Would like to take some dry gear through a sump, wondering if anyone has any experience of using rocket tubes for this and knows how deep they can be taken without leaking? 

Have used them quite a few times over the years with (in general) a fair bit of success. Really need to test individual tubes to see how good they are.  Obviously the o-ring needs to be in good condition (grease it up) & the main flaw is that the attachment point is also likely to be the main cause of failure. What I mean is that you need to really crank the lid closed but then if you're clipping it on to a harness then you're likely to be using the ring/handle and it's easy to loosen the lid this way. Might be an idea to put some snoopy loops around the tube & then put a crab into this?

Other issue with rocket tubes is that they actually have a pretty small capacity (ribs inside them) but they're a bugger to get rid of. Oh, and pretty buoyant so might need to attach a bit of lead to the outside.  And my memory is pretty shite but I know that I've used them down to at least 25m. Well, I say "I know" but what I actually mean is "I think"!!!

Cheers!  Lid security is one of my misgivings, it doesn't take much of a turn.  I was thinking about carrying it in a bag to reduce the likelihood of this.  I noticed that the D5 rebreather uses one for the scrubber...

Martin Groves and I both used rocket tubes for scrubber canisters on our first rebreathers too. They performed admirably: it was the land rover inner tube (cunningly disguised as a counter lung) that was almost my downfall. Nightmare to breathe from with that much resistance....!!

'O' rings degrade when mineral grease is used on them. Silicon grease would probably result in a longer life.

A length of motor cycle inner tube (or any other rubber tube of correct diameter) around the outside of the tube where the seal is gives a bit of 'belt and braces'.

These tubes will work to at least 25 metres. Check the o ring is clean though. I had locking problems and I solved them by using a self tapping screw which goes in the side (from one of the bits that hang down) sufficient to prevent accidental unscrewing of the lid. Remember to take a screwdrive with you although with a flat head screw and diving knife will do. Rember they can be very buoyant if you are sump diving. I had a mini epic in Wookey once taking ten minutes to drag one through a 3 metre long sump! If the kit inside is heavy it's ok otherwise a weight is handy to give neutral buoyancy.