Last weekend I was hunting with a guy that was toting a 6.5-300WSM - He said you either start with a .270WSM or .300WSM casing (same case capacity) and resize the neck to 6.5

He was using Hornady ELD-X and I forget how many grains of RL22 he was using but you could tell there was a ton of case capacity left and I believe he said it was going at 3,300fps; I'm usually skeptical about wildcat rounds and claims that people make but this guy is REALLY into long range shooting and he knows his powder and lead.

https://www.hornady.com/bullets/rifle/6-5mm-264-143-gr-eld-x#!/

I was thinking about getting one made but I always like to get some second opinions from other reloaders or people that have experience with the round, and FYI I'm looking at this as a long range dual purpose target & hunting round.

I’m sure it’s possible. The 6.5-300wby pushes that same bullet to about 3500-3600 FPS.

Speed sounds about right for a 26-28 inch barrel with 143gr with WSM case.
I personally would just run the 145 ELD-X in the standard 270 WSM at the same speed and be happy.
But if you want a 6.5 it would be easier to just do the 6.5SAUM or even easier 6.5PRC and not have to make your own brass and get pretty close to same ballistics with less hassle and even factory ammo in the PRC round.

Yes, it's certainly possible. But like DStroud said, there are other options without making a wild cat type round. The 6.5 GAP 4S (or 6.5 RSAUM, same round) is based on a 7 RSAUM necked down to 6.5mm. It's very similar to the WSM case. The new 6.5 PRC would also be a good one. A 147 grain ELDM at 3100-3150 would be awesome medicine up to elk size game.

But the WSM case would hold slightly more powder (about 4-6 grains) and would yield about 100 fps more. So something to consider. Necking down 270 WSM to 6.5mm would be easy. (I wouldn't use 300 WSM brass, as the necks would get too thick necking from .308" to .264", unless you neck turned them)

6.5 PRC info: https://press.hornady.com/assets/site/hornady/files/load-data/6.5-prc-v3.pdf

The only problem I see with the 6.5 WSM is the velocity of the 140 grain bullet.
The barrel life will be short lived I have 7&mm WSM and the barrel life is short due to the high speed bullet.
From what I have seen all on the long ranges matchs.
The dope on the scope is more important than the speed of the projectile.
A high speed barrel or burn out quickly I would recommend a round with moderate velocity and long Barrel life.
A friend of mine shoot his target rifle 6.5 Creedmore 2800 to 2900 In a thousand yard match.
I asked him why not faster? I have fewer barrel changes!
Remember bullet speed is part the problem, weather and reading the wind are the biggest factors.

This is where I would go as well. Well I did actually with the 6.5saum. Hell of a hunting round. I have mine down around 3025fps and shooting great groups. You can push them but I wanted the best accuracy and thats where mine was best. Took my dall sheep in August at 515yd