Split DirectShow Capture

I am currently using ffmpeg to capture video and audio from direcshow (windows 7):

ffmpeg.exe -f dshow -i video="V string":audio="A string" -r 25 -t 300 -n output.avi

"-t 300" makes it to capture for 300 seconds (5 mins). But i would like to capure more time splitting the output file in chunks.

For example make a 30 minute recording in 6 chunks of 5 minutes. I could do this in a batch file calling 6 times ffmpeg in a loop, but in this way i lose capture time between the loops (1 second more or less). I would like to do it without losing capture time.

Any way i can make this to happen?
Thanks