I am looking to add a buttstock cartridge holder to my Mossberg 4x4 Bolt Action and didn't know if anyone had any input on this topic before I went ahead and ordered one blindly.
Hmm, I think Eagle is reasonably inexpensive for what it is. It is Federal's ATK Tactical division, LE/ATK. http://le.atk.com/general/irl/catalogs.aspx Click on "Eagle Catalog", it opens a pdf catalog, and the shell holders are on page 41. Other places carry those, like Optics Planet for example. http://www.opticsplanet.net/eagle-industries-shooter-039-s-stock-pack-rifle.html These are made in the US so are more expensive than import counterparts, but also include a lifetime gaurantee. I really like these, and would buy one in a heartbeat if in the need for one. I may buy the shotshell version infact.
I bought a few "airsoft" ones off ebay awhile back, they seem to be holding up fine so far. put one on a mauser/PH HB .308 I built and one on a Rem 700 25.06 (main hunting rifle). YMMV