I have a 2.5" rocket, single deploy that uses redundant charges. RRC2, Apogee, then 1000 feet. My normal charge is 1 red cap (I use volume measurements, I have thousands of these little red caps from a previous project). My back up charge is 1.5 red caps. I use 3/8" OD x 1/16" wall, 1/4" ID tubing. They end up being 1.5-2 inches long, including the ends which are zip tied off. I drop both of them right on top of my electronics bay, no shielding except some dog barf tossed on top. I pack the kevlar cord, then my inverted piston on top of that. It ends up being a really tight space.
I've flown that rocket at least 15 times, ~2,500' each time - you can see all the events. I have never had a sympathetic ignition of my backup charge. I've always been able to see (and hear sometimes) the 1000' charge go off during descent when the rocket is under parachute.
I've flow the surgical tubing charges exclusively since Tony shared the idea with the world - mostly in cramped recovery quarters. I've never noticed in any of my altimeter data a sympathetic ignition - I've been able to see the pressure spikes in the data when they go off.
Edward