29/40 question?

I'm looking to get some higher powered casings, but had a question about the 29/40-120 system I already have. Specifically, what does the "40" stand for? There's also 18/20 and 24/40... what's the "20" and "40" mean? Is it thread related?
Based on previous threads, it also looks like I might need different closures for 29-180 and 29-240. Is this correct?
Thanks in advance! Dave

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The numbers have nothing to do with the threads on the casings. The first number refers to the diameter of the casing in millimeters and the second number is the approximate number of newton-seconds of the reloads it will hold.
Thus the 18/20 is 18 mm in diameter and will hold 20 nt-sec "D" reloads. The 24/40 casing is 24 mm and hold 40 nt-sec "E" reloads, though they also make "D" and very light "F" reloads that work in this casing as well. The 29/40-120 is a 29 mm casing for which they make "E", "F" and 120 nt-sec "G" loads.
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This is correct. The 29/40-120 casing uses different closures than the other line of 29 mm casings. For these though there are a large number of sizes of tubes which all work with the same closures, so you can have one set of closures and they will work with any of the tubes which hold, depending on their length, reloads from 60 to 360 nt-secs.
It also happens that the newer 24/60 case uses different closures than the 24/40 case.
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Ah, thanks. The "40" is a little bit of a misnomer then, since the 29/40-120 will hold some G's that go up to ~120 NS.
Thanks, Dave
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