Spitfire has alot weaker tail than P-51..
Like I've said before, spitfire loses horizontal stab with one direct hit into there. (I've experienced it, I've done it for several)
What comes for P-51, I have not noticed any one single problem with its durability or guns (well, inline engine might only problem)
I have done very well 8 kill sorties in a P-51 out of flying enemies, even when having just 2 guns left.
Only thing it needs, is to cool up your wrecking patience and wait for close & good shot.
Also, I have taken some beating in P-51, have flown it few times home with some part missing.
In 109 I can't say that same, it takes couple hits from cannons or so and chopchop..
One damage is much more lethal in 109. (Ie. it can't land without wingtip, like p51 does, even without both wingtips)
Spitfire loses tail parts easily also. (stabs)
N1K2.. thats one tough bastard I say.
F4u also.
C.205 is also almost like 109, you can't really fly it for long without a wingtip (Ack once chopped both wingtips, I had to ditch because even 15 MAN did make it roll due to torque. I had to roll it home on the earth level)
What we have here, is that P-51 takes fairly damage among these other planes, with less minuses from damage, being alot more flyable than other planes.
Maybe I do something wrong in P-51 compared to some of you others, who complains about guns being weak and plane being fragile...
but I feel it as fairly good at the moment.
NOTE: ONE PING that you hear MIGHT be 10 HITS!
So if you get lined up 200 yards front of enemy plane, he dumps 20 hits withing very short time on you, you'll hear 2 pings.
One thing you can do is to have code adjusted.