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How to fill a precharged gun from a scuba tank

by B.B. Pelletier

You would think a subject like this is too simple to bother with, but it’s not. Although I have done several postings on scuba tanks in the past, this is the first time I’ve discussed filling.

Only finger-tight!
A guy once asked to borrow a crescent wrench at an airgun show. He wanted it to tighten the nut that attached his fill hose to his gun (this was before quick-disconnects were popular). That’s like using an impact wrench to wind a watch! Because these filling connectors all have O-rings or other types of seals, they need to be only finger-tight.

Fill slowly
I should have guessed what was coming next. Once the adapter was torqued down, he cranked open the valve on the scuba tank and filled his gun in three seconds. I actually jumped back when he did it, which surprised him. Filling that fast is like filling a shot glass with a fire hose! I asked him if his gun reservoir felt warm, and he told me it always did after a fill. I bet! A fast fill generates so much heat from compression that it can melt the seals in your gun. His steel reservoir was too hot to hold. I wonder how long that gun will last with that kind of treatment?

Always allow at least a full minute for a fill. That’s a fill that goes from 2,000 to 3,000 psi. If you’re filling from zero, take longer. Learn how to open the scuba tank valve so the air flows out very slowly.

You can’t put in more than you have
As you use a scuba tank, the air pressure inside starts dropping. How fast it drops depends on how many guns you fill and the size of their reservoirs. If your scuba tank has 2,600 psi in it, it will only fill a gun that high. The fact that the gun’s reservoir is smaller than the scuba tank has no bearing on the matter. When the pressure in the gun equals the pressure in the scuba tank, the air stops flowing.

Keep all connections clean
Dirt is the enemy of a precharged gun because it defeats the air seals so quickly. A small particle of sand can get on a hard synthetic seal and embed itself in the material, causing an opening at the point the seal is supposed to be tight. It only takes a microscopic hole for pressurized air to leak out. All your connectors and the ports to which they attach should be kept clean. If the gun has a cover for the fill port, use it. This is especially important after lubricating a seal, because fresh grease attracts and holds dirt like a magnet.

Lubricate O-rings and seals
If you have a dry O-ring and you have to twist whatever it connects to, the O-ring can tear. The static ring on a K-valve is very forgiving because there is no lateral movement, but the O-rings on a fill probe are subject to a lot of pulling as the probe slides in and out of place. Keep them lubricated with pure silicone grease and they will last as long as possible.

Any questions?

The O-ring in a scuba tank K valve is not subject to twisting forces.

This Career fill probe has two O-rings that scrape the side of the fill port each time the probe is inserted and removed. This will tear the rings unless they are lubricated. Once you grease them, keep the probe in a plastic bag when it’s not in use.

16 thoughts on “How to fill a precharged gun from a scuba tank”

  1. I was pondering the question of gas pressure just the other day, so, your Blog is timely. Assume a 2600 psi scuba tank. What is the pressure we are trying to achieve in the average guns resevoir? If the like the FX Black Widow it is 3600psi wouldn’t it be impossible to fill the resevoir? Is the pressure in the guns’s resevoir high in order to provide the maximum number of shots at the highest power rating or is it another factor? What is the lowest pressure that will produce acceptable results in the average PCP? How may fills will the average tank provide to the average gun? Finally, a safety question. Aside from the fast fill issue you mention are there any other dangers to be aware of. Thanks for the great entry.


  2. Dry-fire,

    It is usually safe to dry-fire gas (CO2) and pneumatic airguns. Unless the O-rings are positioned where they will blow off, there is little harm that can be done.

    If you plan on doing a LOT of dry-firing, however, it is probably safer and certainly cheaper to do it without gas in the gun.

    I shoot my PPK/S Umarex) pistol dry all the time.


  3. i am a novice airgunner and just purchased my first airgun off this site. i got a BSA scope to go along with it. I dont know if all scope mounts are like this but my rings have pieces of tape on the inside. i was just wondering wether or not to remove the tape on the inside of the mounting rings.
    thanks, chris

  4. hey B.B. sorry to bother you on this subject again but i have one or two more question on the subject of scope slippage. unfortunatly, your last tip did not help my gamo 220’s scope problem. however, i was wondering if mounting an adjustable riser base, such as the first one listed under the scope mount section of this website, would help because of pin slots it appears to have. If this WOULD help i have one final question, the first adjustable riser base listed under the scope mounts section of this page states that it can be mounted only to 11mm guns NOT US 3/8″ guns. i checked on gamos website and they say that all their guns are 11mm OR 3/8″. Does this mean that this base can’t be mounted to my Gamo rifle? whats the deal?

    thanks for your time and help,
    scopestop guy

  5. scopestop guy,

    The U.S. Gamo sales team is being too general when they say their mounts fit either 3/8 or 11mm. Those dimensions are very close, so all cheap mounts are said to fit both. Apparently the U.S, Gamo reps are saying gtheir mounts have sloppy tolerances.

    If you compare that to B-Square, you’ll see that B-Square makes mounts for specific brands, because of small dimensional differences. That’s what a quality manufacturer does.

    There is a difference, however it is more cosmetic than functional. Sloppy bases will look off-center and canted, but because the scope is perfectly round, it can be made to work in them.

    For your other question, ask the Pyramyd Air tech reps.


  6. bb, in read two other posts about accuracy, and i wanted to buy a match gun for use in hunting. my two choices have come to the izh 61 and the daisy 853. i prefer the 853, because it is a pneumatic, but i like the izh 61, for its accuracy. the only problem is, i have a dislike of the tendancy for spring guns to vibrate and lose power. in terms of accuracy and vibration, which is the better choice?

  7. B.B. when i was browsing through Gamo’s website, i stumbled across a airgun labeled Hunter Extreme. This gun claimed a muzzle velocity of 1600 feet per second in .177! this velocity seems rediculous. i was wondering if you new anything about this new gun.
    scopestop guy

  8. sorry for the off topic question but my Gamo has lost a great deal of velocity and e-mailed the Gamo folks and they responded with “Your breech o-ring may be damaged. However, you can order replacements online.” and give me a link to the part. then i mail them about the installation of this part and they respond with “We provide these parts and schematics only to licensed, insured, and authorized gunsmiths.” im confused. any input would be greatly appriciated THANKS!!


  9. Neither the IZH 61 nor the Daisy 853 is suitable for hunting. Both are underpowered for the job.

    If you want to hunt with a match gun, try the Walther Dominator. It was made for that purpose.

    But why hunt with a match gun? For accuracy? There are regular hunting guns that will give all the accuracy you desire – they just are not made for target shooting.


  10. scopestop guy,

    The Gamo Hunter Extreme at 1,600 f.p.s. IS ridiculous – just as you say. It can’t hit a barn wall when shot from the inside. It is pure advertising hype – nothing more.

    Now, the Gamo 1250, as it used to be labeled (the velocity of the same rifle when using the lightest weight lead pellets), is a good spring rifle when used with the heaviest pellets available. I like it even better in .22 caliber with heavy pellets.

    But shooting that new Raptor 5-grain “gold-plated” pellet, the HUNTER EXTREME is a joke.


  11. Ben,

    What you are saying is that people are not talking to each other at Gamo.

    I suggest you deal directly with the dealer who sold you the gun. Dealers are usually responsive, where foreign manufacturers often have communications problems.


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