Looking for a good scope for a 20" 6.8 spc. I am not so much looking for brands, but power. Would the ballistics of the 6.8 be best maximized with 3-9x40 or something more? And if more how much? I am thinking 400-600 yards.

I have a Leupold Mark AR Mod 1 6-18x40 on my 6.5 Grendel.  Currently shooting it out to 600 yards, will take it out further in the future.
While shopping for a scope for my Grendel, I figured that I could always dial a 6-18x scope down to 9x, but couldn't dial a 3-9x scope up to 18x.  Went for more magnification to cover all future endeavors.





A 3-9x scope is plenty for 600 yards, but I like a little more magnification as a "cushion".



Quality glass matters, clarity is at least as important as magnification.



 

What size objects do you want to shoot?  Is this for tactical, bench or hunting use?  These are very important questions for picking out an optic.

Mainly for bench shooting.

I prefer more magnification for bench shooting.  The magnification won't do you any good if the glass is poor quality though.  I would recommend something with at least 15x.  I don't know what your budget is, but Vortex Vipers are normally a decent quality budget option.  Their HS-T 6-24's are pretty good for the money.  If you want to spend more I would look at the Burris XTR II line, followed by the Bushnell ERS, and Vortex Razor in that order.



My 16" 6.8 has a 3-12 as I wanted to keep the low end for hunting use.

I've got a 4-16 on mine.

3x9, but only used out to about 400Y (probably max effective hunting range).

You could also look at a 10X fixed from SWFA if that works for your use case.  

If it is for bench shooting then get the best glass you can afford in a power range of at least 14x.  6-24x50 scopes seem a bargain now, as so many manufacturers are going to wider power range with more complex internal designs to allow for that greater zoom range.  These will be big, long, bulky and heavy scopes, however, especially with the necessary sunshade on the front for high magnification.   At the bench that will not matter.  But, if you are going to use the rifle in the field, you might want to think about something more compact and lighter.

I have a 6-24x50 Burris Black Diamond (no longer in production) that is superb at long range and even 100 yard target shooting where the goal is to shoot sub MOA or as close to it as possible.

But, remember this:  Magnification alone is not good if the glass is not also good.  You just get a big, indistinct blob if the glass is not up to the task.  I'd rather have a 14x scope with good glass that gives me a clear image with sharp detail, high resolution and contrast, than a higher power scope that is blurry or fuzzy or indistinct at maximum power.  Once power goes higher than about 12x, the need for good glass becomes more important, and the higher you go, the more the quality of the glass matters.  That's why good high power scopes tend to be pricey.

BTW:  In contrast to the above, I have a 3-10x40mm tactical scope I am playing with on a long range bolt gun.  I've not shot that combo beyond 400 yards, yet, but I am having no problem shooting MOA with that combo at that distance at "only" 10x.  But, the scope is the limiting factor, even though it has good glass and accurate turrets and reticle, as higher magnification scopes on that rifle take it sub MOA.  Weather permitting, I'm going to move to 600 yards with that scope and see how it does.

I have a 1-4 on mine, but I don't shoot more than 1,200 yards with it. I can do that because it's a 6.8, if I had a 6.5 Grendel I'd need more magnification to do the same. (I'm joking, nobody get mad)

I've found that the 4-14 range scopes give a very good balance of low power for hunting, decent magnification, and size for an AR15. All things being equal, you should also theoretically be able to get a little better quality 4.5-14 for the same money compared to a 6-18. Not true across the board, but I'd say more times than not.

I saw a last post by MS556 and thought for sure it was going to be a Leupold reccomendation, lol.



Like MS556 says, extra magnification doesn't do you any good if it's just a bigger blob.

I'm a firm believer in a 1.5x5 or 1x6 scope for all around AR use. Leupold's German #4 reticle with illumination allows you to dial the magnification down to 1.5, turn on the dot and basically have a close range red dot sight. Dial it up to 5x and turn the power off and you have a good precision scope. At around $600 plus the mount you get the best of both worlds.  

Red dot style sights rule inside of 50 yards for self-defense. The VX-3 1.5x5 illuminated is two scopes in one.

If bench rest and prairie dog towns are the sole venue then a high magnification scope is called for. Living with a high power scope for general purpose duty is far from ideal. They are big, heavy and almost useless from unstable positions. Getting them on a fast moving animal is almost impossible when walking.

The highest magnification I have on my AR's is a 2.5x10. I use a Leupold 24x Target scope for load development and swap it on and off as needed.

NIKON 1-4x German #4 is what's currently mounted on mine.

Also have an AIMPOINT to swap

Why would anyone get mad?? Grendel shooters are so humble and reasonable

So my buddy loaned me his diamondback 4-12x40. I have the 3-9x40 on my .308 and like it a lot. I like the extra magnification and the vortex has some nice glass. So I'm going to try it out once I get the rifle together. I'll let you know how it works out when I get it to the range.

ACOG on one 3-10 on the other

I'm putting a leupold 3.5-14x mk4 on my 6.8 build but only because I already have the scope I'm taking off another rifle.

If I needed to buy a new scope it would be another nikon monarch 4-16x great glass for decent price if you shop around online.

I have an ACOG on my 6.8 pig gun. Micro H1 on my other 6.8

I really like 2-7 power on 6.8's.

I have a 1-4 Trijicon on one and. 4-14 Burris on the other.  Both work well for what the gun was assembled to do.

My 6.8 is dedicated for Whitetail duty, I use a Leupold VXIII 3.5x10.

Scope options really depend on the main use of the rifle.

I think the 2-7 range is great for 6.8. Mine is a whitetail gun too.