Interesting thread. I think most cracks behind the lockplates are caused by setback of the action into the wood. Some actions have more bearing surface designed in the head of the stock, and others don't. If you're around double guns for a while you'll get to know which are more prone to cracking. Once in a while someone will write that cracks are caused by old wood that's tightened around well inletted lock plates. While I suppose that might happen once in a great while, I really don't buy that explanation as a general rule. Take a look at some Manton or W&S flintlock and percussion double guns. All have very tightly inletted locks. Another example is the excellent Syracuse Lefevers circa 1890's. You'll rarely see cracks behind the tightly fitted lockplates. In both examples, the wood is pretty dry by now!
Too much oil squirted into the firing pin holes, running into the wood and softening it? That happens with all makes including Lefevers and Holland & Hollands. If a gun has enough bearing surface in the head, the action won't set back as much as one with lesser bearing area.
Personally I think that glassing a classic sideplate gun to prevent it from splitting is like pop riveting some galvanized sheet metal onto the fenders of a 1933 Packard coupe so they don't start stress cracking if you want to drive it a lot. If I have a shooter-grade gun that is already cracked, the glass route might make some sense; but if it's a collector grade gun I'd sell it off and get another classic double gun I can shoot as much as I want without worrying about getting cracks started. In fact I have done that. IMO of course. Silvers
