Quoted:
By durability I mean something that is reliable in all situations: "over the beach," while firing a large amount of rounds, and can still hit a target at 500 meters or 20 meters for a long duration of time (sounds like something the military would do so would Mil Spec be the right choice? Or are Mil Spec materials a victim of the "lowest bidder" and there is a better commercial option out there?).
With proper maintenance and care taken into consideration what type of barrel would last longer through a worst case scenario?
Since CHF is a process and Mil Spec Steel are the materials (thank you for the knowledge). What combination of materials and production processes used would make a barrel more reliable in a worst case scenario, combat/self-defense situation?
As with everything there are tradeoffs. There is no free lunch, or even an expensive one... there is no "best at everything".
As far as milspec, unless you are a military purchasing or contracting officer, milspec is just words on paper. What are the qualities of the materials, or design, or whatever and does that meet your needs?
As far as ability to fire a ton of rounds before the barrel fails, you're looking at the heat resistance characteristics of the steel alloy as well as the profile/weight of the barrel. A more heat resistant alloy will maintain more strength as the barrel heats up, and resist erosion better as well as allow more shots before catastrophic failure... barrel alloy is as close to no downsides as you get in this exchange. Barrel profile.... well, if you're willing to carry around a bull barrel, you'll get maximum heat capacity and rounds fired before failure. Direct tradeoff there between weight and heat capacity. Barrels also lose accuracy as they heat up... a pencil or govenment/M4 profile barrel group size might start increasing after just a couple mags due to more rapid temperature increase. Stainless barrels are traditionally a little better on accuracy but far worse on heat resistance and erosion... which is more important to you? If you go to a high performance CMV steel, you get much better heat resistance, but also need some kind of treatment or coating to prevent corrosion in the bore... chrome is traditional, nitride process are becoming more popular though and may have some advantages.
The main advantage to HF barrels is greater burst resistance, to prevent damage from water-filled bore or something similar. 5.56mm bores hold water after submersion due to capillary action, so if you anticipate coming out of the water shooting like in
Act of Valor then you should probably get a HF barrel. Otherwise with an AR15 variant, when you come out of the water, point the rifle down at low ready and retract the charging handle about an inch for 3 to 5 seconds to let water drain from the barrel and gas tube, then return to battery and drive on.
So you have to balance accuracy, weight and handling qualities of the rifle, and heat resistance/high round count performance and come up with what will best suit
your needs.
If you want an
opinion... which might be worth the price of admission ($0.00)... it's that, you do not need anything "special" for essentially any conceivable civilian use for a fighting rifle. Even some kind of extreme SHTF/TEOTWAWKI fantasy engagement you would be in will be over one way or another before you would have that many rounds down the tube. Your ability to carry and move with a "fighting" rifle quickly and effectively far outweighs the ability of a rifle to rapidly shoot more ammo than you could reasonably carry on your person.
ETA:
Here's something to consider as example. This rifle has a "lightweight" or "A1" profile barrel, which is really the lightest barrel profile generally available for an AR15 type rifle. It happens to be hammer forged but realistically would have the same results if it wasn't.
This isn't intended as a suggestion or recommendation of that particular rifle/brand or anything else, just an indication that, given a good quality AR15 type rifle, the idea that you need something special or extraordinary in order for the rifle to perform in serious service just isn't true.