20+ FPE Gas Piston Rifle

You can do 6lbs with Gamo since their barrels are mostly plastic, otherwise you're looking at ~6.5-7, and most that can hit/break 20ftlbs weigh 8-10lbs, and all these #'s are w/o a scope.
20ftlbs is kinda in a gray zone, most standard guns make up to ~18, then magnums typically start ~.22.
Based on what you wanted my first choice thought would be a Crosman/Benjamin Trail, Venom, or Titan.
These are all cheaper guns tho, $150 to 200. At $500 you might consider a Weihrauch which are MUCH better, but I think the only guns that will hit that power is the HW80 and 90, and only the 90 is nitro. They also heavy and typically more than $500.
And RWS 350 might be something to consider, it's not a gas spring but can be modded to be basically as nice as one. It's a mid-grade gun with closer to 25ftlbs. I can't remember the weight but it's at least 8lbs, and long at >48".
Also factor a scope in the price because if the one you choose comes with a scope, like a Trail, it will be what most consider just awful.

The RWS 350 is great choice for accuracy and power. It is a bit heavy and doesn't have a piston. But it is a great gun. I use RWS Super Point .22 pellets in mine.

It's a tough decision, which is why many of us have several guns.
If you want good accuracy then it's better to stick with more expensive guns. The cheaper guns like Crosman, Gamo, Stoeger, Hatsan, Ruger etc etc can often shoot well, but typically they don't or need some work to do so. It's a roll of the dice buying those guns. An RWS is a nice upgrade and the 34 and 350 are pretty cool. You'd probably be happier with the 350's power, especially in 22 since the 34 is a bit slow in 22. Just don't get the short barreled model, the short barrel costs power and is much harder to cock so it's pointless imo. Then Vortek sells a kit to make it smoother and quieter which people really like. The gun and kit will put you at ~$500 unless you get lucky with a good sale.
Check out airguns of Arizona, they have RWS and a better selection Weihrauch than probably anyone. RWS also has side lever cocking guns, but I suggest you try one before you buy, I hate them myself, plus an RWS 52 almost took my fingers when it slammed closed. They have a safety feature to prevent that but it skipped right over it and slammed with the force of God, so think about having to put your fingers in there every time to load it... Break barrels are so much easier to cock and load, they're lighter and safer.

north east redneck said:

The RWS 350. It is a bit heavy and doesn't have a piston.

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doesn't have a piston.
You may want to check that over.

Type

I understand that the OP specifically referred to a gas piston air rifle.
Is that a firm decision? I am a proponent of PCP rifles and a Benjamin Discovery has all of what was spec'd. In .22, it will produce the requested 20 ft.lbs. (23 actually)'weight is at 5.1 pounds. The price, with pump, is under $400. And you don't have to pump it after every shot or break the barrel to charge and cock. It runs at 2000 psi and is way less of a job to pump than the big guns running at 3000.
Just saying.
Pete

Chevota said:

slammed with the force of God, so think about having to put your fingers in there every time to load it.

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I had this happen to me on a side-cocker, cut into three fingers to bone through my finger nails. Hurt like heck and totally turned me off on side-cockers.

Drilling holes into the nails to run stitches was weird as heck too.

tnelson31 said:

I had this happen to me on a side-cocker, cut into three fingers to bone through my finger nails. Hurt like heck and totally turned me off on side-cockers.

Drilling holes into the nails to run stitches was weird as heck too.

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Been there not done that, but my son did.
I've an under-lever, told him to hold the cocking lever when loading as a fail-safe.
He didn't and he got the nasty.
Even with break barrels I hold the barrel when loading.
And my son, he was 19 at the time.
I would not have turned a young kid loose with an air rifle.

20 fpe and a gas piston (versus spring piston) is excluding some really nice options in the <$500 range.

If you're after critters with an air gun, it's all about punching holes in just the right place. A 22 with 20 fpe will still not provide 22 LR powder burner shock and awe. Quality dome pellets and 12+ fpe will do the job on squirrels and rabbits, as will 177 caliber. Hollow point, pointed and flat wad-cutter pellets need not apply for penetration and long range (more than 10 to 20 yards) accuracy.

If I had allocated $500, I'd be looking really hard at an R9 (or HW95) in 177 (~14 fpe). This is a lighter weight quality spring piston - still not meeting your 6 pounds, but that's not going to happen anyway. For less money (~$200 from flying dragon air rifles), the BAM/Xisico B26 clone of the R9 (~12.5 fpe) is very nice. I would not get either of these in 22 caliber though - the slower velocity would make trajectory too loopy.

The RWS 350 in 22 already mentioned comes real close to your specs if you want the power (or for less money, the BAM/Xisico B28 clone from flying dragon air rifles), but it will be longer, heavier and recoil more. NO 177 for these - 22 caliber is needed for the pellets to be heavy enough to stay under ~950 fps for accuracy in magnum piston rifles.

Aside, recoil is an issue for any piston air rifle. Unlike a firearm that spits out the bullet before much happens, the pellet doesn't exit before the effects of the piston recoil are over. Some extra weight isn't necessarily a bad thing to help absorb this.

