, it is highly effective for rapid-fire. Federal officials only consider a gun automatic ― and thus illegal to own without government clearances ― if it can fire multiple rounds with a pull of the trigger. That definition allows such devices, which do not involve physically pulling the trigger, to skirt current regulations.

Conclusion

Technology also has now made it easier than ever to forge both legal and illegal gun parts, as seen in this video by Wired:

Paddock may not have utilized all of these weapon modifications. But following his rampage, it’s likely that some of them ― especially the use of bump stocks ― will face increased scrutiny. Sen. Ron Johnson (R-Wis.), the chairman of the Senate Homeland Security Committee, already has said he’s leaning toward supporting a bill to ban bump stocks.

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