Just getting into longrange shooting. A buddy traded me this.

556 sum 1/10 16” heavy barrel with an A2 upper.

http://www.olyarms.com/shop/parts/barrel/barrels-ultramatch/sum16.html

Considering I am just getting started with a Bushnell sports view 4-12 x 40 I pulled off my 30 30.



Taking my out of the equation what distance and of grouping should I even expect?

ULTRAMATCH BARRELS
These barrels are our most accurate barrels, and quite frankly, these are the most accurate AR-15 barrels on the market. These barrels have Broach-cut Rifling. What is that you say? Well, broaching is the method used to create the lands and grooves. For these barrels, we use the stamping "SUM" which means Stainless Ultra Match. SUM barrels are all chambered with minimum SAAMI 223 Remington reamers. These barrels can also be identified by looking down the bore. All of our broach-cut or Ultramatch barrels have only 4 lands and 4 grooves.

3 minute

what are your goals? I am bucking the trend here but I am not a fan of .556 for much past 300 yards. I know it can do it, but its far from ideal given all the modern cartridge options.

I have that same barrel, and it is pretty accurate for what it is.  Mine shoots quite well and is MOA capable out to 300-ish.  I use it in occasional 3 gun matches with a red dot, so I have never done extensive accuracy testing.

Don't bother putting a scope on an A2. I would suggest replacing the rear aperture with smaller ones like an Armalite 0.050"/0.060", or hooded 0.040".

600yds with 69gr SMK if the twist is true or on the fast side. An Army E target is about as small as you want to aim at. Realistically, expect 2 MOA for 5-10 shots at best.

From 100yds - 2 clicks to 200yds, 5 clicks to 300yds, 13 clicks to 500yds, and 18 clicks to 600yds.

Punching paper and steel to see if I can do it.

What weight bullets do you like best for it?

Should I switch it out for a flattop?

750 or so gets tough for shooting out of an AR, with mag length ammo.

Yes.
http://www.jbmballistics.com/ballistics/lengths/lengths.shtml
0.8-0.9" long at most.

62gr BTHP and 69gr SMK ought to play well. The 53, 55, and 60gr VMAX also.

Wow, for me that’s a country mile. I was only hope to hit paper at 500 yard. So this could go even further?

Ideally you want to stay above the transonic zone of about 1300-1050fps. That's where the shockwave goes back over the bullet from back to front, and many bullets can get wonky (look up .308 168gr SMK at 1000yds).

With my local inputs, 16" velocity, and the bullets a 1:10 twist will stabilize well, that would be about 600yds. After that flyers are more likely.

This is correct being stuck with a 10 twist.  If it was faster you could use heavier bullets and squeeze a good bit more out.

Getting ready to test it this weekend

It's a plain jane carry handle upper. You could put a scope on the handle and go for a chin weld but I think you're pissing up a rope trying to use that for anything other than plinking or self defense.

You could buy a stripped flat top upper and move all the parts over and then usefully mount a scope but even then, I'd expect no better than 1.5MOA from it and if you happen to get better than that, count some blessings.