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Greg - you might want to re-check any empty weight less than published XP-51 at 6278. I'm having a hard time conceptualizing the POF stripping out 1000+ pounds. The wing, tail, fuselage, landing gear, surface controls, prop, engine, cooling systems total up to 5737. That leaves out controls, lubricating and hydraulic systems, electrical and communications and fuel systems, battery, instruments, starting and engine control systems as well as any trapped fuel/oil, armament support (ammo bays, gun mounts, etc)
It looks like you referenced America's Hundred Thousand (great book) page 328 Table 48. That's referencing Merlin Mustangs. Table 47 just below it shows a P-51A empty weight (dry) of 6433 lbs or 555 lbs lighter than the Merlin Mustang but still 633 ibs over the 5800 lbs (no doubt a "ballpark" number) but one easily imagined after removing the (using the similar Merlin numbers) Armament (270 lbs), Armament Provisions (246 lbs) and Communications (163 lbs) . Actually, 6433 - 679 = 5754 and we still haven't addressed the armor plate (some of which is probably impractical to remove for CG or structural reasons).
You just took the liberty of subtracting guns from the Empty Weight - which is not included in Empty Weight until the Basic Weight build up. I wasn't trying to address the changes made by the Hinton's - just quoting a Basic
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Empty weight for P-51A - without guns or fuel or bomb racks is 6574 pounds but includes trapped fluids. I put that out there because that would begin a true base for the Hinton P-51A unless they completely flushed the tanks, resevoirs and lines.and referenced that The P-51D is 6956 without guns or 85 gallon tank or racks. The 32 pound racks are not shown in Dean's book until the build up to minimum combat Basic Weight with trapped fuel.
Typically "trapped fluids" is not included in Empty weight, nor was it included in the quote posed for XP-51 which did not have armor plate. Armor plate is included in the published 'integral' empty airframe for A/B/C/D and A-36. My data comes from NAA and differs from Dean's on the weight of 'trapped fluids' which included hydraulics supply in lines as well as fuel and oil.
I am sure Not arguing with Greg's quote from John Hinton.
I was just curious where the key strips of airframe occurred to take 800-900 pounds out. Armor plate, trapped fluids would pull 225+ pounds out of the P-51A published Empty. New, non bullet resistant windshield glass would take a few, installing a smaller set of wing tanks would strip some. The armament provisions at 246 is one obvious major chunk but only 1/2 of the delta.