Man it sounds like that Quest C6 shouldn't have been certified to begin with. I mean if it behaves like a C3 someone could make the fatal mistake of thinking it could work in a heavier rocket, and result in dangerous condition of rocket going down under power! I'm not surprised, Chinese motors won't be consistent batch to batch and like I said, they might make decent consumer fireworks but a model rocket motor must be under a much stricter quality control. If it's a C6 it shouldn't be more than .2 newtons off... There is so much to making bp motors that a lot can go wrong, and when the objective is cost cutting.. you get the picture.
I don't know their customer service but Estes has very good customer service. Never had an engine fail except for one staging incidents when I was 14 where the sustainer stage blew up and destroyed the rocket (I think it may have been my fault), and Estes has replaced it free of charge. Only one motor failed but they were nice enough to send a whole pack... All the Estes motors are also highly reliable. Very consistent batch to batch and it works every time. If anything happened it was always the rocket, rather than the motor's fault. I never tried Quest but assumed they should be similar in terms of quality, but maybe the few times I tried quest back then they made them in Germany (who is known for quality)
Has a motor ever had their certification revoked because the market sample is far too different than submitted (or the designation was too far from what the manufacturer claim)?
I'm not an expert on the NAR certification requirements, but I've had a lot of the same questions you bring up. Evidently, the manufacturer/importer seeking certification submits the motor with THEIR designation of what the motor is... IOW, if Quest says to NAR "I submit my C6 motor for certification" then it'll end up being called a C6, even if it's more like a C3 in actual performance... Basically, it makes the NAR "certified" motor designation basically meaningless... because you're not getting "the real picture" of what the motor is. You're right, basically... if you stick a Quest C6 into a heavy/draggy rocket that flies okay on an Estes C6 with a different thrust/time curve and think they'll perform the same, you're going to be sadly surprised... To REALLY get an idea of what the motor's performance and suitability is like, you really have to look at the thrust/time curve, to get actual data of the post-ignition thrust spike (determines how heavy a rocket the motor can quickly "kick" into the air and get up to an aerodynamically stable velocity) and then the average thrust, and the burn duration. Simply based on the longer burn time, for an equal amount of total impulse (total power delivered from the motor), the thrust (peak, average, or both) MUST be lower, since total impulse is basically thrust times duration (area under the graph of thrust levels over a given duration). IOW, if you have a 20 n/s total impulse motor, that motor can have say a thrust of 20 newtons for 1 second duration (20X1=20) OR you could burn the motor at a thrust of 10 newtons for 2 seconds (10X2=20). Simplified example, but you get the idea...
Anyway, personally I'm of the opinion that NAR should determine what the final motor designation should be, not the manufacturer submitting the motor for testing... that would clear things up better. Quest submitted their motors as "C6's" because they want to be seen as 'equivalent' in performance to the Estes motors they're competing against... despite the fact that the motors perform very differently due to their thrust levels and burn durations as previously mentioned. IMHO, it's putting "marketing" ahead of giving the consumer valuable information determined by the actual testing.
Many people have used the Quest motors and never had as severe a heating problem or case erosion problems as I've experienced... though there HAVE been more than a few people report having had similar occurrences. NAR certification DOES include an "external case temperature" measured during static certification testing, and obviously it passed, or the motors would have never been certified. Periodic statistical analysis of a certain percentage of motors imported or produced is supposed to verify that the motors continue to meet the certification and reliability requirements batch to batch-- the exact details of this would be for others to answer... It's a given that a certain amount of "faulty motors" will get through (since it's absolutely impossible to test every solid rocket motor, since they are destroyed in testing-- statistical batch sampling is the only way) but even when the Quest motors perform "as advertised", the heavy sooting and dirty burning characteristics tend to really make my rockets filthy with "motor gunk", which is why I'm not particularly thrilled with them. Estes motors create crud as well, but burn "clean" compared to Quest motors (Chinese motors, specifically, to be clear).
SO, from a "reliability" standpoint, the Quest motors evidently meet the NAR standards for certification or they'd lose their certification... just like with Estes or anyone else. Estes motors aren't immune to a certain number of faulty motors slipping through and catoing or otherwise malfunctioning, to be sure, but in my experience, I've had FAR less issues and burned a LOT more Estes motors than I have Quest... (IOW, I've had as many or more problems with Quest motors as I have with Estes, and I've burned a LOT more Estes motors!)
Anyway, that's been my experience...
Later! OL JR