Self sealing DSMB's are better than the old fashioned open ended type.
I carry a large self sealing DSMB, and a reel (as a backup). The last few years I have been using a gas cartridge type and a finger spool - which is much easier when on CCR, as my primary DSMB.
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I heard that these are dangerous as they can go-off and drag you with them to the surface, don't know how true are these rumors.