m4 barrel that accepts m203

[urban spec-ops]

hi all

i have started re-building an m4 i had in my collection, metal body upgraded internals etc and thought.....

i have an m203 barrel mount style one ( not ris ) lying around i will add that

i ordered a mounting kit from ca but when it arrived it appears even though the mount kit fits the m203 the barrel seems non compatible.

the groove at the front that houses the m203 front mount the barrel is still too thick and the back mount does not fit over the part of the barrel that bulges right where the barrel/receiver nut goes

is there a special barrel available for this type of set up as if there is i seem to be struggling to find it?

many thanks for your time

urban