I am looking to get a Daniels Defense barrel.  (all of these are mid-length gas systems)

What would the difference in these rifles?  From what I understand, and please correct me, and tell me under what circumstances/conditions would these difference become apparent.  I do see a lot on conflicting information, and to stir things worse, these are cold hammer forged barrels.

The 18"  would have a slightly higher velocity giving it a longer lethality range, and flatter trajectory.  I am not sure what the typical velocity would be because this is a mid-length gas system in all the barrels, and as I understand, the the bullet velocity takes a hit after passing the gas port, and that is why there is a small change from 14.5 to 16" carbine gas systems.  But the propellent is still burning in a  longer barrel.  At  what distance and what sorts of ammunition would this become noticeable?

There should be no accuracy difference based on barrel length.  Would there be a difference in accuracy based on barrel physics and harmonics between them?  I believe the 18" S2W barrel was designed to be a little like a HBAR/BULL. and to be purposed as a designated marksman rifle.  At what ranges would the SW2 barrel begin to stand out? (I am sure that having a higher velocity and flatter trajectory will also come into play).  Would there be a difference between the LW and GP?

Heat.  I really do read a lot of conflicting information about this.  I am not using these barrels for automatic use, nor will be be trying to see how fast I can unload magazines into a target.  I would say the S2W 18" barrel will be the best concerning heat.  But, between the GP and the LW, the LW has a thinner barrel at and after the gas port. Under what conditions would a cold hammer forged LW barrel begin to demonstrate deficiencies from excess heat?

They bores are lined differently, but I don't think that will be of a concern.

Thanks for any input.  On paper the barrels seems very similar, perhaps they are, but I am sure many more people have experience with them.

Barrel length as well as significant differences in weight, balance and handling qualities.

Anytime you pick up the rifle.

What does hammer forging have to do with anything?  It's just a method of manufacturing the rifling in a barrel.

Irrelevant to velocity.  IIRC you get (on average) about 100fps for the extra 2" of barrel with an 18" vs a 16", I could be wrong on that though.

Not to any appreciable degree.

Huh?  Yes there is velocity increase in a 16" vs a 14.5.  Bullet velocity increases along with barrel length out to quite long barrels, but the rate of velocity increase decreases rapidly as barrel length increases.

No it's not.  Propellant is mostly burned by the time the bullet exits the cartridge case, after that it's just hot, high pressure gas expanding.

Yes.

Accuracy is accuracy... its the same at any range.  All depends on your requirements.

Likely not in accuracy, they are the same profile from the breech up to the gas block.  The government profile is thicker at the gas block forward to the muzzle, so it's a little heavier and balances more front-heavy.

Define "deficiencies".  In the Lucky Gunner tests they shot a 16" midlength gas lightweight profile barrel with about 500 rounds essentially as fast as they could swap mags and pull the trigger, and managed to set the polymer handguards on fire.  In extreme barrel testing-to-failure, barrels will burst generally a few inches in front of the chamber, that's the hottest place on a barrel.

You're comparing an extremely heavy profile barrel with the lightest profile barrel available.  So there's a bit of a disconnect here.

What is the purpose/mission for your rifle; what are you going to do with it?   What are your needs so we can suggest how to best fulfill those needs.