I have a DPMS "post ban" rifle and considering on installing a DPMS A1 flash hider. While doing a little research, a local gunsmith said that adding a flash hider would change the harmonics of the barrel and could affect the accuracy of the rifle. Is this true

not on a DPMS rifle.
The FS dinks with harmonics at sub MOA numbers.
so if your groups are 3/4-1MOA you won't have problems.

now your gun smith can easily mess up the barrel if he installs the FH wrong.
he can turn a 1MOA gun into a 3-4 MOA gun.

MOA is minute of angle.  1 minute is about 1" at 100 yds.

good luck.  find a good smith!

I tend to believe that idea, that a flash hider isn't exactly the hot set up if you're interested in accuracy.  And if you think about it, when was the last time you saw a flash hider on a dedicated target or varmint rifle?
On the other hand, you have to wonder, "How much does it affect accuracy?"  I have a post ban Bushy w/20" chrome lined barrel.  This rifle is plenty accurate enough for me and it serves as my test bed for testing all my handloads.  I'm actually glad that its muzzle isn't "garbaged up" with stuff it doesn't really need.  On the other hand, my new Super Light 16" carbine is going to get a flash hider (pretty much just for the Hell of it and because the Liberals don't want me to have one).

Quoted: I have a DPMS "post ban" rifle and considering on installing a DPMS A1 flash hider. While doing a little research, a local gunsmith said that adding a flash hider would change the harmonics of the barrel and could affect the accuracy of the rifle. Is this true

Yes. Find a good smith that knows the formula for cut back. Also watch that they do not crush the barrel while threading the muzzle. And for gods sakes do not damage the crown (sticking steel in the muzzle, ie die guides)!

If it does effect accuracy, it won't likely be noticeable. Go for it. You will have to re zero though.

The Army Marksmanship Unit installs the flash suppressor only hand tight with the aid of loctite.  They have found if you torque it down that it effects accuracy.

overtorque too.  I have one of those Guatemalan barrels where the FH was torqued so tight you can see where the lands and the grooves jog by a couple degrees at the base of the threads.

That barrel is good for maybe 6-8" tops.  I took it off pretty quick.