I've got a 1985 vintage HBAR which, of course, uses the large pins.  I just acquired a relatively new Colt Law Enforcement Only CAR-15 upper (yes, I'm LEN--it's legal), and wanted to put it on the old HBAR lower.  I purchased the proper pin so the small-pin LEN CAR-15 upper would match the large pin lower, screwed them together and when I tried to close the gun,there was about a 1" gap between the upper and lower at the rear of the upper.  Seems as though the frame on the old Colt HBAR lower is not machined to match the newer Colt CAR-15 upper.  Seems kinda silly to me that Colt parts don't interchange.  

Is this normal?  Any way to fix it?  If not, can anyone suggest a good complete lower for me to use with the Colt CAR-15 upper?

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dahugo,

Are you using the offset conversion pin? Small center section, larger outside sections?

If so, when you close the upper the pin rotates. Take the nut off the conversion pin and close the upper. Reinstall (LocTite if necessary) the nut. Every time you open and close the upper the pin will rotate slightly.