I have a large stockpile of 70gr TSX and Mk262 ammo. I am building an 18” precision 5.56 and I need to get a barrel that will stabilize them both. Does a 1:7 seem like the way to go in order to stabilize the TSX? And will it shoot the 77gr well?

And what are the suggestions for precision barrels that offer a 1:7?

I've shot tons of 77gr in 1/7 and 1/8 barrels and there are gobs of people who do the same...zero issues there.  I've never shot the 70gr Barnes before, but I'm 99% sure a 1/7 or 1/8 would work just fine with that load as well.

I would suspect if you're looking for precision barrels you'll also see a lot in 1/8 twist...don't rule those out.

Lots of folks have good luck with the Larue barrels.  That would be one place to check out.

Barnes recommends a 1-8 twist or faster for the 70 grain TSX bullet.  Their new 78 grain TSX requires a 1-7 twist or faster.

It depends on your barrel, but I’d expect 1/7’s and probably most all 1/8’s to do fine. For those unfamiliar, while just 70gr, due to its metallurgy it’s still pretty long and should be treated as one usually treats 75/77’s. ETA they’re actually longer than a 77 SMK IIRC.

I have shot a lot of them through my 1/8 LaRue barrel. Very accurate.

1/8 should be fine.

Others with far more knowledge than I can chime in but you should be more worried about chamber vs twist.   A "wylde" chamber vs a 5.56 NATO chamber would be better on a "precsion" rig.  However, If you're handloading that may be a moot point because you could adjust COAL on your loads to suit your barrel.

I have a 1/8 Wylde that shoots 77gr just fine.

EDIT:  Also, you may want to look at an ODIN works barrel.  Affordable barrel and most are pretty accurate. They have a 16" DMR iin Wylde chamber with rifle length gas system that caught my eye. I'll be shooting suppressed so cycling should not be an issue.  With the longer gas system (ie shorter dwell time) it should shoot smoother with less of an impulse.

I have 2 1/7 twist barrels . a 16 inch and a 20 inch it appears the faster you push the 70 grain tax the better they fly . I don't know your intent for the bullet but they do fubar deer pretty good . barnes has load data on their web page .

I load a lot of Barnes TSX bullets in multiple calibers.  I really like them.  They are long for weight, do you must take that into account bin barrel twist.

In .223/5.56 I’ve loaded everything TSX from 50 to 70 grains.  I didn’t know about the 78.  That must ne very long!

All I’ve  loaded perform great in slower 1:9 except the 70 grain bullet.  It needs 1:8.  None need 1:7, although I’m sure it would work fine.  I can see how a 78 TSX or TTSX would need 1:7.  I’m not sure you could push the 78 fast enough to benefit from it except with a much longer barrel, slower powder, and maybe a .224 Valkyrie barrel and cases.

The 70 TSX is actually slightly longer than a 77 SMK.

It shoots (as do the lighter ones) quite accurately in good barrels, right at one MOA.

The 70 TSX will keyhole badly from my 1:9 barrels.

For deer, honestly, the 55 and 62 grain loads work a bit better, due to higher velocity at ethical deer hunting distances.  They all will crush bone, expand nicely, and push right through.