Yes they are common.
The tapered is the standard model usually 4 & 6 inch.I believe they made some longer in the past but they may have been a heavy tapered target model.
The most common bull is a 5.5 inch the 6.78 inch would be the second most common.They also made a slab side competition model in a 6.78 & 5.5 inch.
Over the years they have made several different barrel lengths.
I have a 4 inch bull & a 6.78 government target bull a 5.5 & 6.78 slab side & a couple of other MKII's.
Hope that helps you out some.