I'm looking at getting an AR10 in 308.  I want a 20-22 inch barrel.  I don't care if it's heavy.  I'll lug it to my private range and shoot.  It'll mostly be a stationary gun.

So with that length of barrel, free floated AND a suppressor.   That's a lot of force right at the barrel nut / receiver interface.  Seems like a pencil barrel would be much worse once it heats up.  

Thoughts?   Honestly I'm trying to learn.

What's the question?

Will there be a damaging moment on the barrel/receiver joint?  No.
Will there be a accuracy change when the barrel heats up? Maybe - depends on the barrel.
Will the barrel droop because of the weight? Theoretically - yes.  Practically - not worth noting.

My scar 17 opens up a bit after it gets warmer / hotter.  That's a 16 inch barrel with a suppressor.

The longer the barrel, the more who will occur at the detonation sequence. There are a few different profiles and contours that help mitigate this, but you'll never eliminate it. It is what it is. 14" is a sweet spot for barrel whip. Short sand chunky is the best solution.

However, when watching under high speed, things settle down rather quickly. About 1 second the energy has transferred into the rest of the weapon. Maybe a bit more, or less depending on setups. This is why rapid fire is so inaccurate.

Anyways, the tapered type contours are going to help control barrel whip the best. Triangular type works pretty well too. Fluting can help when done correctly.

As for the extra stain at the barrel nut, it's practically nonexistent up to 24". But with 308, 21" is maximum powder burn i believe. The gains are not with the bulk trade off. Unless going with a bolt gun.

Gotcha.  I'm reading about POI shift with a suppressor.  I wondering if that has more to do with the weight than ballistics effects of the suppressor.  I'm just thinking.  

So if I wanted an ar10 in 308 and a 20-22 inch barrel (or 21).  What barrel profile would be best to use with a suppressor?  I have a Saker in 762, and it's heavy.

IMHO, you're not gaining much past a 18" barrel in 308. Also, yes, if you put a can on the end of a 20+" barrel that's free floated, you are going to get POI shift after it heats up (that goes for almost any length barrel, but the longer, the more pronounced). If I were you (canning it), I'd do a 16" Free float set up with a medium taper barrel (so as to not be overly heavy, yet provide enough strength for repetitive firing with the weight of a can). YMMV.

I'm not so sure the POI shift with a suppressor vs. without is primarily or solely due to the weight of the suppressor.  Sometimes just a muzzle device can affect POI in some cases.  In some cases just the miniscule effect on the bullet exiting the barrel by any means will affect POI.  Don't get me wrong...I'm not saying putting weight on the end of a barrel doesn't affect POI...it does.  Even a short, stubby, beefy, pistol barrel's POI can exhibit POI shift with a suppressor, and it probably has more to do with the pressure curve and other "black magic" physics issues that can affect a bullet as it leaves the rifling at the muzzle crown.  It's not a simple "one issue" deal IMO.  No doubt that barrel harmonics is a big issue due to weight and contact with the barrel.  It's just not the only one for sure.

On barrel nut integrity under fire, there may be some AR10-style platforms out there that have wimpy barrel, barrel nut, and barrel nut extension issues, but I'm not aware of them...and if there are they are probably not on most people's radar as a good .308 rifle candidate.  As to the pencil barrel/heat issue, I think it takes some fairly decent rapid fire conditions to make this a big issue...and under conditions where you're pumping out rounds that fast, is accuracy the all consuming goal at that point.  Maybe a belt-fed .308 would be more appropriate.

In a semiauto, AR10-style weapon, I wouldn't go over 20" in a barrel.  When long range accuracy and effectiveness become that important, a precision bolt rifle like the Ruger and others will probably be a better choice.  This newer generation of chassis, precision, bolt rifles are pretty cool and offer quick followup shots as they're magazine fed and have short rotation bolts, AR furniture options, etc.  I have a homebuilt chassis bolt rifle and an Armalite 20" barrel .308 AR10.  The AR10 is a fun and decently accurate rifle, and it would be my choice for a stationary location, defensive weapon like a "Firebase Gloria" scenario...a Vietnam, trench warfare style of situation where big, longer range, semiauto/full auto firepower might actually be a benefit.  My two AR10's were my first AR platform guns...a 16" and a 20".  Now that I have a bunch of AR15's, bullpups, and a precision chassis bolt rifle, they're probably the lowest on frequency of use or choice in SHTF scenarios.

Thanks.   Helps me understand better.   I appreciate it.

What distances will you be shooting?  AZ has a lot of wide open spaces allowing up to ELR (1000yds and beyond).

Will that be true foor you, and what types of targets will you be shooting?

I've owned 5 different AR10s, LR-308s, and LR-260s, 3 factory guns, 2 custom rifles built by GAP.

3 of them were/are exceptionally accurate and reliable rifles-the ArmaLite 24" AR10T, and the 2 GAPs, one a 22" Obermeyer fluted .308, the other a .260 Rem Bartlein 22" which I have held onto.

The .260 Remington GAP is a laser even past 1000yds.  Makes the .308 pointless for me, so I sold all my .308 rifles and components.

Wind drift and drop differences are game-changing.  Impact on steel wiith a 130gr or 140gr from the 6.5mm is boring even at 1000yds, compared to very low hit rate with a .308 even at 800yds.

If your range is shorter, like 600yds, then the .308 can have a decent hit rate, but if you ever get a chance to see them side-by-side like I do regularly, the difference is night and day.