Looking to get into longer range shooting 800-1000 yards and I want a gas gun, looking to buy a seekins sp10 in 6.5. Are there better options than that in the 2500.00 range?

I agree (not that I'm an expert).  If you're looking at Seekins, go for it

A Seekins or a used LaRue, if you can find one. Both great guns.

My old LW barreled OBR is an absolute shooter.

My 18” 6.5 UU has shot many sub 1/2” groups. A thousand yards was easy with it.

6ARC > 6.5CM

Small frame that’s lighter, easier to shoot accurately, less recoil, cheaper ammo, cheaper to reload. There’s no reason to get a 6.5CM over one for your uses, only disadvantages to it.

You can buy off the shelf Hornady match ammo and it’s dead nuts consistent to 1200 yards out of an 18” gas gun. The Seekins DMR’s in 6ARC is awesome too.

Build.

A) Buy a quality BCG.  

B) Send the BCG to a quality barrel maker and have them make a barrel headspaced to YOUR bolt with gas port placement and size suited to your final caliber and muzzle device configuration… Bartlein, Lilja, Craddock, McGowan, Kreiger, Hart, Christiansen, etc. (I went with Craddock/Bartlein on mine and they even nitrided it.)

C) Buy a quality trigger like a hiperfire hipertouch reflex or better.

That’s like 99% of the accuracy side of the equation for a gas precision gun; barrel, BCG, headspace, and fast light creep free trigger. Stick it in whatever receiver set with whatever accessories you want.  Aero M5E1 builder kit is a good bang for your buck.

Especially when you get off the beaten path of 5.56. Oddball calibers have their own learning curve, and large frames even more so. It’s not insurmountable or something but it takes some additional experience and knowledge.

I just got done working all the kinks out of my Grendels. They now run reliably and shoot VERY well, but that’s not the case if you just crank them together and run with it. I enjoy troubleshooting and fine tuning. If one isn’t willing to do that, you’re better off buying factory for sure.

6 ARC has enough gas to get you to a 1000 and be supersonic.  However, the wind will kick your ass with the lower velocity.  My 6 Grendel is a 100+ fps faster than a 6 ARC and it struggles when the wind starts kicking up.  Thats when my 6 Creedmoor really shines over the shorty Grendel.  3100+ fps with a 105-109 gr bullet > 2800 with a 105 from my rifles.

And I know that my 6 isnt a 6.5.  Youll get better ballistics from a 140 from a 6.5 Creed at 2700 fps vs a 105 at 2600 from the 6 ARC.

At OP:  Buy or build the 6.5 Creed.  Make sure its got a decent barrel and an adjustable gas block.  Factory ammo is plenty, and reloading is easy for it.