If 5.28 x 1.78 blows out

[mubhaur]

Of the barrel, what next size should I try for the breach seal of .22 pcp.

 

[Verve]

Blows out from which side?

What's the pellet probe OD?

 

[mubhaur]

The OEM size is 5.28 x 1.78 mm

Internal dia of brass thimble is 5.79 mm

Probe dia is 5.42 mm

Clearance is around 0.37 mm

 

[mr.grimm]

I use a 009 o-ring. Bought a package of 100 for about 8 bucks from Mcmastercarr.

 

[Verve]

Main thing is the diameter of the groove o-ring sits in. Doubt it is the o-ring thats the issue, it's likely that the grove is not machined to spec. 

5.28+1.78 = 7.06mm

That's more than sufficient. Check the groove. Does the o-ring sit even all around in it? slide the probe in and out and check if it catches the o-ring and moves it out of place.