Hw 57 pop up o ring size?

[HW777]

I bought an assortment of rubber O rings in a multi pack, pm me your address and I'll pop some in the post to you, there's bound to be one that fits.

[Black Beard]

The o ring only stops the breech from popping out all the way. It'll work fine without it if you don't turn the gun upside down.

[Strimmer]

I bought an assortment of rubber O rings in a multi pack, pm me your address and I'll pop some in the post to you, there's bound to be one that fits.

Thanks for the offer there, I've a few different assortments myself, with one being close enough but was hoping somebody knew the actual size as I'd imagine it's a common service part that would need changing.