I think trajectory on 25 caliber is just too loopy with so many good 22 caliber options out there to choose from. And speaking of choice, you'll have a better selection of 22 or 177 pellets than 25.

tnelson31 said:

I had this happen to me on a side-cocker, cut into three fingers to bone through my finger nails.

Drilling holes into the nails to run stitches was weird as heck too.

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You know in the movies when someone gets kicked in the crotch and everyone watching recoils and groans "OOOHHHH!!!"

I bet everyone here just did the same thing reading your story. O…M…G. Not cool

John: I believe springers losing power in cold is primarily do to the grease thickening up and slowing the piston as the seal plows thru the goop. The mfg often uses sticky grease so this is a real problem, even if warm temps. Anyone with a coil gun, especially in the cold should tune the gun and use non-sticky grease and the problem more or less goes away. Another thing is altitude hurts springer power so maybe some of that story came from people going to the mountains to shoot and assumed it was the cold.
Also the guys selling nitro springs say a lot of things that are bs to push their new product, one of which is cold hurts a coil but not a nitro. It's true to a point because most nitro springs are stronger so they lose less power.

John: Yes I meant the nitro (at least in a Crosman) has more power, excess power imo, so a little resistance from grease has little to no effect on power. The coil spring in a Crosman is a bit weak to make full use of the guns bore/stroke, so every little thing costs power. This is why it's important to use thin non-sticky lube in a coil gun, not just for low temps, but period.
Since I believe the pressure in a nitro is already more than needed I don't believe the cold is going to change anything when pressure drops a bit. This excess spring strength is also why nitro guns break more scopes, and that gets worse at altitude. So nitro guns should be tuned to minimize that problem, meaning fixing leaks. The less the gun leaks the less severe the reverse recoil is. Other guns with different bore/stroke combos and spring strengths will differ, but the generic Crosman is very common and a good example.
The theory the nitro guys are trying to push is the steel in a coil responds slower in cold, but between the minimal temp change, minimal movement and speed involved I don't think temp is much of a factor, it is imo almost entirely in the lube.
I think the ultimate spring would be a gas spring that the user could adjust the pressure and rate. Then we could adjust it to just make peak power and no more, or whatever less power we wanted. Also to adjust for leakage. The standard Crosman nitro's that I have do have a tiny fill port, but making it happen is another story and I haven't bothered yet. It would be interesting to play with the spring rate to see if power could be maintained with less overall cocking effort and reverse recoil.

The bore/stroke, the springs strength, the pistons weight, transfer port, dead airspace, barrel diameter and length, and the pellets weight/friction/stiffness are all factors. The nitro in a Crosman is imo too strong, meaning it's more than the gun can use so it doesn't make use of all that extra spring strength. Like a car that just starts to spin the tires at 400hp, so going to 600hp isn't going to help.
If the coil version compresses the air all the way, then compressing it all the way but faster like the nitro does the same thing but is harder on the gun. Or fi grease slows it it's still as fast as or faster than the coil so no real felt loss.
Now make the chamber larger and it has more air to compress and that will make more power.
If the pressure in the nitro has too much power, say 500psi too much, then a 2500 dropped to 2200 from cold is still too much so the new power out the barrel will be the same. I'm sure it isn't really the same, but increasing the pressure wont give you the returns you want. Say a coil spring barely making peak power in a Crosman, then drop spring strength in half you'll lose half the muzzle energy. Since the oem spring has nearly maxed out the power of the gun at 18 it means you could double the spring pressure but probably only make 19-20. Doubling again probably won't add anything at all, but what will happen is those stronger spring will be more than twice as hard on the gun/scope. This is why magnum guns don't simply have stronger spring, they have to add bore and/or stroke to get it. With more volume they can now take advantage of a stronger spring.
The the crosman peaking at say 18flbs, with a spring capable of making 22 in a larger gun, isn't going to do much better than 18 in the gun. It does however allow the spring to have hangups like grease and cold because those reducing the spring by 10% it still makes more than the gun can make use of. Now a coil barely strong enough to make 18 under ideal conditions gets cut 10% by cold sticky grease will drop the guns power 10% to match.
There are many variables, but that's the gist of it.
So I tune coil guns to get the best power, and tune nitro guns to have less excess scope abuse. They get more power too because the tuning bumps the potential of the gun itself up, but both that bump and proper tuning reduces the shock of the stronger nitro to save the scope. It also helps accuracy in both guns.

And yes, my HW90 had an optional pump to adjust the pressure, but I never bought one. People claimed they could drop the pressure a bit and see almost no change in power, because it was overcharged. Apparently many dropped the pressure quite a but, like 30% but the power drop was nowhere near that. The power was still good but it was much easier to cock and a much nicer shooter. As mentioned with a Crosman coil gun if you dropped spring pressure 30% you'd lose 30% power because the spring was not stronger than the gun could use. Make sense?
So imo the Crosman should have the same nitro, same stroke, but a 3-4mm larger bore and it would be maybe a 22-24 ftlb gun. No extra cocking effort, plus it would be quieter, smoother, and not break scopes. But I don't work for Crosman so it probably won't happen.