More pics..

Here is a shot of the stainless probe simply pushed into the .177 barrel, no breech to confuse the issue



Here is a brass .177 extended probe



Here is the two probes side by side, not a great picture I am afraid and hardly scientific. Not easy to see but I used the lined paper through the bolt holes to centre.
The brass bolt has more brass behind the handle and doesn't reach so far into the barrel



I'll try to measure the distance between the bolt handles and the o-ring grove as accurate as possible for each probe.