The Russian Defense Ministry said in a statement that 11 people were killed and 15 were wounded when two men opened fire on a Russian military firing range.
The ministry said in a statement that the shooting happened in the Belgorod region, which is in southwestern Russia. Two men from a former Soviet republic were killed in a shoot out with volunteer soldiers.
The ministry said it was a terrorist attack.
Hundreds of thousands of people fled Russia after President Putin ordered Russian forces to increase their presence in Ukraine.
Over 220,000 people have been called up as part of the effort to recruit 300,000, according to Putin. He promised the work would be done in a couple of weeks.
There was confusion about who should be called up for service in a country where most men under the age of 65 are listed as reserves.
Even though the Russian leader declared that only people who had recently served in the military would be subject to the call-up, activists and rights groups reported military offices rounding up people without any army experience.
Some of the newly called-up reserve soldiers posted videos of themselves being forced to sleep on the floor and given rusty weapons before being sent to the front lines.
Some people who were sent to combat without proper training were killed.
The authorities promised to improve the situation after acknowledging that the mobilize was often disorganized.