A B727 CARGO FLT FLIES 400 FT UNDER HIS MINIMUM ALT ASSIGNMENT WITH THE WRONG ALTIMETER SETTING 15 MI NW OF DFW; TX.

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FLT PLAN SHV-AFW. ARRIVING IN FORT WORTH AREA; DURING DSCNT; ALTIMETERS WERE SET TO LCL AFW ATIS RPT AT 29.95. DURING DSCNT; ATC DID NOT GIVE A LCL ALTIMETER SETTING UNTIL CLRNC ALTDEV NOTICED. LAST ASSIGNED ALT WAS 3400 FT. ATC NOTIFIED US MODE C INDICTED 3000 FT; 'ALTIMETER DFW 29.55; MAINTAIN 3000 FT.' ATC DID NOT GIVE ALTIMETER SETTING WHEN ALT ASSIGNMENT BELOW FLT LEVELS. SUPPLEMENTAL INFO FROM ACN 504332: I RECORDED THE AFW ARPT ATIS ABOUT 100 NM OUT. THE XMISSION WAS FAST AND HAD SOME STATIC. AT FIRST; THE ALTIMETER SETTING WAS NOT CLR. SO I LISTENED TO THE BROADCAST 3 TIMES AND THOUGHT THE SETTING WAS 29.95. IT WAS ACTUALLY 29.55. I PREPARED THE LNDG DATA CARD (WITH THE INCORRECT ALTIMETER SETTING) AND PASSED IT TO THE PLTS. AT TRANSITION ALT; THE PLTS SET THEIR ALTIMETERS TO 29.95. WHEN THE LOW ALT SECTOR CTLR WAS CONTACTED HE DID NOT GIVE US THE LCL ALTIMETER SETTING. WE WERE ASSIGNED A MINIMUM ALT OF 3400 FT MSL. AFTER A COUPLE OF MINS AT THIS ALT; THE CTLR GAVE US THE CORRECT LCL ALTIMETER SETTING AND INFORMED US WE WERE 300 FT BELOW OUR MINIMUM ALT. THE CORRECT SETTING OF 29.55; ONCE RESET; GAVE US AN INDICATED ALT OF 3100 FT. NO FURTHER DISCUSSION OF THE ERROR. WE HAD NO TFC OR TERRAIN CONFLICTS AND WERE AT THE INCORRECT ALT FOR ONLY A MIN OR TWO. I THINK IF ATC HAD GIVEN US THE LCL SETTING WHEN WE TRANSITIONED THROUGH FL180; WE MAY HAVE AVOIDED THIS ERROR.