I'm a bit confused by this. Muzzle brakes don't come with SDN6's. Flash Hiders do, and not ALL the time (depending on when the can was made.) Is it possible it's a 90T, or something else off?
There are four different main situations I get, and I'm guessing most other dealers and manufacturers get.
1) The 'unclear' scenario. "The thing for the one side of the place is backwards so when it does the second part it won't work."
2) The 'wrong' scenario. "I put it on and it just came off with the can, the torque was enough." (meanwhile, it's barely hand tight.)
3) The 'not an expert' scenario. "Something isn't right and I don't know what it is, I need some help."
4) The 'correct' scenario. "I'm a machinist and thread barrels and manufacture a bunch of parts. The piston is marked 1/2x28, but threads onto my 1/2x36 barrel, that houses a 1/2x36 fixed barrel mount. I hit it with a thread gauge and it's reading 1/2x36."
The BEST solution for all of these is to bring in all the necessary parts. If someone REALLY knows what they're doing, you can get away with not having everything in front to find a solution most of the time. 95% of the issues are people that don't know what they're doing--- which isn't a problem. We all learn about things, no one is born an expert at everything. There's absolutely nothing wrong with needing help and learning the ropes. The remaining 5% are either confusion "Well this 1911 barrel IS 5/8x24" (while it's really .578) or it flat out is something wrong with a part (out of spec, etc).
As a blanket statement, I want to see all the parts in front of me if I'm trying to offer a solution. Otherwise, it's just guessing.