Neal,
I have some experience with adopted gun dogs.
Five years ago, my wife and I adopted an AWS that had been severely abused and neglected. He showed no indication that he had been trained and, even at 2 y/o, he didn't know his name.
Obviously, my expectations were low and I decided to adopt him as a companion, not as a hunter. That was the correct attitude. He was very difficult to train as he didn't respond to positive reinforcement and would shut down at the slightest hint of negative reinforcement.
I stopped at basic obedience but he's been the most wonderful companion you could ask for.
As for quirky behaviors, you can expect a few. I'll give one example -- Morgan is absolutely terrified of dark-colored Jeep Cherokees. He's fine with light-colored Jeeps and any other small SUV, but if he sees a dark Jeep, he freaks out.
Morgan's background is the worst-case scenario. If you are patient, you can find mature pups that are in the rescue network through no fault of their own or due to unfortunate family circumstances. Over the years, I've seen quite a few chessies with gunning experience in need of placement. I'm sure you can find plenty of labs as well.
For any dog that is in the rescue network, regardless of age, approach them as you would a puppy. Expect timidity at first. Let them respond to you before engaging them. Further, don't expect them to respond to commands. Even if they know commands, they don't know or trust YOU.
I think some of the advice above is way off base. Leave guns, blank pistols, dead ducks, etc. at home. Base your decision on social skills. Again, assume the dog is a blank slate -- like a puppy -- and leave any hunting development up to a progressive training regimen.
Good luck.
