I was offered a nice 1912 Rouble (Napoleon Defeat), it looks genuine to me and dealer has a lot of experience and guarantees thats its authentic. The only problem I find is that the weight is 20.2g instead of 20g, which is stated in catalogs. I know Russian copper and silver coins were weighted in bulk and weights could vary for individuals coins, but thought that Commemorative coins had better weight control.

Can genuine Napoleon Defeat rouble have a weight of 20.2g?

I do not think it is possible for 1912 rouble. It could be a case 150 years earlier.

Keep in mind that this is an error of 1 percent, and possibly would warrant careful re-zeroing of the scale used to get this result. Or double check on another scale.

just my .02 roubles worth.

what if the scale is not 100% accurate?

Then you can be in the money, but better check with a more accurate scale.

According to Uzdenikov difference of +-0.18g was acceptable for roubles minted after 1850, it doesn't mention anything about commemorative coins. So, I guess Roubles with weight of 19.82-20.18g would still pass quality control even if they were selected individually. What do you think?

agreed.

Also consider the individual clerk, placing coin after coin on a balance, all day long, weary eyed, reading the pointer between the little marks, saying to himself "good enough" with a reading on the edge of high or low tolerance, while sipping tea. Or his boss telling him to put the "heavy" one into the bin with all the rest.

Could be the scale...

It is not clear where Uzdenikov obtained the figure of .18 gram but an official St. Petersburg Mint statement
of early 1887 indicated the following:

1) Prior to 1886 the silver rouble (20.731 grams) had a remedy (allowance) of .133 gram, meaning that it had
to weigh between 20.598 and 20.864 grams. How far this rule extended prior to 1886 was not indicated.

2) Beginning in January 1886 the rouble remedy was .062 gram, meaning that the weight was between 19.934
and 20.058 grams. (The official standard weight was 19.996 grams.)

Planchets were, generally speaking, not weighed by hand. Automatic weighing machines had been introduced
to most world mints by about 1880. Any planchet outside the remedy was rejected by the machine.

The law of 1899 does not specify individual coins of silver but rather gives a remedy for 1,000 roubles‘ worth.
It seems likely that the law of 1886 determining remedy for individual silver coins was still in effect in 1912 but
this is uncertain.

Judging by the above, 20.2 grams does seem rather high but the problem may lie with the scale rather than the coin.

RWJ

I finally got more precise scale and the coin appeared to be 20.07 , I already sent it to NGC. I thought my old scale was precise, but it was obviously not. Thank you for suggesting to check it with another scale, I was about to return the coin.

How about some pictures?????????????