There are a lot of myths floating around about "assault guns". Currently this term usually means some variation of the AR-15.
For the last few years variants of the AR-15 have been the most popular rifle. There are millions of them out there but fewer that 700 people are killed by rifles annually. Not only is this not the weapon of choice for most murders, it comes behind knives and other sharp weapons.
It is not a military weapon. It is made to look like a military weapon. You have to pull the trigger for each shot (semi-automatic). Military weapons have an option to keep firing as long as you hold the trigger (full automatic).
There are several reasons why the AR-15 is so popular. Some people like it because it does look like a military weapon. Others like it because it is light-weight and dependable. It is also versatile. I have heard it described as a "platform" because it is so modular. It is possible to swap out most of the parts. You can change the stock, add or remove the grip. There is a lot of controversy among shooters about the best type of sight. This is easily changed and you can use different types of sights depending on the type of shooting you are doing. Some people want to be able to shoot different caliber bullets. Normally that means buying multiple guns. With the AR-15 you can swap out the barrel and a few other parts. This saves a lot of money over buying an entire second gun.
A lot has been made of the high-capacity magazine. There is some magical thinking involved here. Magazines were developed to make reloading faster. The State of New York seems to think that limiting a magazine to 7 rounds will keep a shooter from carrying multiple magazines (or keep him from carrying illegal high-capacity ones). In an open environment that might give people time to run but in a closed space like a school or theater it just means that the shooter has to change magazines more often. The Aurora shooter did reload multiple times.
Sports shooters do have a legitimate use for high-capacity magazines. People at a shooting range go to shoot. They usually have a limited amount of time and don't want to waste it reloading. Neither do they want to pay extra to carry multiple magazines.
Some people have insisted that these guns are more deadly than regular guns because they were designed for military use. They claim that these guns were designed to kill people. Actually, military weapons involve a series of trade-offs. They have to be light enough to be easily carried. The standard 9mm NATO is not the most lethal out there. I remember a lot of complaints when the army switched from .45 to 9mm that the new ammunition did not have the same stopping power. A shotgun loaded with deer slugs is much deadlier but it is painful to shoot and not as accurate at longer ranges.
Ironically, military ammunition is less lethal than sporting ammunition. According to the Geneva Convention, it has a full metal jacket. That makes it pass cleanly through the body and makes the wounds more survivable. Hunters commonly use hollow-point bullets which mushroom on impact and create bigger wounds.
Historically, the military has been slow to implement advanced in gun technology. Sportsmen had rifles centuries before they were standard issue to soldiers. At the turn of the last century, the US military did not want to army its troops with rifles that could accept five round clips. They felt that soldiers would aim more carefully if it took them longer to load. That attitude continued in Germany through WWII. Similarly, semi-automatic and full-automatic weapons were considered too expensive and delicate for field use well into the 20th century.
So, we have tens of millions of sportsmen being told that their preferred weapon is too dangerous to be owned based on misinformation and false urgency. Is it any wonder that there is significant push-back?