Cartomizer (combined cart and internal atomizer) aka carto, holds 2x or more juice vs an atomizer cart, needs juice less often, provides a more consistent vaping experience for your favorite juices. however they have limited lives. They cannot be restuffed and are difficult to clean and dry to get additional life out of them. I only put juices I know I like in cartos, and I keep the carto damp for a week or sometimes two until the performance is too degraded and then I start a new carto.
Slim 510 can only use standard resistance cartomizers and atomizers, not the hotter LR (low resistance) ones (those need batteries 450mah or larger like fat battery models and mods.
Boge are the most popular cartomizers. I find mine last the longest if I use thin juices (20% or less VG) that leave little or no flavoring sediment to clog the carto (typically clear lighter juices)
And note - if you chose auto batteries, then you have to be extra careful to make sure a carto is not going to leak before putting it back on the battery because auto batteries can be damaged or killed by juice leaks. Manual 510's are totally sealed metal on the threaded end so they are safer from juice leaks (and you can also use a drip tip on the atomizer if you have a manual battery - a convenient way to test juices)
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How I fill Vapor4Life KR8 cartos and Boge 510 cartos
Pop out the soft cap with an unbent paperclip in the mouth hole, very careful not to catch stuffing or the wire loop.
Shake and open the eliquid
Hold a bit of tissue on the threaded end hole, tilt the carto, and add 18 drops per ml of capacity into the stuffing (Boges and most KR8 cartos are about 1ml).
Wait 15 seconds or so then see how many more drops the carto stuffing will soak in easily.
Clean up the threaded end, tilt open end down into tissue, puff into threaded end to clear air channel (Boges and the KR8 cartos I use have a defined center air channel and I want to see light through that)
Sop up any loose juice above the stuffing
Cap and let it settle - I lay it on different sides for 15-20 minutes to make sure every bit of stuffing gets damp so I won't singe the carto. if you use thicker juice than me (I use 20% Vg or less) then you may need to wait much longer.
While vaping, if the vapor gets a bit light or a bit dry or the carto is getting hotter than normal, I take the carto off the battery and add eliquid. I sometimes use a plastic or delrin drip tip as a mouthpiece on Boge and KR8 cartos (not to drip into since the juice would head for the central air channel)