SignessSG wrote:So what length past the tube are we looking at here? I just put a new Nordic extension on my 870, and went with the manufacturer's recommendation of 16" past the tube extension for the spring length to start (at the longest). I've read posts of people going as low as 12". Anyway, took it shooting last weekend, and it fed and ejected flawlessly. I figure if it ain't broken, don't fix it; but, it concerns me a little that that much length may prematurely wear out the spring from over-tension.
It really depends on the specific spring. Different springs have different stiffnesses, solid & relaxed lengths, diameters, coil pitches, etc (and manufacturers are really bad about actually giving customers those specifics); so there's no hard rule for how long springs should be relative to the length of the tube. The main thing is to be sure the spring is stiff enough to feed all the shells reliably. Based on my testing (which isn't really complete, I had to put it on hold while I was finishing my thesis) as long as you have 1.5 - 2 pounds of spring force or more with the magazine empty, you should be good.
Interesting, were you reducing the tension until you were able to out pace the shells ejecting from the mag? If so what was the first sign of failure, a shell halfway out of the magazine caught by the lifter?
Kentactic wrote:Interesting, were you reducing the tension until you were able to out pace the shells ejecting from the mag?
Among other things, yes. Like I said, I've had it (and basically everything else in my life) on hold for the past few months while I was finishing my graduate research and writing my thesis, so my testing and research on mag springs really isn't complete. But from what testing I have done, a 1-lbf empty spring force seems to feed flawlessly from a short tube configuration, and 1.5 lbf does the same from an extended configuration. I have a lot more to do on the project, and I'll post a write-up or video when I can make more confident and detailed statements.
