In my opinion, if you are looking for an investment then you need to consider value "out the door", individual desirability, and long term worth of the brand.

Value "out the door". You can purchase a firearm new but as soon as you take possession it becomes used. The value of that firearm suffers an immediate decline that usually takes years to recover from. So I suggest you are best off purchasing a second-hand firearm for any but the very longest term investment. You can often find "as new" and "unfired" second-hand guns that retain their "out the door" value. But if you purchase a used, but not abused, firearm you then you have the option of shooting it yourself without reducing the value. Shooting it becomes sort of a dividend on your investment.

A firearm purchased as an investment should also have a high degree of future desirability. Obsolete calibers, faddish styles, and bizarre mechanism will appeal to fewer potential buyers than standard calibers, classic styles, and well established mechanisms. I don't think you can go wrong with a double rifle in .375 or .470. The .375 isn't going away anytime soon and most new large bore double orders are for the good old .470. I believe a fine sidelock will always be more desirable than a boxlock, with the possible exception of Westley Richards' droplock design. Classic sights, stock shapes, checkering patterns, and engraving will never fall out of fashion.

Regarding brands, I suggest you choose a well known firm, long established, that has a very good chance of being in business for the duration of your investment. There have been some excellent firearms made by failed companies, their brands having fallen into obscurity. The value of most obscure maker's firearms have tended to hit plateaus, neither falling nor rising over time. Additionally, you will want to send the firearm to the original manufacturer should it need some sort of repair. For example, British made Rigbys are fine guns and considered by most to be a good choice for investing but if your gun needs repair where will you send it? Your only choices would make an "original" gun less than original. If you send your "original" Holland & Holland to the maker for repair then what you get back is still all Holland & Holland.