It might look good.
The F2A-2 Buffalo with 4 guns had a fixed useful load of 628lbs over and above the empty weight, to which was added 1443lbs of disposable load. for total useful load of 2061lbs.
In the fixed useful load was the 200lb pilot, 151lbs of communications/navigation gear, and 277lbs worth of guns. The disposable consisted of 83lb of oil for one engine, 270lbs worth of ammo and 1080lbs of fuel (180 US gallons)
Take your 581lbs and subtract even 130lbs of radio gear and you have 451lbs, subtract 277lb of four .50 cal guns and you have 174lbs.
.50 cal ammo is about 20lbs per hundred so 174lbs equals 145 rounds per gun. Now perhaps they didn't use quite as much oil and traded the weight for more ammo? Using more fuel would require full oil tanks?
USN used some strange (to us) accounting in the early years and often had Normal or light loading's for fighters and considered a fighter with full internal tanks and full ammo (or even full gun load) as overloaded.
Now please note that the F4F-3 has a weight of 286lbs listed for the same four .50 cal guns and F4F-3s also carried 7.9lbs of pyrotechnics (flares) and a 13.7 lb gun camera in the fixed useful load.
What they were figuring for gun and ammo weight if "normal" and overload condition fo the F5F would be interesting to figure out.