How do you prevent/halt Spencer butt cracking?

[DJ]

Mercifully, none of the Spencers I acquired in the past have had (or developed) butt cracks.  However, I recently picked up an M1865 rifle, and it has very tight but detectable cracks starting to develop along the magazine tube, starting at the buttplate.  In their current state, they aren't a problem, but I would not want them getting bigger. 

Is there a good way to keep the cracks from spreading?  I would like to keep the rifle in as original condition as possible, so something like sanding it down and refinishing would not exactly work.

Thanks--

--DJ

[Herbert]

the cracks genrally start because there is rust around the magizine tube, it will have to be striped down and the magizine tube cleaned and coated to prevent further rust ,not a easy job once the rust starts to grab the stock,do not put oil in there to try to free it (it will make it tighter)just gently tap the magizine tube & the butt stock with somthing that will not mare the tube or stock wile trying to unscrew the magizine tube ,be pactiant this can be a long job or it may come easily (unlickly if rust is present)when the magizine tube is removed ,thourly clean the rust from the hole and coat with boiled linseed oil and leave for a few days the cracks oftern close up by them selves leaving a almost invisible mark,if not I would get it seen to by a antique restorer,it deservs it the 1865 rifles are not very common + nealy all were Canadian issued to prevent the US Irish invasion that they were woried about with good cause

[DJ]

There isn't any rust--I think the wood is drying out.  I would like to restore it, but I don't want to create an oily mess that glue won't stick to if the cracks need to be repaired at some point.

[Herbert]

If you think the trouble is from the wood drying out it will have to be oiled at some time,this is probly a cass were a profesinal antique wood restores nohow would be welcome,they will allso probly put you onto Kramers Antique Restorer,this is the best oil mix For antique wood I have ever used,good luck

[minerotago]

Without seeing it if it is, as you say the butt drying out then I would glue it now to halt any further cracking and then rub it down with Linseed oil ( Boiled Linseed not raw ) I would give it several coats to really protect it and ensure that the first coat soaks into the grain and pores of the wood.
I have done this with many rifles over the years and it does work wonders. Many years ago I got a BSA .22 for rabbits and the last owner kept it  on the seat of his car in the sun for years and it was very badly cracked but Linseed oil took care of that and it never got any worse, indeed the linseed soaking in actually closed up the cracks considerably.

If any of you get the impression that I place Linseed oil over anything else - I do!