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Matus Tejiscak you wrote: What am I doing wrong? Any ideas? Could my eye's idea of what two concentric circles should look like be drifting over the hours? Or is it the hardware?

Dear Matus
The first thing you’re doing wrong is worrying about it.  You must remember that Mr Link produced the A12 as an inexpensive mass-produced solution to equipping the rapidly expanding USAAC quickly and efficiently, just like with his IF Trainer.  Your A12 might now be 80 years old, so despite Geoffery’s care and Mr Link’s tricks to reduce it, it might have picked up some backlash.  Unless you’re planning to use it to take you to your last hope of finding water in the Sahara, treat it as what it is, ‘a fun machine’. 

On the other hand, that +14’ looks a bit suspicious.  Where are you getting your Hc from?  If you’re using an app primarily for maritime, have you a half semi diameter left in somewhere.  Try using it at night against a star like Vega or Sirius.  Another possibility is make sure you keep the bubble dead centre in the chamber. In theory, if the curvature of the chambre roof is equal to the focal length of the optical system, your sextant should be relatively tilt proof, but I’ve noticed they’re not, possibly because of a change of fluid from the original. 

Finally, don’t be tempted to adjust anything which can be adjusted with an Allen key.  Should you remain tempted, it’ll be a Bristol screw, and you’ll need a Bristol key, which is more star-shaped and costs about 10x as much as an Allen key.  With luck you’ll find Geoffry’s left a couple in the box.  Think long and hard about adjusting the index mirror though, because unless you’ve got the steady hand of a surgeon, you’ll end up with a greater fixed index error than you started off with; well I did.  DaveP