Popular Science Shows DIY Sentry Gun in May 2014 Issue
In the May edition of Popular Science, there are several interesting articles, as always. For the NIC, there's a relatively easy method of setting up a sentry using a Stryfe and a passive infrared sensor. Full instructions are here, for those interested.
There are other blasters you could use, but the Stryfe is easy and inexpensive for experimentation. Not to mention compact. Mounting a Rapidstrike might lead to some space and balance issues.