AM's impact on firearms involves two dimensions: new manufacturing methods for established companies, and new possibilities for the making of do-it-yourself firearms. In 2012, the US-based group Defense Distributed disclosed plans to design a working plastic 3D printed firearm "that could be downloaded and reproduced by anybody with a 3D printer."   After Defense Distributed released their plans, questions were raised regarding the effects that 3D printing and widespread consumer-level CNC machining   may have on gun control effectiveness.    
I took summer courses (one in the three-week session each, on-campus) last summer but not as an ALM candidate. I was working towards my three pre-admission courses into the ALB program.
Last summer was the first time they had two sessions. Prior to this, I believe it was one giant seven-week session. And as @Cats mentioned, you can't take more than two courses in the summer. And I think you also couldn't take, say, one seven-week course and one course in either of the three-week sessions together, but I could be wrong.
Anyways, the three-week sessions were brutal . There were readings before the course started, one, and two, and assignments were due after the three-week session ended. This means that I had to pre-read for my second session while concurrently studying/writing papers during my first session AND work on final papers when my second session started, which was already reading-intensive.
One thing to note is that library privileges end shortly after your last class. You mention how you're interested in satisfying your pro-seminar course and courses within two sessions. If, like this year, you have work due after your session formally ends, you will not be able to enter the library and check books out for your final assignments. We brought this up a couple of times in the summer but the administration said they could not grant extensions. So if you have work due after your in-class meetings end, you need to finish your assignments before your library privileges are taken away. But if you were already a ALM/ALB candidate before the summer, you could certainly exercise these privileges, but until then, they will end shortly after your class finishes.
Again, this was the first time Harvard Summer School split their summer into two sessions so things might change next year given the feedback from this past summer.