Small pistol of some type

[Old Painless]

I'm sure some of you can tell me what this is.

 

[RMH]

Powder strength checker?

 

[Acorn Mush]

Powder strength checker?

I believe you are right. Perhaps the OP can post a couple more pictures that are larger.

 

[Blackpowderart]

Pizza cutter side arm

 

[Rifleman1776]

Powder strength checker?

yes

 

[Uncle Miltie]

Eprouvette, to test the strength of powder.

 

[hawkeye2]

eprouvette: A measured charge of powder was placed in the chamber and the cover closed. The wheel was numbered and when it was fired the cover would be blown open and the relative strength of the powder could be measured by how far it was blown open (the number on the wheel).