I recently purchased the subject guage to determine the most accurate bullet setting depth. The one point I do not understand is that on a rifle with a magazine shell feed, the limiting factor to how far you seat the bullet is based on the length of the shell magazine. If you seat the bullet out to far the shell will not fit in the magazine or feed properly.
Can someone clear my head on this!

You are correct. The thing is that not all magazines are so short that you cannot load the bullet out far enough to get close to the lands. Out of 18 rifles I reload for, only 3 have magazines that are the limiting factor. You can also pick a bullet that has an ogive closer to the tip of the bullet so you can load it closer.

The trick with the Stoney Point is learning how to get the same measurement everytime. The way I do it is to set the bullet back inside the modified case pretty far, get everything set up with the case in the chamber, and push the bullet with a light force (the same every time) in until it stops, lock the push rod there. If I try to do it like the instructions say with bumping it back and forth with a cleaning rod down the muzzle, I get different measurements everytime.

Takes some practice.

When I was using my Stoney point OAL Gauge, I used a wooden dowell down the barrel to dislodge the bullet from the lands. Just back and forth, gently, until it feels like the same pressure each time.

Be careful not to push the bullet in too hard. Steel lands are harder than copper jackets, and will score the bullet, making subsequent readings very innacurate. I test a bullet once, then not again, though I will shoot them after that. If you feel it "click," then you know you've scored it.

Overall, I've stopped using the Stoney Point OAL Gauge as the measurements never got particularly repeatable. The RCBS Precision Mic, which does bullet depth as well as headspace, is much more repeatable.

I still use the Stoney Point Bullet Comparator, which is a good way to transition the measured bullet over to paper, then to die measures. The Comparator allows you to measure a length from the cartridgehead to the bullet ogive, a very repeatable measure.

Jaywalker