O.T. but for securing shims in hooks try Permatex High Strength
Sleeve Retainer. (NAPA part #765-1149). It has 3000 psi shear strength & it's used to secure valve seats & valve guides in automotive cylinder heads. You can get it at NAPA & other automotive parts suppliers.

I did one gun with this about 6 months ago & so far the shim has stayed in place & I'll have a better idea of how well it works once sporting clays weather reappears this spring.

Loctite has some similar products.