I've been planning a new rifle build for a while, and have had the lower for several months. My existing rifle is a 20" modernized A1, ad dead-simple irons-only 300yd rig that is my go-to for general purpose work. I'm a bit torn between two options for the next rifle, which is intended to be a longer-range system for shots out to 600yds.

The two points I'm undecided on are the optic and barrel length. Stock will be an ACS-L; I've about made up my mind there. Free-float M-lok rail, also decided.

Optic 1A: Primary Arms 1-6x24mm ACSS
Really just a step up from a garden-variety carbine, but fast and better for CQB.

vs

Option 1B: Leupold Mark AR 3-9x40mm, mil-dot
I'm interested in learning how to do 'real' long-range shooting with dope cards and everything; this optic is less point-and-click but would be a great way to familiarize myself with the mil-dot system.

Option 2A: 18" barrel
Slightly less velocity, moderately better handling

Option 2B: 20" barrel
More velocity, better balance

Any input? I'm willing to consider any combination of the above major attributes.

Get a 18-20" LW50 barrel treated with their LifeHD process and lmk if it's the god of AR barrels.

Whichever route you go. Spend a couple extra bucks. Get a leupold vxr patrol 3-9. You'll be a lot happier with that in the long run

Why do you say that, exactly?

I've owned both. Sold the Mark Ar relatively quick. I still own the Vxr.
Don't get me wrong the mark will do it's job. But the vxr will stick around on your rifle a little longer. I guarantee it.
Mil adjustments imo we're more precise. Especially when returning to zero. Glass was cleaner. Just a better scope all around.
You get what you pay for when it comes to anything optics related

Keep in mind both are 2nd focal plane. so measuring range always needs to be done using the max magnification.
Either way. Both are low range to mid level scopes. so there will always be room for improvement.

This.  The glass is about two levels higher in quality than the Mark AR.  Think VX-3 compared to VX-1.  That might not matter much inside 300 yards, but makes a big difference at 600, where resolving of detail matters.

As to barrel length, a 16" will allow high bc ammo to remain supersonic to 750 or so.  Consider barrel quality as more important than length from 16" to 20".

I'll take a 16" precision match barrel any day over a so-so rack grade 20" barrel.  A good 16" Recce can get the job done easily at those distances and is far more versatile, lighter,  and handles better.

My Recce.  Yes, ACS stock.  VX-R 3-9x Patrol, 16" Lilja M4 Navy SEAL recon stainless match barrel,  Daniel Defense free float, SSA-E trigger, Harris 6-9" bipod and a few other enhancements.  Very capable at 600.  OP's build might look something like this if he choses a 16" precision build.

I've shot 500 meter steel with an 8X optic.  Worked fine.  I traded up to a 15X.  Why?  Twice as good at that range.

1-6s are great optics, and gaining the ability to use up close is going to be an advantage.

I'd do a slight change on your optic choices though, and take a good look at the PST Gen2 1-6 from Vortex.  The reticle is identical to the Razors.

You'll get a center dot which may be an advantage to you if you like Aimpoints.

Go with an 18" barrel IMO.

For me at least, shooting to 500 yards and beyond is really difficult with a 3-9X scope. Yes it can be done but I would rather have at least 15X for 500 yard shots. I really like the Bushnell Tactical Elute G2DMR 3.5-21X50 for shots from 50 yards to 800 yards. Or the Gen 2 version of the same scope.

I shoot 600 yard matches. I would want at least 8x for that distance. I would also go with 20" barrel, but, as pointed out already, quality matters more than length.

Eye box is important if you're doing anything other than laying perfectly still and have all the time you want to shoot. Get a scope with plenty of exit pupil at the high end. I use a 3-12x42 and dial it down to 8x when slung up prone because it's too hard to stay in the box at 12x.

You also want consistent turrets so you can dial up known distance zeroes in a repeatable manner.

At 600, a whole lot more than magnification comes into play with your scope.

When using a scope I would consider a different stock.

Something like Magpuls UBR where your cheek weld stays the same no matter the stock length