BUILT IN! Which is smaller than the 1/4" commonly used on low pressure impact wrenches, and tools. it is consistent with high end paint ball type fills and tanks.
Air requirements are reasonable, up to 3,000 psi for a full fill, in the Benjamin Bulldog 357's hidden 340cc tank.
Are in fact contradictory terms. If you want a 4500psi compressor, they can be had from about $200 (used, when you can find them), to over $20,000+ (with the average in the $300 to 3,000 range). The differences are:
a) speed of fill (volume per minute)
b) quality of air (how it's filtered)
c) filtration system (filtering air for breathing)
d) manual use vs auto-shut off
e) power options (gas vs standard electric, vs 220)
f) self-servicing ability
g) longevity of running BETWEEN required services
Or you could use a high pressure hand pump (It's about 100 pumps per 1,000psi of air in the Bulldog's 340cc hidden tank, and it's recommended to not allow the gun to shoot below about 1,200psi... which means the average re-fill would be about 150 to 180 pumps with a hand pump).
A HP Hand Pump will cost you about $50 to 300, depending on the brand, and type. The better ones are generally about $150 or more (I have a cheap Chinese model, and the Benjamin brand... both work well, but the Benjamin is tougher, thicker, and seems a bit stronger, like it should last a lifetime). OR, you could get yourself a SCUBA tank (generally max 3300psi 80 to 100cf) or SCBA tank (4500psi, 3 to 87cf), or even an HP compressor. I have each of these, and appreciate being able to fill my own tanks, and filling my aGuns from the tanks.
The downside with a hand pump is EFFORT, physical work refilling. 150 to 200+ pumps EVERY TIME you want to REFILL it, and it TAKES TIME. But, it works without any power, and is the perfect backup for any doomsday-er.
Since I was a certified scuba diver, I already had a bunch of tanks... and, as long as you keep at least 100psi in them and moisture out of them, the average scuba tank can be refilled thousands of times, and last decades. I personally have some that are OVER 30 years old, and still 100% functional! The downside, they will only fill/re-fill the Bulldog to the full 3,000 psi about 8 to 17 times, depending on how low it is, how big your tank is, and how much it was filled to at the beginning. My 90cf 3300psi tank was able to do much better than the smaller 80's @3k.
The downside with the SCBA tank is they have a MAXIMUM LIFE EXPECTANCY of 15 years. PERIOD! Unless you own a compressor and are willing to fill them yourself. The upside, they can be filled to 4500psi... so if you get the 9L (87cf) tank, which is about $500 to 700 with the gauge and hose, you would be able to get a pile of refills out of that one tank. Downside, you have to have someone to fill it (a friend, or scuba shop... the later costs about $8 to 15, depending on geography... regardless of the fill amount.
You can get a good functional high pressure compressor, that can fill up to 4500psi, for about $300 to 600... which can also work off an inverter in your vehicle).
That is what I did, because of HOW the local 'scuba shops' charge: $8 for scuba tanks, $12 for HP Carbon Fiber tanks, regardless of the amount of air needed OR THE SIZE OF THE TANK. Plus, they will not refill any tank out of Hydro, they force yearly VI's (visual inspections), and won't touch a CF tank over 15 years old.
There are different TYPES of HP Compressors. mine (in the photos) works to 4500psi, and AUTO SHUTS OFF at a 'dialed in' pressure, or temperture. Those 'blue holes' in the front top have hoses that go to a little 12v pump and return hose, to pump cool water through to function as a 'radiator' for the unit, allowing larger tank fills (volume).
Remember, DO NOT RUN YOUR TANKS DRY... that allows condensation, which ruins tanks!
If you like numbers, you can find a handy 'tank to tank fill calculator' here.
(usually about 1200 psi)... fill as much as you can with your scuba tank...
Then TOP OFF the rifle with your high pressure tank, or the hand pump.
Working it similar to a 'cascade system' for refilling scuba tanks, and it will increase the number of fills you can effectively get.
The 'average' scuba tank is 80 cf (cubic FEET), with a max fill of 3,300 psi.
