Many semi auto rifles, like the civy version of the M-16 can be easily modified to full auto weapons.
Anyone who knows anything about weapons will agree that full auto fire is nearly useless in assault rifles anyway. Multiple well placed single shots in a short time is what makes assault rifle more effective for warfare than earlier bolt lock rifles that require the operator to cycle next round in. With magazine capacity of dozens of rounds (usually around 30) and gas-operated cycle, an assault rifle is just as dangerous whether it has full auto enabled or disabled, because practically everyone with any weapons training will forgo the full auto option anyway in all but very specialized situations.
If you think about it, a nutcase with good aim can pop off a cap in 30 people's ass in a minute, while a nutcase who watched too many rambo movies will spew all 30 rounds from the magazine in less than three seconds and be less than likely to hit any but the first few rounds with any semblance of accuracy.
So, that said, for the feared worst case scenario, a civilian model of M1 carbine or M16 rifle or any other similar weapon is just as dangerous, if not more so, than the full-auto military version, since it doesn't offer the possibility for said nutcase to use the full auto option in the first place. For the purposes of limiting this sort of threat, limiting the magazine capacity of civilian weapons is one option to reduce lethality and the other is limiting civilian weapons to manually cycled weapons (ie. the operator must cycle the empty shell out of chamber and new round in).
I live in a country that has technically bigger firearm density per capita than the United States of A (based on registered. However, this mainly consists of hunting weapons - shotguns and rifles, rather than personal sidearms or full auto disabled assault rifle models (which are mainly used by people involved in voluntary defence associations, which actually co-operate their courses and events with the Finnish Defence Force to some degree) and the majority of violent crimes does not include firearms. In most cases of armed robberies, manslaughter et al, a bladed weapon or some improvised weapon such as a hammer, baseball bat or other blunt object. So, I wouldn't say the amount of guns per ce is the problem, but if the population continuously misuses their privileges, then the privileges should be reduced.
Note that privileges are not the same thing as rights. Privileges correspond to certain responsibility. For example, a license to drive a motor vehicle requires that one does it according to the traffic rules in a a responsible manner. Failing to do so will cause the loss of the privilege to drive a car - it is not a self-explanatory civil right to drive a car.
Early in the history of automobiles, their use was not licensed per ce - the owners of automobiles simply applied for and typically were granted to use their vehicles on public roads. The first country to start using tests to issue driver's licenses was Prussia, though they were mainly concerned about drivers' mechanical aptitude to maintaining their vehicles.
As the number of automobiles in use rised, fatalities started to occur and eventually all drivers were required to have passed a driving education and test to have a license. And so the privilege to drive a car with no special permissions was lost from the populace, replaced by a license-regulated privilege that has to be gained.
No one I know thinks of this as a bad thing. However, it is exactly the same as gun regulation; irresponsible use of automobiles led to fatalities, which led to tighter control over who is allowed to drive one. Why is there no public outcry against controlling government in this case? After all, the American people are about as enthusiastic about their cars as they are about their guns...
Aside from that, I know very little of day-to-day American politics and make random guesses.
