It depends to an extent on what you are trying to do.
no
No. 9mm and .40 are not interchangeable.
6.8mm / .270
That is '6.35mm' or .25 auto. Value depends on condition.
http://www.industrial-enclosures.com/html/conversion-mm-inch.html
Short answer, a 9mm is smaller in diameter and lighter.
Roughly .275 or .28 caliber. Take MM divided by 25.4 (mm/inch) and you get caliber in hundredths of an inch.
40mm = 1.575 inches.Direct Conversion Formula40mm*1 in25.4 mm=1.57480315in
9 caliber is 0.09 inch, smaller than any standard ammunition made. 0.09 inch = 2.286 mm. Perhaps you were thinking "What caliber is 9 mm?". 9 mm = 0.354 inch = 35.4 caliber.
.68 caliber is ABOUT 17mm.
Probably 32 ACP. Have it examined by a gunsmith.
This is hard to determine- possibly a small pistol I have that only shoots confetti. In terms of an actual firearm, the Swiss Mini Gun is a minature working revolver that is 2.34 mm caliber (rimfire). That is ABOUT .09 caliber.