I've spent 90% of my time in AH Beta, AH 1, AH2 in the P-51D mustang. Its my ride of choice for the MA.
Firstly, guns. I really fail to see why people say the guns on it are weak. For someone who can use em properly, i assure you they are anything but weak. Try a 4-gun P-51B, then you'll appreciate a 6-gun package! The 6x50 cal package is brutal. I often take aircraft out at high-deflections in single passes no problems at all. The 50cal has a beautifully flat trajectory, a high muzzle-velocity, and high ROF. Now mult it by 6, and you get a more than capable guns package, which is easy to use, easy to hit with, easy to predict where your rounds are going to land. Now, while it lacks the laser-like 1-ping 'insta-death' (tm) of some of the cannon armed a/c, it is still a brutally efficient killer. You actually have to *learn to aim* your guns and concentrate the hits at a point, not just spray haphazardly in the hope 1 shell will do the killing. The 6x50cals is a real mans' gun package
As for convergence, its a personal thing, but you will find you will need to concentrate them all at one point, rather than spread em out. Much more effective IMO. Also i set mine all at 325. You want to set them at your typical firing point for your flying style. There's a temptation to set them out at 450 or 500, as most BnZ in the pony, and they want to fire as soon as possible, and rarely have the luxury of closing in from dead astern to under this range as your intended victim normally sees you coming miles out. I film most of my engagements as habit, and one thing i noticed on reviewing them, is actually most kills are from high-AOT snap-shots at fairly close range (around 300). But that's more a function of my style - YMMV.
Now to your climbing tactic. Lets put it on the table: the mustang is outclassed by numerous a/c in the raw climbing ability department. So why anyone would rely on it as a valid and sure-fire tactic is beyond me. ALWAYS fly to your strength which exploit your opponents weakness.
Trying to simply out climb your average MA opponent in a mustang is a sure-fire trip to the tower. Now lets also be clear that there's two types of climb: sustained, and zoom. Relying on sustained climb rate is not a sure-fire safe tactic. It may work against some a/c and sometimes you may just be lucky, but in an even-E climb contest the mustang will loose on average more than it will win. To be successful at that tactic you need a clear and decisive climb rate advantage. Zoom climb on the other hand the mustang can do just as well, if not better than most a/c in the arena. Relying on this tactic is bread-and-butter stuff for the Energy Fighter and/or BnZ'er. The key is correct relative Energy assessments (easier said than done most of the time).
Flaps may delay the inevitable stall, but they also suck E out of you in the form of drag. Use them sparingly and use them appropriately. At the top of your hammerhead or in a scissors where an overshoot would be deadly etc.
The Pony is a brilliant Energy fighter. Perhaps not as good as the 109K or the spixteen (luckily no-one flies the spit 16 as an E fighter). Ill dispense with the usual "you need alt advantage to be successful" drivel, as thats really standard advice for any engagement, so we'll take that as read. There's no one-way to engage all a/c. Some will require an aggressive high-speed angles fight, while others may require a more patient and progressive E-attrition style of fight. The pony can do both pretty well, though the later is lots harder to do than the former.
The pony shines at high-speeds, if you can keep the fight nice and fast where it counts the pony will reward you in spades. Its a extremely maneuverable a/c at those speeds thanks to its brilliant instantaneous turn rates. More than capable to taking on spitfires, zekes, nikis, 109s etc in a high-speed turn contest. Just don't get it too slow, or you'll be meat on the table. This means always leave a buffer of alt under you to keep the fight nice and fast if you want to play the high-speed angles-fight game.
The E fight, well, its the same advice for any a/c, so i wont get into specifics here, as i don't think theres anything particular you should know to E Fight in a pony with that doesn't apply to any other a/c.
Lastly note the B and D have quite different characteristics. The B is more stable at slow speeds, and is a monster above 15K - it really is extremely capable up there, but is let down by the 4x50cals. The D is in its element between 6-13k. This is where its strengths are at their strongest relative to common MA a/c. The 51D is slower than a La7, Tiffie, and Yak 9u below 5k, and slower than the 109K4 at all alts while it has WEP, and slower than a 190D below 7k. The D gets outclassed by the B above 15k.
