This slide set (LINK BELOW) may be useful to those who have not seen it before.
I’ve come across this slide presentation given at a WHO conference before but it’s good to review. Bruce Weniger provides some background on the unsafe 1st generation jet injectors and compares them with newer models that were being tested for safety. I wish we had the audio recording but to access the slides, please click below. The DOD withdraw of jet guns in 1997 is mentioned.
The slide above appears to relate to this study, A model to assess the infection potential of jet injectors used in mass immunisation.
(abstract only):
Jet injectors are needleless injectors that penetrate skin with high-pressure fluid. They have potential advantages over needles and syringes in mass immunisation programs, but concerns over their capacity to transfer blood-borne viruses have been a barrier to acceptance. Hepatitis B infection can transmit in 10 pl of blood; detection of such low volumes presents severe difficulties to such assessments. A model to assess jet injector safety was developed using injection of an inert buffer into calves and assaying the next injector discharge, representing the next dose of vaccine, for blood using a highly sensitive ELISA. Four injectors were tested: two with reusable heads and direct skin contact, one with single-use injector heads and one where the injector head discharged at a distance from the skin. All injectors tested transmitted significant (over 10 pl) volumes of blood; the volumes and frequency of contamination varied with injector. The source of the contamination was consistent with contamination by efflux of injected fluid and blood from the pressurised pocket in tissue that is formed during injection. This insight should inform the design of safe jet injectors.
The jet injector designs just keep coming. Bad idea originally; bad idea now.
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Veterans Claims Help 
A recent book chapter on the subject of jet injection, including the safety concerns for the old Ped-O-Jet® multi-use nozzle jet injectors (“MUNJIs”) used widely by the military and public health authorities in the 1950s into the 1990s, is downloadable from here:
http://bit.ly/Vaccines6thChap61 (case-sensitive URL; not copyrighted as the work product of a U.S. Government employee co-author).
The first of a series of 5 YouTube clips from a more recent CDC audiovisual presentation relevant to the subject (2009-01-07) is available at: https://www.youtube.com/watch?v=P0V-BpHBI9s
The entire series may more readily be accessed by searching YouTube for “Cutaneous Vaccination”.
A recent Lancet editorial on a new generation of safer disposable-syringe jet injectors (DSJIs) is available here: http://bit.ly/Lancet2014v384p642 (case-sensitive)