[pavel_lishin]

I'm not familiar with guns - what's a sear?

Without knowing that, I'm gonna go ahead and make a comment that may not make sense: I think the idea is that it shouldn't be trivially simple to convert a semi-automatic rifle into an automatic rifle. E.g., it can't be done by putting a paperclip into some part of the mechanism, or slightly filing down a metal part.

If getting a full auto sear is a non-trivial process (which it seems to be) and making one from scratch isn't trivial either, then gun manufacturers seem to have gone to reasonable lengths to prevent their guns from being converted to full auto.

[bmelton]

A sear is, basically, a piece of metal. It's part of the trigger assembly. Speaking simplistically, it is the thing that gives the trigger pressure -- it holds back the striking mechanism until your trigger pull releases it.

https://en.wikipedia.org/wiki/Sear_(firearm)

This is an animation of a semi-automatic sear (in which the sear is the green part): http://mcb-homis.com/blog/trigger-animation.gif

This is an animation of a fully automatic sear (in which the sear is blue): http://mcb-homis.com/blog/m16animation1ww1.gif

Taking out a semi-automatic sear and replacing it with a fully automatic sear is trivial. Converting a semi-automatic sear is slightly more difficult, as it requires precision, and a supplementary catch (to keep the firearm from always firing always).

If people aren't committing crimes with fully automatic firearms, it isn't because the process of obtaining fully automatic firearms is difficult, because it isn't. You can get butt stocks that replicate fully automatic fire for less than $50. Fully automatic fire is just silly to use. If you miss with the first shot, you're almost certainly going to miss with subsequent shots, so for anything requiring accuracy (which is almost everything you would do with a firearm short of perhaps intimidation), converting to full auto makes it less effective.