To be efficient 24" would be good for being efficicient. One of 20" is a bit short for getting range. Based on historical data and use.
The question is efficient and not optimized length.
One historical point of practical use was the Westley Richards used by the Boers. They found 24" barrels the best overall. Good enough to to cause a up to a 40 to 1 kill ratio against the British army in the 1st Boer War. The Brits had long barrelled Martini-Henry's.
In the last battle the Boers killed 92 Brits with 78 of the kills being head shots. So the 24" barrel proved to be both accurate and provide enough power to do the job.
BP | Bronze Point | IMR | Improved Military Rifle | PTD | Pointed |
BR | Bench Rest | M | Magnum | RN | Round Nose |
BT | Boat Tail | PL | Power-Lokt | SP | Soft Point |
C | Compressed Charge | PR | Primer | SPCL | Soft Point "Core-Lokt" |
HP | Hollow Point | PSPCL | Pointed Soft Point "Core Lokt" | C.O.L. | Cartridge Overall Length |
PSP | Pointed Soft Point | Spz | Spitzer Point | SBT | Spitzer Boat Tail |
LRN | Lead Round Nose | LWC | Lead Wad Cutter | LSWC | Lead Semi Wad Cutter |
GC | Gas Check |