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Easy. Is it a double or single shot? If it is a double, look for the proof marks on the barrel flats (the bottom of the breech end of the barrels. The following comments refer only to the marks on the flats. Marks elsewhere are rarely proof marks. If it is a single shot, look for these marks on the side of the barrel.

Since it is a .500, it should say .500EX somewhere. The "EX" means that it is an express rifle, either black powder or nitro.

If it is a .500 Nitro Express:

You will find the legend "80 Cordite, 570 MAX". You will also find the Nitro Proof mark, which will look like an upraised armoured arm holding a scimitar over the letters "NP", or the words "Nitro Proof". This is the full .500 Nitro load. Also, the .500 Nitro Express was sometimes proofed for the "Tropical" load intended to duplicate the normal load in very hot climates. This load is 74 Cordite, 570 MAX. If it is not marked on the flats for one of these two loads, it ain't the full monty and you should NOT fire standard .500 Nitro Express ammunition in it.

If it is a "Light Nitro" .500 Nitro Express:

You will find the legend "70 Cordite, 480 MAX" or "75 Cordite, 480 MAX", together with the Nitro proof mark or the words "Nitro Proof". This is the "light" nitro version which is sometimes encountered. This is not the full .500 Nitro Express loading but is not the same as the "Nitro for Black" loading, which is less powerful.

If it is a "Nitro for Black" .500 Express:

You will find the legend "52 Cordite (or 53, or 55), 340 MAX (or 440). This would mean that the rifle is a Black Powder Express, but proofed for the smokeless equivalent of the black powder load. This is NOT a Nitro Express.

If it is a Black Powder Express:

You will not find a reference to Cordite (a smokeless propellant) nor a Nitro Proof mark. You may find the legend "500EX 136-340".

The maker's engraved legends on top of the barrels, on the action, or even on the buttplate (of the old BPEs) should also tell you what you have. If it says ".500 Express", it probably isn't Nitro. If it says ".500 Nitro Express", then it is.

Hope this helps.
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"Serious rifles have two barrels, everything else just burns gunpowder."