Out of proof gun sales are against the law for a reason in England. Safety is the main reason. If a gun is in proof it is assumed that it should be safe. I know that a in proof gun may fail but the odds go up when a gun is out of proof.
If the gun is out of proof have it looked over by a gunsmith trained back in Great Britian or one who can evaluate the gun to see if it is worth the time and effort to make it right. Do not assume that a out of proof gun is a good deal, some out of proof guns can still be a good buy but most are money pits.
I worry more about on face and how thin are the barrels. A gun that has not been honed to death still can be unsafe if the barrels have been heavily filed when refinished. I looked at a gun, still in proof by the bore measurements, but it had one area of the barrel that was .012 which is much to thin to shoot with my hand almost over the area that was paper thin.
Back to your question. Do not buy a 10K gun that is out of proof unless it is worth a great deal more than that if in proof. If you buy a gun that would be worth 30-50K if in proof for 10K then by all means buy the gun. Then you can have it sleeved, lined or have a new set of barrels made and still be ahead of the game.
A poor sleeve job is a eye sore and the gun takes an extreme hit in value. A perfect sleeve job will still be a 20-50% discount to most buyers. Some will never even look at a sleeved gun.
Another thing to think about is the difficulty that you will encounter later selling the gun. If you pay too much for a gun it will be almost impossible to sell later. I have seen too many people buy a "name" and forget to evaluate the gun without the name when buying. That worn out Boss will not bring top dollar when you try to sell it. If you are going to spend 10K then figure the cost of inspection by a good smith to be cheap insurance.
