That's correct Lou

Irrespective of engine power limits and wing strength, Mg's simply were not reliable enough for use on wings, always being prone to jamming. If a cowl mounted Mg jammed, the pilot could reach out with a hammer to try and 'knock' the jammed round free. If it was the top mounted Lewis, again the pilot could reach the gun to attempt to clear the jam.

Rather risky to do o when the Mg is wing mounted.

Good point

Actually, the aft was aluminum tubing with fabric covering on the Hurricane. The early models only used sheet aluminum on the forward fuselage. Later it was used on the forward fuselage, wings, and the fin, and stabilizers, with fabric covering the fuselage aft of the cockpit, and the control surfaces.

One thing too, about WWI arms, is the way guns jammed. Guns mounted away from the pilot were dead with one jam. With them mounted in front of the pilot, if a jam occurred, the pilot had a chance to clear the jam. In WWII planes, with wing mounted guns, once jammed, they were jammed. The early 4 gunned P-51 sufferred from this problem. Plus the guns have to be charged before firing, and then in the cold skies over Europe, how do you keep the guns from freezing? Especially in the cooler times of the year.

Wing construction too was a big issue. As was powerplants for aviation. Most of those old aero engines didn't use ringed pistons. They used pistons that had oilgrooves for lubrication, which effects the power output. Also, the size of the pistons were huge for the horsepower they developed. Pison rings allow for higher compression, but with higher compression, now you need a higher octane fuel, or the engine will knock itself apart over time. Like the P-38 the Italians captured. They insisted on flying it regularly, but because they used a lower octance avgas, compared to the Americans, and the British, the engines didn't last long.

There is a lot of other factors too, but I need to go and finish planting corn and beans out in the back acre.

Probably too many nagging issues in a WW1 craft for them to cope with at the time.

High drag and loss of lift with it fitted on the wing anywhere but in the middle - the wings were often thin so embedding inside the wing would likely be impossible.

There are other issues like reloading mid flight, fixing gun jams as mentioned above, and feeding the ammo - drum based would be best if on the wing, inside the wing belt-fed would need ammo storage, feeding mechanism adding more weight in the wing. Also poor access for maintenance, reloading on the ground and so on.

One main issue of the time I'd think is the extra weight, and even more weight from making the wings stronger to hold the guns and ammo.

No idea if this was the case but seems likely thinking about issues in WW1 craft already there.

The Austrians in 1916 Had an aircraft with a Vickers Type Machine Gun with its ammo, mounted on the upper wing in it's own nacelle, that might have knocked out the problem of drag. But It wouldn't help with the weight, there's just no way to get around that one. At least until more powerful engines came along, but by then the War was over

Using 20-20 Hindsight, that setup would allow incindary ammo early on