The mandrel aligns the cores, yep. The instructions I read seemed to indicate the grains are inserted individually:
From the pdf doc:
Pro75/98® Moonburners
M520-P, M795-P, M840-P, N1100-P, N1560-P
Special instructions for loading (supplement standard Pro75/98® instruction sheets)
located at
https://www.pro38.com/pdfs/Moonburner_supplement_segmented_V1.2.pdf
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Step 4: Install mandrel in grain #1
Slide mandrel in first grain (grain #1) (see image on the right). Note the
orientation of the grain as indicated by the arrow on the grain inhibitor.
Step 5: Apply adhesive on grain #1
Using the brush, apply adhesive on outside and top of the propellant
grain.
Step 6: Install grain #1
Slide grain into the case insulator using the mandrel to push it all the way
down.
Step 7: Apply adhesive on following grain
Apply adhesive on the next grain. The adhesive should be brushed on
the outside as well as the two propellant faces.
Step 8: Install following grain
Noting the orientation of the grain as indicated by the arrow, slide the
grain over the mandrel and push it all the way down. Make sure the grain
is properly seated on the previous grain by pushing down on it using a
piece of plastic tubing, airframe or yard tool handle.
Step 9: Repeat
Repeat steps 6 and 7 for all other remaining grains. The last grain does
not require application of adhesive on the top surface.
Step 10: Clean up case liner
Wipe of any excess adhesive using paper towel along the top edge of the case liner to ensure that the nozzle will fit
inside the case liner once the adhesive is cured. Also any spilled adhesive on the outside of the liner should be wiped
off.
Step 11: Remove the mandrel
While pushing down on the top grain remove the mandrel. Ensure that the grains do not move during mandrel removal.
Step 12: Adhesive curing
Set grain aside for a minimum of 2 hours before handling it and keep it upright at all times.
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However, the thought of gluing them together OUTSIDE the liner is interesting! Hummmm...
Speaking of hummmm, thoughts about the Gorilla glue? Or should, I (I can't help myself...) not monkey around with that?!?