At least for now.
Took it to the range this morning and tried it out and did some sight adjusting.
Part the First: worked flawlessly.
I'd expected it might shoot low, instead it was high. Quite a bit. Dropped the rear sight to the lowest notch and that put it dead-on with the practice load at 30 yards. Did have to drift the rear a bit to the left; it's close enough that I'm not going to mess further with it until make a witness mark or two and have a more-solid rest and better light. Plus some of the full-power hollowpoints want to try it with.
Update: Just struck me why it was hitting so high: when I bent the magazine tube so it would fit into the front barrel band without pulling down on the barrel, it got rid of the downward tension that'd always been there before. Thus it started shooting much higher than before.
I'd wondered about the effect when I changed it, but it'd slipped my mind.
Part the Second: with the magazine spring at original length the tube holds seven rounds, and the last is pretty tight to push in, which brings me to the modification: shortening the spring.
Here's how it looks with tube, section cut off, full-length spring, follower and end cap
put the end cap on the bottom, tube on top
drop in spring
slide in brass rod
slide on cutoff section
compress spring all the way to the tube end
raise the cutoff section just enough to slide a thumbnail in and catch the spring
remove cutoff, carefully remove spring so I didn't lose the spot, and measure. Immediately.
The part that fit in the cutoff section was 5.5" long(cutoff is 3.5"). I reduced that to 5" even and cut it,
bent the end* to make it level instead of a sharp end sticking down, wiped it all down and reassembled. Which consists of
drop in the follower
drop in the spring
carefully work the spring into the tube
put the end plug on without letting the spring come shooting out
install the screw that locks the plug in place.
Tried it, and it now holds eight rounds and feeds with no problems. Which latter was a great relief since I can't glue a new piece on the spring if it was a bit weak.
One more thing I definitely intend to do at some point is replace that yellow plastic follower with one made of either brass or steel. Read somewhere that someone makes one, if I can't find that I'll make one.
One thing I might do at some point: this stock gives the carbine a 13" pull, which it a bit short for comfort for me; if I can find a stock a bit longer might have to pick one up and fit it.
*Actually bent both; it had just been cut off of a longer section or wound to length and cut with not cleanup on the ends.