As Paul said, there is a large variation in the quality of many guns.
That makes it almost impossible to give a "list" of guns to avoid.
There are a few things for potential buyers to look at when buying a older factory made gun.
The first thing may raise a few eyebrows on the sellers but the lock should be removed from the gun.
Looking at the interior of the lock you should see a tumbler bridle. Many of the Spanish guns (and others) that were imported by many different import companies in the U.S. were made without this important feature.
For those who don't know what a tumbler bridle looks like, I'll try to give a brief description of it.
A bridle is basically a part or feature that entraps something.
The "tumbler" is the partially round thing that the mainspring acts upon. It is the part that turns when the hammer is moved. It has at least one little flat place on it for the sear to engage and this is usually on the side towards the rear of the lock.
The tumbler bridle will look like a little bridge that covers part of the outside flat surface of the tumbler and it has a hole that is located in line with the hammers pivot axis. It is usually retained with 2 or 3 screws.
The tumbler will have a round projection that is in line with the hole thru the lockplate and this projection goes thru the hole in the tumbler bridle forming a outer bearing that gives much needed support to the tumbler.
Locks that do not have this tumbler bridle have a tumbler that is standing proud of the lockplate with nothing supporting the side that is furthest away from the lockplate.
Without this tumbler bridle and with nothing supporting the outer end of the tumbler, the hole thru the lockplate is the only thing that works as a bearing and is the only thing that keeps the tumbler straight.
As the tumbler has the mainspring that produces a lot of force pushing against it, if the tumbler has only one bearing in a soft steel lockplate it doesn't take long for the bearing hole to wear out. When this happens the tumbler will get cocked and the sear that keeps the lock at full cock will not engage its full cock notch properly.
This can lead to big safety problems and there is no easy fix for it.
While the lock is out of the gun you will see the sear arm sticking out like a sore thumb.
Use your thumb to push up on this arm. It should move freely with a moderate amount of resistance from the sear spring. If it is very hard to push up try to determine if this is because the screw that holds the sear in position is the problem or the sear spring is just too darn stiff.
Both of these can be easily fixed but it is good to know about a stiff sear.
With a flintlock, many of the poor guns have a frizzen that is not hardened properly.
While most sellers would become upset if you took a file to the frizzen to see if it was hard, most wouldn't know what you were up to if you took your pocket knife out and scraped the corner where the frizzens face meets the edge towards the barrel.
If your knife can cut into this edge the frizzen is too soft. If your knife doesn't cut into the metal, if it leaves a bright shiny surface but no chips the frizzen is probably OK.
With the lock installed back into the stock, try the trigger.
If the trigger pull is heavy and the sear spring was very stiff or the sear screw was overtightened you can fix it by fixing the stiff spring or by loosening the sear screw 1/2 turn.
If, on the other hand, the sear spring was about right and the sear arm moved fairly easily the problem with the trigger pull is that the trigger pin is in the wrong place. This is a design problem and it is not easily fixed.
If the owner will allow you to remove the barrel then do so.
Look at the wood that was under the barrel at the breech end. A little fouling residue around the lock area is fine but if there is a lot of fouling at the rear of the barrel it indicates that the breech plug was leaking rather badly. This is not good.
Also, look at the rear of the barrel where the breech plug is installed.
Some guns were made with the breech plug brazed in and these will often have a small area where the brass brazing alloy is exposed.
A brazed in breech plug is dangerous so the existence of any brazing alloy is an automatic reason for rejecting the gun.
While I'm talking, be sure to run a oiled cotton cleaning patch on a correctly sized cleaning jag down the barrel.
A heavily rusted bore is bad but a light coat of red rust on the patch usually doesn't represent a problem.
When you run the oiled patch down the bore take note of the force needed to move the cleaning/ram rod. If, at some point it suddenly becomes very easy to push and then returns to the former force you have discovered a bulged barrel.
Because most muzzleloaders barrels are so thick, these bulges usually can not be seen by looking at the outside of the barrel but they sure can be felt when running this test.
I could go on but these are the BIGGIES so I'll leave it at that.
