It is not possible to retrofit factory choke tubes to fixed choke barrels. See Briley option below.
Extra Full choke tubes only available for 12 gauge.
Screw-in Choke markings are as follows:
00 = CYL = Cylinder = 0.00 mm (.000")
0 = SK = Skeet = 0.13 mm (.005")
1 = IC = Improved Cylinder = 0.25 mm (.010")
1+ = LM = Light Modified = 0.38 mm (.015")
2 = MOD = Modified = 0.51 mm (.020")
2+ = LIM = Light improved Modified = 0.64 mm (.025")
3 = IMOD = Improved Modfied = 0.76 mm (.030")
4 = F = Full = 0.89 mm (.035")
5 = XF = Extra Full = 1.02 mm (.040")