That’s about as long as they make them, you know, “poke your eye out with that thing”. That’s a four foot pipe and I imagine your right, the little bit of powder should be about all burned up by the time it see’s the muzzle.
If you are building a custom you can get a longer barrel, there is a rifle over on the ALR forum with I think a six foot barrel, over all length is eight feet of rifle. Need a special case and a full size truck to transport that one.