I don't think so. Many consider it to be a waste of time since the rifle length gas system works better than the carbine length or mid length. There are plenty of conversion kits out there if you are dead set on gas piston but I can tell you from experience that in semi auto mode you will likely never be able to notice any advantage to the gas piston over direct impingement system. Just my two cents worth.
There are several rifle-length piston kits out there, but I have not seen any fully-assembled rifle-length piston uppers. (I may have missed them, but...)
Like you, I wanted to have a rifle-length piston upper. Mostly, this was due to wanting to reduce the amount of time spent cleaning my rifle. But, admittedly there was also the "novelty" aspect of having a rifle-length piston-AR.
I've installed a couple of AR-15 piston kits before...an Adams Arms, and an Osprey Defense...both were carbine-length. Of the two, I like the Osprey Defense slightly better, as it's a more simple design.
That said, the Adams Arms is a very good piston system too. And, they do offer rifle-length piston kits as well.
Anyway, I decided to go with an Osprey Defense rifle-length Exo-coated piston kit for one of my builds last year. The installation is straightforward, as long as you feel comfortable removing your front sight base and you have the right tools for the job. However, I do recommend that someone has some "wrench time" around an AR-15 first, before tackling the job.
There are some nuances that you have to be aware of. For instance, for some reason, I was having fail-to-feed issues with the rifle while using standard GI 20-round mags. But, when I swapped them out for GI 30-round mags, it ran flawlessly.
Now, I think I've isolated this down to the follower not pushing-up the rounds hard enough. In fact, I noticed that the bolt carrier group was actually pushing the rear of the cartridge back down into the magazine. Again, this was only with the 20-rounders. Once I put 30-rounders in there, it ran flawlessly.
I have picked up some 20-round PMags to test them. But, I haven't gotten to try them in the rifle yet...too many other "hobby" projects have gotten in the way. Besides, when I ran 30's, the rifle was a beast...I did several mag dumps to verify this. :ar15:
Now to put it all in perspective...
Piston AR's are definitely something I like. That said, they can be finicky at times. And, you do have to watch for carrier tilt. Bottom line, when you add a piston to an AR-15, you're fundamentally changing the Eugene Stoner design. Moreover, you're adding MORE parts to your rifle. And, as it turns out, these parts are generally under extreme stress, considering the nature of what they are responsible for.
So, you're essentially adding some "complexity" to the design of the rifle. Does this mean that you should forgo a piston-AR? Well, no. But, you should consider the pros and cons of having a piston-AR. That's all I'm addressing here.
Lastly, as long as you don't mind cleaning your AR-15, the direct impingement (DI) system will serve you well. And, unless you're shooting thousands of rounds without lubricating your firearms, a DI rifle will be reliable. At least, that has been my experience with my DI AR-15's.
So, what does this leave you with? Simple...it's a personal preference. If you want a piston-AR, I say, "Go for it!" You'll definitely save some time cleaning after your range sessions. Plus, there's just something cool about them.
I have what is left of an Osprey Defense gas piston kit with both carbine and mid length rods for it but, In an attempt to remove it so I could put it on a different rifle I broke the piston (actually I guess it would be the cylinder) where it fits into the gas block and they want $125 for the replacement part to fit it. If anyone wants the kit I will give you a deal on it and you can buy the replacement parts direct from Osprey.
It's your money but, if you want a 20" gas piston AR why not just buy the 20" rifle of choice and install the piston kit? You will save several hundred dollars over the route you are talking about and believe me, a 20" rifle is just as capable of fighting off Zombies as a 16". I have never seen so many people get so excited about 4 inches, except maybe for a few females, if you know what I mean. The Osprey kit is an excellent set up. Absolutely trouble free. The problem I had with mine was caused by me, not by the product. I broke it because I was too cheap to buy the proper tools to try and remove it. In fact, I was trying to remove it from my 16" Bushmaster to install it on my 20" 6.8SPC build. If it wasn't for the lack of desire to spend the $125 for the replacement parts, I would have it installed now. Mainly for the "coolness factor".