The difference is in the neck
This is a great question and topic for anyone interested in possibly owning a 22BR. I wanted to chime in to provide additional perspective.
The older, and original, 22BR Remington child calls for chamber dimensions of 1.520". With the "original" 22 BR Remington, and the "original" 6BR Remington, the 1.520" length is the spec.
Today, the 6BR Norma has become relevant to the competitive long-range (300 meters and beyond) target shooting community. The difference in the length is in the case neck, between the current 6mmBR Norma spec and the original Remington 6BR spec -' with the former being the longer. The remainder of the case dimensions are practically the same for all practical purposes.
Enter stage left: the highly accurate and fast-footed 22BR Remington. Since there is no "official" 22BR Norma spec, the original 22 BR Remington spec currently holds "true". However, today, an attentive gunsmith and shooter wanting to create a new rifle build would want to spec a "modified" 22BR Remington chamber so that the length would/could/should be adequate to manage the1.560" length, so that the shooter could use, with ease -- without trimming new brass, the readily available 6mmBR Norma/6BR Remington brass -- whether manufactured by Norma, Lapua, and dare I say Remington, too. (I use all three brands -- successfully -- between the 6BRs and 22BR that I have)
I have the 22BR in an older Rem 40x, and I have to trim, trim, and trim today's quality 6BR brass down to fit the original chamber length of 1.520". The best thing I could do is have Mr. Gun Smith ream the chamber neck, in the Remington 40x 22 BR, another 0.040" deeper. However, I never like leaving my toys anywhere overnight if I don't need to.
When buying older 22BRs, and likely some new ones, assume you will have the shorter 1.520" oriented chamber -' especially if the barrel is slow twisted for lighter bullets in the 40-55gr weight class. Again, this is a great subject, and is not clearly presented many places.
Stay safe, and trim the cases properly so you can avoid the costly issues associated with excess pressure. The 22BR is a very hard one to part with once you have one.