How much barrel is required?
For doing what?
I have 44 Mags with barrels as short as 3 inch and as long as 24. Both do the job they were intended to do.
I have 45/70 from 22 to 28. The problem with short 45/70’s is burning all the powder.
"The difference between stupidity and genius is that genius has its limits" Albert Einstine