There's really no way to make a semi-auto firearm that impedes full auto modification. Full auto is the default behavior. It's the trigger disconnector that makes a handgun semi-auto. That said, full auto isn't very useful. It's inaccurate and eats up ammo quickly, necessitating reloads.
1. https://en.wikipedia.org/wiki/Firearm_Owners_Protection_Act#...
This isn't true for any modern rifle or pistol design- the absence of the disconnector just means the gun stop working (hammer follow). You need additional parts to ensure that the gun works properly in full-auto; this is accomplished by a secondary trigger (the 'auto sear') that trips the hammer/striker after the bolt closes.
>There's really no way to make a semi-auto firearm that impedes full auto modification.
The only designs that work this way are all already classified as machine guns in the US, so they aren't made/sold there any more.
The original comment was about handguns specifically, and was likely a reference to auto sears on Glocks. The combination of striker-fired and reciprocating slide makes it very easy to trip the sear at the right time, giving you better reliability than simply disabling the disconnector. This tweet has a useful diagram showing how Glock auto sears work.[2]
The ATF did restrict the manufacture of new guns that fire from an open bolt (since they are trivially made full-auto), but old open bolt guns aren't classified as machine guns.
1. https://www.youtube.com/watch?v=qc2xpGQ_8w8&t=250s (just FYI this guy swears when his gun accidentally goes full auto)
2. https://twitter.com/keegan_hamilton/status/15074309018771292...
https://en.wikipedia.org/wiki/2017_Las_Vegas_shooting
The fully-automatic question comes up because of pragmatism and practicality. The guy used 1000+ bullets to fire upon a crowd. You physically cannot do that if you only had semiautomatic.
Luckily very few mass shooters have plans that are so well thought out.