If scuba tanks are taken care of, never ran out of air for any length of time, and not in and out of the heat (which creates condensation, and water causes pitting... pitting can cause weakness and degradation)... an aluminum scuba tank can easily last 30 to 50 YEARS. I personally have a dozen from the early 90's that are in perfect shape, most of which I'd still dive with today (after a VI, and fresh fill).
I have selected a couple tanks specifically FOR MY COMPRESSOR, and aGun fills. Knowing they will NEED to be VI'ed & refilled by a scuba shop (or better filtered air) before used for scuba diving.
A SCBA tank is usually EITHER 2650 psi max fill, OR 4500 psi. They can be steel or carbon fiber. The Carbon Fiber are the ONLY ONES capable of being filled to 4500 psi, BUT will only be refilled by a scuba shop until it's 15 years old, and they won't hydro them after that (at this time, but they are attempting to get the law to allow extending them to at least 30 years, with 5 year hydro's).
You *might* be able to squeeze a bit more life out of them if you a) get it hydro'ed toward the end of the 15th year... or you learn to refill them yourself.
Regardless, they still have to be VI'ed and Hydro'ed like scuba tanks...
Tanks should be VI'ed (visual inspected) every year, with a Hydrostatic check every 5 years (where they put the tank in a special 'reinforced tank' - then purposefully OVER FILL IT to make sure a 'full tank' is safe DURING TRANSPORTATION, in a vehicle, and worst case 'normal use.'
then if needed, because my scuba tank was below say 2600psi... I will 'top off the rifles tank' with a high pressure SCBA tank (fire rescue, 4500psi).
However, in an emergency, I could refill (or top off) the rifle with a high pressure hand pump. Use hand pumps as a last resort. It works well, but takes about 100 full pumps per 1,000psi of air in the Benjamin BullDog. Note: even putting just 1,000psi in the rifle from a tank will save you pumps (and time), if you top off using a scba tank or hand pump.
Maximize Air Fills
Cascade System Style of Fills
but TOP with those, to maximize the number of fills I can get from my tanks. The higher volume, lower pressure (3300psi max) scuba tank is the 'rock' and the higher pressure (4500psi) scba tank is the pea gravel locking it altogether as a 'top off' tank.
It's math, and logic, and it allows me to maximize my air use for the maximum fills between having to refill the tanks. It took me a little while to learn that trick, but it works well. Plus, I can quickly, and EASILY fills many times off a standard scuba tank (you will need a way to fill, which is NOT INCLUDED).
Boyles Law (on Gases)
Saving Time & Money
I selected ONE of my scuba tanks as the 'high volume - low pressure' tank... then have the HP tanks for the 'top off' - and seldom have to run my compressor more than once ever couple times of shooting a bunch. The scuba yoke is for a k-value, but there is a DIN adapter that can be purchased from third parties, and the yoke works just like a first stage. It has a pressure gauge, and hose. Remember, slow fills (about 10 to 20 seconds) is far better than a 'slam fill' (2 or 3 seconds)... and always BLEED THE LINE before disconnecting. Slammed air is HOT, and hot air is less dense. Less dense = fewer shots per fill.
So, when one of my scuba tanks drops below 2,800 psi (I usually have them filled to at least 3,300)... I USE THEM to 'start' the fill on the aGun, then use the HP SCBA tank to TOP IT OFF. What that means, I put in 340ci of air to 2,xxx psi (what ever the 'scuba' tank is), then just a few cubic inches (ci) to top THE HIGH PRESSURE (if below 2,800psi). This procedure makes my HP tank last WAY LONGER, because I'm not using the 'volume' from it, just mostly the 'PRESSURE' . Make sense?? In addition to pressure, VOLUME MATTERS. This is a similar logic to how scuba shops use 'cascade systems.'
The cascade system allows small cylinders to be filled without a compressor, and allows a low-capacity compressor tank to be refilled MANY MANY MANY more times. And extends the periods during which the storage cylinders can be recharged. It's ultimately 'the biggest bang for the buck.'
