I've had a couple of these over the years - and foolishly sold them.

I was always told it was used to clamp over the end of the barrel (having removed the nose-piece) and protected the crown from damage.
It was used for clearing serious blockages (like 4x2 stuck in barrel or 'squib load' resulting in bullet half way along barrel). Armourer could then uses a brass rod, slide it down the guide and hammer away to clear the blockage.

Peter Laidlericon may have another application - but thats what I was told it was used for.

Here is a pic of one of mine - you can see the diameter reduction to support the 'brass-rod'

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