Simple Fix for a Leaking S&W 78G Piercing Cap

Two days ago, I gassed up the S&W 78G with a new 12g cartridge. Heard a faint gas leak from the cap after piercing. It wasn't a fast leak so I went about shooting my targets in the basement range. After about 30 shots, I set the gun aside assuming it'd need some attention. The next day, it was still holding pressure. Even so, I didn't remember replacing the piercing pin seal when I overhauled the gun a while back. Figured it was simply time.

Got another 8 or 10 shots off to empty the gun completely of CO2 then removed the cap.



The outermost o-ring seals the gas tube. It's not the one leaking.

The piercing needle is in the center of the cap and has a tiny o-ring that seals around it's OD. To get to it, I aligned the side hole and drove out the roll pin.

With the roll pin removed, the components of the cap are extremely straightforward.

Unscrewed the small bushing to reveal the o-ring.

Yeah, I probably could have done the entire fix without removing the roll pin, but I wanted to clean and inspect all the parts.

With a dental pick, the seal was out. It's lost it's elasticity. A few small cracks.

After all the years in the gun, it's difficult to accurately measure. A #006 o-ring looks like a good match.



Reassembly was simple.

Smeared the o-rings with a bit of Buzzy's Slick Honey. Gun works perfectly.

Note: I often get emails about where to find Slick Honey. Local bicycle shops can source it from the largest parts wholesaler, Quality Bicycle Products. QBP part numbers are: LU2004 1 oz--about $8, LU2005 2 oz--about $11. Super thin, seems to almost melt between your fingers. A little of this stuff goes a loooong way. Supremely tenacious, hard to displace. Will not harm o-rings. We use this stuff religiously in the shop on suspension fork bushings and seals to reduce stiction. QBP does not sell to private individuals.

More stuff soon.