The Germans historically used air-cooled Maxim LMG 08/15 machine guns on their Fokker biplanes and triplanes. The guns featured in the movie are either one of two mockup kits. The first is the blank firing version which is achieved by using two submachineguns inside a "shell" made to look like the Maxim. The second is the flame shooting variant, (using gas), that is more faithful to the design of the original Maxim but only fires a gas flame. The revealing mistakes for the blank firing version are easily spotted: The cocking handle on each MG doesn't move when the gun is fired unlike the original. The brass also ejects through the right side of the gun and actually from the middle of the cooling jacket as opposed to the original. Another giveaway is that the barrel of each MG is seen portruding from just above the exact center of the cooling jacket. (In reality the barrel was at the bottom of the jacket.) Another strong hint that the movie MG's are a mockup is the presence of stick magazines inserted very near where the movie-version MG's is seen ejecting brass. The mockups might are the main reason (at least in part) why the pilots are using a handle to fire the MG's even though the historical aircraft used buttons attached to the flightstick in order to fire the onboard weapons.
Lewis gun (mockup)
The British fighter craft and recon plane is armed with the Lewis aircraft gun which is revealed to be a mockup on all the planes. Besides the general (incorrect) form of the weapon, the main revealer that it is a mockup is that the ammo drum does not rotate while firing.
The Lewis Gun is replaced in some scenes by what appears to be a genuine (but unknown) LMG. It is modified to fire flames only from the muzzle and the hose is visible in most scenes.
Vickers Machine Gun (mockup)
The British are also seen using the Vickers MG, though they all seem to be mockups made to shoot flame only.
Rifles
Lee-Enfield No.4 Mk.I
Lee-Enfield No.4 Mk.I is carried by German troops. The rifle is also seen in the hands of Bruno Stachel (George Peppard) during his own time in the trenches. The movie was shot in the Republic of Ireland and there was a lack of genuine German firearms (with one exception) for the shoot so older Irish Defence Forces weapons were substituted for German weapons.
Lee-Enfield No.1 Mk.III*
In a reasonable move made by the producers all the British soldiers are carrying the older Lee-Enfield No.1 Mk.III* with it's original sword bayonet to distinguish it from the German soldiers who (due to lack of German authentic weapons) carry the Lee-Enfield No.4 Mk.I with it's shorter bayonet.
Lee-Enfield Mk.I (probable)
During the hand to hand combat scenes at least one German is seen with what appears to be an older Lee-Enfield Mk.I.. The rifle is identifiable via the magazine cut-off switch visible and the portruding barrel which none of the later Lee-Enfield models had. The bolt action handle, (for the combat scenes), seems to have been removed though the bolt itself seems to be in place.
Handguns
Luger P08
The Luger P08 can be seen briefly carried by a German 2nd. Lieutenant.