We have a new deep range and I was going to pick up a 18" upper to have some longer range fun but still plan on shooting bulk 55gr most of the time (steel inside 200-300 yards)

I know 1:7 and 1:8 with heavier bullets are going to get me the best long range results, but I could still shoot 62gr I think with 1:9

Should I stick with a 1:9 or go to a 1:8? Will 1:8 be that much less accurate at 250 yards? With 1:8 (from what I understand) I could jump up to ~70gr if I ended up getting into longer range

General rule of thumb we use is 77gr for that distance. 55 gr we shoot out to 2-300 but anything further we use 77gr. Will 55 gr shoot that far? Yes obviously. But not as accurately and won’t have much report on steel targets compared to 77 gr. Some people shoot 69 gr and have loads they love with that projectile but 77 gr is easier to find off the shelf match Ammo so I would recommend a gun set up to shoot 77 and it’ll do fine with 55 shorter range.

Barrel twist is determined by bullet length not weight. 55gr flat base can be stabilized in 1:12. Over stabilizing, too tight of twist, will have a greater detrimental effect at long range as it hinders the bullets need to nose over at the top of the tragectery, keeping the nose high, exposing the side of the ogive the relative airflow.

Please define "accurately"

The 55 FMJ is not accurate in my opinion, usually hovering around 2 MOA.  A 69 or 77 SMK is capable of 1/2 MOA or better.

If you are competing, I would pick a different bullet and the longest acceptable barrel length, but if you just are trying to hit a 12" circle at 500, the 55 FMJ can get you there.

A 1:8 twist will give you more bullet options.

A 1:9 barrel will stabilize 68/69s just fine, and IN MY EXPERIENCE at 500' ASL/standard atmosphere will also shoot 75gr Hornady HPBT accurately.

Reality is 68/69s will do well to 500yd, but if I'm buying new I'd choose 1:8 and run 75/77 HPBTs.

Personally I would go 1-8 or 1-7. The 1-7 will still stabilize the 55gr bullet and still stabilize the 77-80gr bullets. You will have to single load the 80’s. I have a 1-9 that shoots the 60gr Nosler bt and the 70gr Berger really well. The Nosler 69 CC is very accurate in another 1-9 twist barrel that doesn’t like the Berger or the 60gr BT. I’ve killed prairie dogs to 400 yards with the 53gr bullets from my 24” bushmaster. But wind kills my accuracy past 400. Good luck

Your issue with shooting accurately at distance with 55 grain .223/5.56 is mostly going to be dealing with wind drift.  Even at just 200 yards I was getting a couple inches left or right with 62 grain stuff with not more than a stiff breeze.  One of the reason I went to 6.5 Grendel for those purposes.  Grant it, you get more drop at those closer ranges compared to .223 but it's much easier to range your target than it is to constantly and accurately read the wind.

12" circles even at 400 yards are enough for me, I believe the new range steel is 12" circles -- They apparently will start competitions that I am hoping to get into (I do uspsa now) but for now I just want to push past 200-300 which is all I really messed around with.

My question was poorly worded, I really just wanted to know 2 things,

What twist is best for 55gr bullets and what twist if I shoot heavier 70ish -- and the third question was kind of "is there a happy medium where I can shoot semi-accurate out to 250 with 55gr but have a little edge with heavier (which it looks like people say 1:8 would be that twist?)

This is my complaint with Savage rifles. I want to use heavier bullets but their 223 rifles are set up with a 1:9 barrel.

I shoot 52g ELDM and 53g Vmax out to 500 yards (1:12 Barrel). The 55gr Hornady soft points are more accurate in my rifle than the FMJs. My barrel wont stabilize anything heavier, not even 62g.

Bulk 55gr ammo is trash for accuracy. Expect 2+ MOA with a decent barrel.

To answer your question: Military fields 1:7 barrels which can shoot 55g just fine.

Try the Berger 70gr VLD or a 69gr like the Nosler CC or the Hornady 69gr.

1:7” and 1:8” twists are more utilitarian, as they can run a wide range of ammo.  The slower twists can be more beneficial to the 55gr for accuracy, but you shouldn’t really be buying a barrel to specifically use with 55gr bulk ammo for the purpose of precision shooting because the 55gr FMJ (and 62gr FMJ for that matter) are not match loads and buying them for accuracy makes little sense other than simply having nothing else available at the time, which is a supply issue.  If you legitimately want longer range accuracy or capability, set the gun up to run a proper match load like a 68-77gr match bullet.  The 1:7 or 8 twist barrel will get the 55gr bullets out to 500yds, and I’ve done it numerous times.  Just don’t expect miracles because the bullets aren’t going to perform any regardless of barrel twist.

I have made regular hits on steel at 500 yards with PMC Bronze 55gr out of my 18" White oak 1:7 barrel.  I believe they were prairie dog targets.  I never shot at paper for groups, so I don't know, but it was not an issue hitting the steel.