For wood to metal: Gorilla glue, loctite E-120HP, K&G epoxy, Gold shafting epoxy, Acraglass.
(avoid anaerobic curing adhesives, jb weld)
For Major gap filling: Acraglass is designed for this very purpose.
(avoid gorilla glue or any other polyurethane expanding types)
For metal to metal: Loctite speedbonder 324, JB weld, PC7.
(avoid any CA's)
For metal to man made materials (i.e. G10, micarta, etc.): Loctite 324 speedbonder or any overall use ones.
For overall use: Loctite E-120HP, K&G Epoxy, Goldsmith golf shafting epoxy, Acraglass.
those that FAILED the test miserably:
Devcon 2 ton
Devcon 5 ton
All superglue including rubber additives
Anchor tite marine epoxy
West systems
Acragel.
Keep in mind the above test was done by extreme torture testing and strength was the ultimate goal. Test that were performed was dishwasher, intense hammering, freezing, dropping, bending, rotating, submerging.
However strength may not be the #1 critical issue needed when putting brass tubes in pen blanks but coverage and strength does indeed help greatly in blowouts and the lifespan of the product.
The other test showed that bead blasting provided a greatly superior surface for adhesive bonding. Also tested was the sanding materials (as we see pen tubes from PSI) and untreated. Sandpaper was marginally better than no prep work at all.