FWB P70 has me stumped.

Richely said:

Blue breach seal bottom, white breach (barrel) seal top? Seals fitted uniformly and with no crease, nicks, cuts? Genuine FWB seals have rounded ridge one side and flat other, perhaps try reversing if all else is good.

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Yes, white on the barrel and blue in the bottom. The blue had the slightest of ridge but made no difference. But I did just add a thin oring same diameter under the blue. The blue one still seated and seems secure. No more pop from the breach. And with the big RAW LDC it is nice and quiet other than the normal P70 spring twang they all have.

Thanks for getting the old noggin thinking.

Maybe the blue oring was out of spec?

Shooting this morning and all was good until a shot made a loud crack again. Found that the small oring I added had been removed from under the FWB seal and launched with the pellet.

Discovered that a QB transfer port seal is the same dimension as the FWB seal. Sliced a thin section off the qb seal and all seems good again. If this goes pop I may go the old qb route of using a piece of fish tank tubing.

While I had it apart I found which spring on the p70s makes the annoying twang. Anyone that has one of these knows what I’m taking about it is awful. Added some clear spring tar and no more boooooiiiiiinnnnngggg.

Rifle is shooting 730fps with JSB 10.34 for just over 12ftlb.

Richely said:

Rifle was designed to shoot 7.5 joules (5.6 fpe). Yours having been modified to shoot 12 fpe may be why you‘re blowing out the seal.

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I think I may have just ended up with an off sized breech seal. My other P70 done up by AZ shoots 840fps with 10.3 for 16 ftlb and has for its entire life. But you may be right about being just over stressing the design.