My son just got older model 342 in 22 hornet. Dont know the year. What I am wondering is when did they change from the .223 bore to the ,224. I dont want him shooting .224 in it until we can figure out what the dia is. The serial # is behind the scope mount so we have to remove it to get it. Also is there any other way to measure to tell. I know that 1000th of an inch difference is hard to measure.