S500 FAC Barrel Leak

[Evas117]

Hi Guys. I have an AA S500 FAC 4.5mm 30 Joules. I have a problem with air rushing from the breech back to my face after each Shot.
I thought it was the barrel Seal so i changed it with a new one. Part Number FP121. But the problem still continues and im thinking the loading bolt could be thinner and does not seal the breech very well?
Any suggestions?

Thank you from Cyprus.

[RichieRich612]

Is it the original loading bolt? If it is then the only cause will be the o ring if it's coming from the breech.
Another o ring to check is the transfer port.

[Evas117]

I changed again the Barrel Seal for a new one and put a new Brass shim and a new S610H Seal. The problem still remains. Lots of air coming out off the breach hitting me in the face

[Blackbaronfish]

I can only think of the breach seal , transfer seal (use a delrin one) or a blocked barrel

BBF

[TenMetrePeter]

I did read of users fitting “oversize” breech seals which for 4.5/.177 guns is (I’m pretty sure) size BS008 or 1/16 inch by 3/16 inch ID. Nitrile Hardness 70

Use a blunt end cleaning rod or similar up the barrel, level with the seal, to help fit them

[Archer50]

Unless something is actually broken, the only causes I can think of are:

- Breech seal - use a BS008 o ring and Molycote 55, which makes the rubber swell very slightly.

- Transfer port seal - should be delrin on an FAC gun. Check and see if it is damaged and make sure it is properly seated.

- Brass transfer port holder not properly sealed to the barrel - use purple or blue loctite.

- Barrel and transfer port holder not sealing properly in the action. I have had a lot of trouble with this, on several guns, even with the addition of the two o rings Air Arms sometimes do fit, and sometimes don't. If this is the problem, try using plumber's tape around the barrel. In the end, on one gun, I used purple loctite as a last resort. It worked perfectly, and I was able to get the barrel out after a week using a hair dryer, but no guarantees.

If the gun has worked without an air blast before, it is very unlikely to be anything to do with the pellet probe.

Good luck, and I hope it's one of the first three - they're easily fixed.

Alan