Welcome to this forum. There is lots of similar questions asked so do a bit of reading and most questions will be answered.
First off, what do you want to use the rifle for - hunting, competition, varminting? Heavy, light, small, big? How far is long range to you? All of this clarifies the best option.
There is an explosion in LR shooting interest so options are almost limitless.
The 30-378 is a great LR cartridge but with a minimal end use not covered by many other choices.
From the 223 on through to the 338 super magnums, there are alot of options.
If all you are doing is plinking/varminting and competition, the 6BR/243, and 6.5 Mystic/260AI variants will cover the bases from 300yds to 1000m. The 223/308 will also do the same but drift alot more in the wind.
For further distances or to reduce wind drift further but increase recoil and costs, the 7mm (like the 280AI/7 Mystics and 7WSM/RM) and 30's (like the 300WSM/WM/RUM/Wby) rule.
6mmBR.com, benchrestcentral have alot more info and you can see rifles in all sorts of applications.
Let's hear what you really want out of this rig...
Jerry