22LR to 20 calibre
About 30 years ago an Australian, Arthur Langsford, owner of Myra’s Sports Store in Broken Hill came up with a barrel that used .22LR ammunition that passed through a forcing cone swaging the projectile down to .20 calibre. He has since passed away and the barrels, I believe, are no longer produced.
Apparently there was a considerable gain in velocity due to the increase in pressure while the bullet was forced into the smaller barrel. The pressure increase was apparently well within the design and safety limits of the actions that he used. Penetration was assisted by having a longer bullet of the same weight. On small game where penetration was not a problem the only gain would have been a slightly flatter trajectory. I believe he also experimented with .17 calibre barrels but could not get consistent accuracy so he shelved that idea.
I have been considering this type of barrel to use in a switch barrel setup but am unable to find out any further info such as how the forcing cone was cut and to what dimensions. If I do find out I will post the results.
Iggy
Ian McLaren