As of a day ago, MakerBot's Thingiverse Web site hosted the plans for a key component of an AR15 semi automatic rifle. Anyone could download Michael "HaveBlue" Guslick's design for the lower receiver, and if you had a 3D printer you make one yourself.
Those plans, and plans for other firearm components have now been removed from Thingiverse. You can access Guslick's old listing, and you can also find it on the Pirate Bay and elsewhere, but the printable STL files have been removed from Thingiverse, and the listing no longer turns up when you search there.
As to what compelled the sudden change, a MakerBot spokesperson said the following:
MakerBot's focus is to empower the creative process and make things for good. MakerBot Desktop 3D Printers make innovative products, new tools, models, fashion items, works of art, and 3D things of all types. MakerBot's Thingiverse website is designed to be the best place to get and share downloadable 3D "Things." Thingiverse's Terms of Service state that users agree not to use Thingiverse "to collect, upload, transmit, display, or distribute any User Content (ii) that...promotes illegal activities or contributes to the creation of weapons, illegal materials or is otherwise objectionable." If an item has been removed, it is because it violates the Thingiverse Terms of Service.
It seems MakerBot has decided to exercise that right.
Update, December 19, 3:09 p.m. ET: MakerBot attorney Richard McCarthy responded to a request for further comment, saying "Thingiverse has always been, and is currently, evolving...as is the company as it pursues innovation and growth. We have always had the discretion to take action for policy violations. Recent events served as the impetus here to take immediate action (and there were several) and reiterate or emphasize the site's focus on creative empowerment for products that have a positive impact."