Boyle's law is a gas law, stating that the pressure and volume of a gas have an inverse relationship. If volume increases, then pressure decreases and vice versa, when the temperature is held constant. Therefore, when the volume is halved, if there is the same AMOUNT of air, the pressure is doubled; and if the volume is doubled, without increasing the quantity of air, then the pressure is halved.
Another trick for aGuns: You can purchase 4500psi Carbon fiber tanks used for cheap - especially if they are close to or actually OUT OF DATE... I have three, and they work perfectly for aGuns, all older than 15 years from the MFG date, therefore a scuba shop will NOT refill them. PERIOD! And while I wouldn't want to breath off them, because I DO HAVE my own high pressure (4500psi) compressor, I was able to get this three tanks for about $100 each ($300). They are PERFECT for refilling the rifles, even more AWESOME when teamed up with a SCUBA TANK for the 'large volume' - saving the 'high pressure' SUBA tanks as the 'top offs.' The way I use the tanks for refilling my air guns, is similar to a 'cascade system' - do you understand how they work?
SUBA tanks really vary in BOTH size and pressure. DO NOT EVER WASTE YOUR TIME OR MONEY on the small ones, or those that don't fill to at least 4200psi (there are some that are only rated to 2650 - those aren't good for aGuns). There are a variety of SIZES - from little 'pony bottles' (only a few hundred 'ci' - to 9L (90 minute) bottles that are equal to about 87cf (cubic feet). On land, the average person consumes about 1 L (liter) of 4500psi air, every 10 minutes. So, what I did, was check with the local FIRE STATIONS... to see if they had any 30, 45, 60, 90 minute 4500psi SCBA TANKS that were going to EXPIRE (since carbon fiber is only 'rated for' 15 years... and a scuba shop won't refill any older than 15 years. A 45 minute tank is about 60cf - at 4500psi - max fill; BUT The 9L SCBA tanks are 87cf (AWESOME... but they retail new for about $700, but can sometimes find them for around 500 if you don't mind pulling them from china). I've only seen a couple used in that size, and they sold for about $350. But, OMG, if you can refill them, they last the longest and best. Our local scuba shop charges $12 to refill the HP tanks, regardless of the amount of air needed, whereas they only charge $8 for a scuba tank fill (even if 80, 90, 100, 110cf tanks)... usually cold filled to 3300psi if you ask.
My scuba tanks are 80's & 90's CF 3300 psi tanks... my carbon fibers are 30, 45, and 60 minute tanks... my HP compressor is a water cooled 4500psi (under $500).
Yes, I also have a hand pump that will fill to 4500psi. Yes, the 'pumping action' is tiring (by hand) - as it's about 100 pumps per 1,000psi, but in the event of a power outage, it's 15 to 30 shots on one full pump. "Zombie Apocalypse Weapon" - can't get powder, primers, caps... then what??
The 'Benjamin' brand hand pump tanks right at 100 pumps to refill the Bulldog from 2000 to 3000psi. That is about 100 psi per pump in the stock 340cc tank bulldog's have.
The Benjamin pumps would be a whole lot of exercise to fill a bulldog (i.e., 300 pumps for a FULL fill, 0-3000, in the bulldog... which is a whole lot of work). But, without any power... and just to 'top off' it's a solid back up option.
I will tell you, that while I'm 60 years old, I'm still a solid 6'4"+ and about 260lbs... and while the pump isn't real hard for me, it's too much for my little wife to operate. It's the last 2 to 3 inches, where the pressure is 'pushed through' that makes it a PUSH for most people that can't put their 'weight' and 'strength' into getting it done.
On my 90 cf Scuba tanks, filled to 3300 psi, I can get about 17 'usable fills' (to 2600 psi or better) for the Benjamin Bulldog, before I have to engage a scba tank or hand pump to 'top off.' By employing the combo scuba & scba tanks, I can get a whole lot more.
Using the greater volume of the scuba tank to get it as high as possible, then the higher pressure tanks to 'top off.'
Here is a really awesome FILL CALCULATOR, though it's set up for a 'single' tank... https://www.airgunsofarizona.com/Tanks/FillCalc.html