So I'm planning on doing another 308AR build
had to sell off my old one for home owners problems.

I had an AeroM5 18 inch with a carbine stock set up, a 4-14 power scope and all the BS added on offsets bipod yada yada

my thought at the time was the 18 inch will be a nice compromise in weight and balance. but it was was still very nose heavy and the carbine stock setup + the scope made it a pain to get a decent repeatable sight picture.

The main focus of my next build is going to be a hunting rifle with ranges up to 700 yards

would it be ideal to go 18 inch with an fixed stock ? Or 16 with a carbine? Is the loss in velocity still going to be worth the weight and possibly less lethality on deer/elk sized animals at 700 and under?

I cant find a ballistics calculator that will allow me to change the barrel length on it for a base like dope card.

Im not wanting to switch to 6.5CM as 308 is easier to find and cheaper to find some 7.62 to go blasting

The plane/height of the top of the rifle and carbine buttstocks is basically identical...at least they are on my 10 AR's in AR15 and AR10.  Cheek weld can be quite different depending on the style of carbine buttstock you were using.  Some are slim, and some have a bigger profile...like a Magpul ACS.

I'd also start by asking what scope height rings or mount you were using on your first setup.  That can make a huge difference.

Ergonomics due to facial structure, head shape, eye relief, etc., etc. can be hard to chase sometimes for a perfect fit, but it's out there somewhere.

I had the ACS stock on the 1st one I had. The scope and mount combo were the Aero SPR and primary arms R grid
On lower power it wasn’t hard to get nice and tight on it, but when dialed up I felt like I was chasing the eye box.
I felt as though I had to adjust my stock or change my cheekweld on it.

Weight is also a factor when choosing the A2 or Carbine buffer assembly

I've experienced the same issue with that optic and an ACS-L on a 5.56 rifle. I think the optic's eyebox just shrinks dramatically at higher magnification.

I’m thinking my next optic is going to be an AccuPower 2.5-10

Trying to do some research now on the eye box for those.

Just trying to narrow down fixed or carbine stock
I would like to build the rifle up to be a 308MK12 build

I have a MagPul STB stock on my 6.5 Creedmoor AR. In position, there's no difference in stability between that and a fixed stock, the STB has storage compartments, and the collapsing feature is nice for transport, and also gives me LOP adjustment for use with heavy clothing or different shooting positions.

Velocity of a 16" vs 18" at 700 yards...it makes a real difference.  For every 1" of barrel, you are looking at a 100-200 FPS difference at the muzzle, depending on the ammo.  At 700 yards, that's about 30-40 inches of drop difference (ballpark since I don't know your ammo, barrel length, altitude, temp, etc.)

Google "Trijicon Ballistics Calculator" and you will find what you seek.  It's a free excel spreadsheet that allows all the data to be plugged in, then it spits out some great data.

There is no way it’s 100-200 FPS per inch. More like 20-30. I’m too lazy to link, but just google 308 ballistics 16” and do the same with 18”. Rifleshooter has a ballistics by the inch article with 308 and it’s a very small change from 16-18.

Purchase Strelock Pro app for your phone. Plug in numbers. It’s a super simple to use app, that is always with you and can store your data. I consider it mandatory for anyone shooting past 300 yards.

With a few exceptions, anything semi-auto tends to get an adjustable stock setup simply because shooting prone vs. bagged on a bench vs. field positions (sitting, offhand, etc) work best with slightly different LOP.

All AR based rifles using variable powered scopes need an adjustable stock.

I mount the scope on its highest magnification in the prone position. Using a bipod is OK, but I prefer to be slung up.

Once the scope's eye relief is properly set for high magnification prone use you can adjust the stock anywhere you want to get good eye relief in other positions or on other magnifications.

A fixed stock does not allow for a lot of options.

Sopmod stocks are cheap these days and have the best cheekweld you can get

based off of those numbers, If I chopped 8 inches off of my buddy's 24 inch 6.5 creedmoor gas gun. We would only be getting 1850 to 1900 FPS MV or at its worse 1100FPS? your on crack.