For a gun class, low profile is the way to go...Thermobak, MULE, VIPER, Ambush will serve you fine. HAWG, Motherlode or BFM are just way too big for that purpose.
The Camelbak hydrolock system is very secure. There is a physical valve that one moves to "off" to close the tube off. It's very secure. I wish I could say the same for the Hydrastorm product.
I was out running around the border one weekend and was using a Hydrastorm Torrent Extreme in my backpack. This was the first real "hard" use I had given it. About 1/2 hour into our excursion, I was sucking about 1/2 air and 1/2 water. I thought I hadn't bled the bladder completely so I stopped, removed the unit from my Becker Patrol Pack and bled the air out of it.
About 10 minutes later, same thing.
Turns out it was the push/pull (on-off) bite valve that is the heart of the Hydrastorm system...it was leaking air into the unit at the point where the valve came "out". Unless I pulled tightly on the valve to "seal" it against the vavle-stem, it would leak air. Needless to say, I was less than impressed.
I've another Hydrastorm unit (Tsunami) that this hasn't happened to, but I've only worn it for light hiking around Phoenix.
In addition, I find the screw-tops of the Hydrastorm units even harder to thread on than the Camelbak ones (their "older" longneck reservoirs, not the new Omega). And that's saying a lot.
Once I get rid of my last Hydrastorm unit from my inventory, I am eliminating Hydrastorm units from my items for sale.