The P-38 may not have been a heavy fighter but this sure as hell is heavy eneough:
XP-38:
Four 23mm Madsen Cannon with 50 r.p.g.
Four .50 Colt-Browning MG 53 m.g. with 1,000 r.p.g.
YP-38: 37mm Oldsmobile cannon, two .50 and two .30 machine guns
P-38: 37mm Oldsmobile cannon and four .50 machine guns.
P-38E:
20mm Hispano M1 Cannon with 150 rounds
Four .50 Colt-Browning MG 53-2 m.g. with 500 rounds per gun.
P-38L:
20mm Hispano M2(C) Cannon with 150 rounds
Four .50 Colt-Browning MG 53-2 m.g. with 500 rounds per gun.
And now lets compare it to the Beaufighter and the Bf-110
Messerschmitt Bf 110 C-4/B
Armament:
Nose Fixed firing forward
Four 7.9mm MG 17 Machine Guns with 1,000 rounds per gun
And
Two 20mm MG FF Cannon with 180 rounds per gun
Rear Gun in flexible mounting in rear cockpit
One 7.9mm MG 15 Machine Gun with 750 rounds.
Messerschmitt Bf 110 C-5
Armament:
Rear Gun in flexible mounting in rear cockpit
One 7.9mm MG 15 Machine Gun with 750 rounds.
Messerschmitt Bf 110 D-3
Armament:
Nose: Fixed firing forward
Four 7.9mm MG 17 Machine Guns with 1,000 r.p.g.
And
Two 20mm MG FF Cannon with 180 r.p.g.
Rear Gun: In flexible mounting in rear cockpit
Two 7.9mm MG 15 Machine Gun with 750 rounds.
Messerschmitt Bf 110 G
Armament:
Nose: Fixed firing forward
Two 30mm Mk 108 Cannon with 135 rounds per gun
Two 20mm MG 151 Cannon with 300 rounds(port) and 350 rounds(starboard)
And
Two 20mm MG 151 Cannon in Schrage Musik installation (Firing Obliquely forward)
Or
Two 7.92mm MG 81 in rear cockpit
Optional:
Waffenwanne 151Z ventral tray housing two forward firing 20mm MG 151 cannon
ARMAMENT:
Four 20mm Hispano cannon fixed under nose.
Ammunition: 60 rounds per gun.*
*Initially hand loaded drums, later belt fed.
One .303 in. Vickers K aimed by observer.
Fighter Marks, in addition to above:
Four .303 in. Brownings fixed in starboard wing.
Two .303 in. Brownings fixed in port wing.
So with the exception of the Bf-110G, and some Fighter marks of the Bristol Beaufighter, the P-38's firepower was no different then either aircraft. The Bf-110 would outfly a Beaufighter and the P-38 would outfly both of them and the firepower was about the same. How about that.