The easiest way is to buy some lead "torpedo" sinkers that are slightly larger than what the bore size is supposed to be (caution: a lot of sinkers now are zinc, make sure you get real, pure lead). Grease the bore well - don't fill it up, just a good, thin layer on the sides. Put a little grease on the sinker, drop it into the chamber, then drive it through and out the muzzle with a steel rod a little smaller than bore diameter. Completely degrease the "slug" and measure.
Being an 8mm bore, swaged lead bullets for .32 caliber pistol will also work. Make sure they're cold swaged soft lead and not cast (most cast bullets are harder than pure lead).