Woo hoo! Some measurements!
I couldn't locate my calipers, so I had to make do with rulers and various other measuring aids. The mag I measured is an older one (serial no. CF10xxx), a powerfeed left mag body with a standard A.I.R. valve.
The first thing I measured is the depth from the rear of the Mag body to the retaining ring inside the which the main spring rests against. Using my precision calibrated plastic chopstick as a depth gauge, I measured 2 -13/16" from the rear of the Mag body to the retaining ring.
I then measured the thickness of the fully compressed main spring. This worked out to be darn close to 3/8".
I next measured the thickness of the bolt flange which engages the sear. It's pretty close to 5/32".
Using my chopstick again, I measured the depth of the bolt plunger inside the bolt. It's recessed about 3/16" from the rear face of the bolt. This puts it about 1/32" ahead of the front face of the bolt flange.
We now have enough information to calculate the position of the bolt plunger relative to the rear of the Mag body when the main spring is fully compressed. First we subtract the compressed main spring length from the Mag body depth; this gives us the position of the front face of the bolt flange relative to the rear of the Mag body. 2-13/16" - 3/8" = 2-7/16". Since the plunger is about 1/32" ahead of the bolt flange, we add 1/32" to our previous result. The position of the bolt plunger relative to the back of the Mag body is right around 2-15/32".
Now we need to determine the location of the powertube tip relative to the back of the Mag body. I first measured the length of the portion of the A.I.R. valve that rests inside the Mag body (from the step right behind the on/off body to the face on which the bumper rests). It's right around 1-7/16" on my Mag. I then measured the length of the powertube, which is rignt around 1-1/4". Adding these two values together gives us 2-11/16", which is the position of the end of the powertube relative to the back of the Mag body.
If we compare the plunger and powertube tip locations, we see that the back end of the plunger is about 7/32" down inside the powertube tip.
Looking at the inside of the powertube, we see a taper. I tried to measure the location of the taper's beginning; this was the least accurate measurement I made. It begins right around 3/16", give or take 1/32". So the tip of the plunger sits darn close to the beginning of the powertube tip's taper.
My bolt plunger has a tiny step on it. As best as I can tell, this step is about 1/64" high (eyeball estimate), and it's about 3/32" from the plunger tip. I could not measure it with the tools on hand.
After reassembling the Mag and firing it a few times, I noticed that the valve body does not rest up against the back of the Mag body; there's a tiny gap there measuring right around 1/32". This extra gap puts the back end of the plunger around 3/16" down inside the powertube (instead of 7/32").
From what I can tell, there isn't much distance between the beginning of the taper inside the powertube and the bolt plunger tip, even when the main spring is totally compressed. What this says to me is that a tiny change in the relative positioning of the bolt plunger and the powertube tip will result in significant changes in airflow through the powertube. Of course as you stated above, if your main spring is a bit on the strong side and thus your airflow is a bit on the low side, you'd crank up the valve pressure to compensate for the decreased airflow (which would also change the relative position of the plunger/powertube due to the increased pressure in the dump chamber).
It's a fascinating little balance going on in there. I think I'm going to have to chalk this one up to the elves. ;)
BJJB