It all depends on what your budget is...
50bmg will cost around $6000 for single shots or $12000 for semi auto
If you are trying to save $$ and still be accruate at 1000 yards, I would look into 300win mag, 338, 30-378. they are all made to shoot flat and purpose was to hit targets at 1000 yards +.
you wanna go cheaper, then get a rifle that has good rating for its accruacy.. for eg tikka T3 tends to have good ratings on its accuracy. u can prob get one around $1000 for Stainless caliber. but then you need a scope that is capable to hit targets beyond 1000yards and you cannot cheap out on scopes. I have tested leupold, Huskemaw, Nightforce scopes and for my personal use i like Huskemaw.
If you dont have a budget set, i just built 30-378 weatherby mag with jewel trig, custom mcmillan tact stock, Krieger match grade barrel 10 twist and so on. It costed around $8000 including scope and atm it shoots around .2-.5" grouping at 100 yards. ofcourse it does not always shoot that good. Human errors will kick in here n there.
It should get in 6" grouping @1000yards without wind and flat land and capable to 1600yards kill.
Pros for this rifle is its my own build with personal taste and shoots great.
Cons Hard to get brass and very expensive. 25 brass are $99+tax
I have tried 300winmag also for long range. there are so many choices with 300 wm and it is way cheaper then 30-378 of course.
I also believe there are no Beginner + accruate long range rifle. from my personal experience no "super custom build" is accruate unless the person shooting it is. Shoot lots and try different reloads cause every gun has difference taste on powder measure, grain of bullets, overall Length of ammo for each gun.. there are so much to cover if you wanna be accruate @ 1000 yards. for most part it is not the quality of the rifle but its more with the person and what loads you are using..
You can even get away with accurized 300wm T3 ar 1000 yard range if you know what the gun likes.
I always say this to others but rather then buying 1000yard grade rifle first, ask ur self if you have time to reload, test powder measures, different kinds of bullets and so on.. if you want to get into it, then welcome to long range world! I can gladly help you with my knowledge with long range whenever you need but i cant really recommand you what to get.
Here is what general thinks tho. Krieger Match grade barrels have many "positive reviews"
both 300wm, 338Lapua are known to shoot very well on long range with right loads. 300wm many people uses around 150grains bullet and 338 lapua of course heavier rounds around 200grains.
For long range they tend to use hotter loads then your .223 so buzzle brake will help tons. My 30-378 with 110grains of powder will feel like .223 with huge boom..
Hope few things cleared your mind and let me know if u live around kootenays or bc we can go shootin together for you to try long range before you buying one!