You can't put moisture in dry wood without swelling and cracking. Wood normally dries at an ever slower rate to a moisture level set by the air around it, so museums control moisture levels, keeping them constant for preservation, to prevent cracking. The lignin in the wood also shrinks as it ages, causing a "dry" look. Plus military stock surfaces usually aren't finely sanded and smooth.
In other words the wood is old and wasn't treated very well, so you have what you have.
Lemon oil is really a cleaner and polish, not a wood finish. use "boiled" (modified, polymerized) linseed oil instead. Wipe some on, let sit until slightly tacky, 20-30 minutes, then rub, wiping it off. Repeat until it looks OK them repeat 1 or 2 times a year forever.
If you have a sporter you'd do likewise, except for very fine sanding and filling the pores, to get a deep, semi-gloss, glowing finish.
Tung oil works good too IF you get a decent brand. But all these oil finishes are highly modified and vary a lot from brand to brand. Danish oil, for example, seems too thin to build up a good finish.