In general, I think that all aircraft in the game are too vulnerable to airframe damage from small caliber bullets. Unless you get a "golden BB" scenario where a small amount of damage causes an increasingly severe chain of failures, all you're doing is punching a hole the diameter of one of your fingers through a tough skin of aircraft aluminum or plywood. For a .50 caliber/12.7mm bullet, you're just punching a hole the diameter of your thumb.
It takes a lot of holes of those sizes to tear apart something as big as an airplane on their own. To help the failure along, you need the forces of gravity and air resistance.
To really start tearing an airplane apart, you need something that will make a big hole, like a 20mm HE cannon shell which will blast a hole the diameter of a man's fist, or a 30mm HE cannon shell which will blast a hold the diameter of a man's head.
.303 caliber
https://s-media-cache-ak0.pinimg.com...982c3fd093.jpg
.50 caliber/12.7mm
http://www.118ahc.org/Resources/Newsome50cal-65.jpg
http://i82.photobucket.com/albums/j2...tz/spinner.jpg
20mm & .303 caliber
http://i.imgur.com/EAwcYfz.jpg
(Notice that the He-111 in this picture took dozens of small caliber bullet hits - look at the fuselage.)
20mm AP
http://i.imgur.com/lQYtUdR.jpg
20mm HEI
https://www.upload.ee/image/4123820/20mmHEcardoor.jpg
https://encrypted-tbn3.gstatic.com/i...R8c4krw6pv6QpA
https://encrypted-tbn3.gstatic.com/i...R8c4krw6pv6QpA
Test of 30mm (L) and 20mm Minengeschoss ammo:
http://i.imgur.com/0A6e0J2l.png
https://youtu.be/ZoLLDi-M3fk
In an enclosed space, the 30mm cannon could do a lot more damage:
http://farm1.static.flickr.com/206/4...e07050e0_o.jpg
http://www.airwar.ru/image/i/weapon/mk108blenheim.jpg
37 mm
http://i44.tinypic.com/6z30k8.gif
Video of .50 caliber vs. modern aluminum plate: