... Now, having some kind of M256 gun co-axially located in a turret could give a nasty surprise. A current circa 2014 A.D. HEAP round will penetrate armor class 66, and a Soviet/Russian Reflex missile fired from an M256 equivalent gun will penetrate armor class 113, according to figures I found on this board and Wikipedia. The current M1/Challenger II/T-80/T-90/Merkava/Leopard II turrets do not require megawatts to operate. The pirates would not take that lying down, of course.
Real-world weapons don't necessarily translate well into Traveler-ese. The M256 120mm tank gun, for example, has a kinetic energy penetrator rated with a short-range penetration well in excess of 60cm - depending on the source. It's Striker rating would be something in the 48 to 49 range. Striker, however, gives a TL8 120mm hypervelocity KEAP round a penetration of 39 - which is about half that of the real world 120mm.
(I have not seen a source claiming it could penetrate 320 cm of steel, which is what a rating of 66 translates to.)
The Reflex (aka AT-11 Sniper), meanwhile, is a missile fired through a 125mm gun. It lists in Wikipedia with a penetration of 70-90 cm of steel - Striker ratings about 48 to 51. (That might be off a good bit, but I don't think anyone's claiming 18800 cm of steel, which is what Striker rating of 113 translates to. Striker ratings are - what is that word for when they follow that steep curve?) Without knowing the mass of the Reflex warhead, I can't say exactly what its' Striker equivalent would be, but to mimic the size and range of the missile, I'd ballpark it around a Striker rating of 42.
(It becomes obvious that Striker could use some errata work to bring its weapons in line with modern realities, but it's difficult to do something like that without throwing entirely out of wack most of the Striker-designed vehicles that are out there.)
Nonetheless, a 48 penetration, or even a 42, is quite enough to poke 120mm or 125mm holes in a ship's hull - assuming the target's within a couple or three kilometers. Much more than that, and an alert pirate-pilot can hit his drives and either cause the hit to be taken somewhere other than where you pointed or cause you to miss entirely.
(Actually, I'd suspect that he'd notice the rather daring phallic symbol attached to your turret well before he approached within your effective range, and would direct you to point it elsewhere. Whatever time it took to turn and point the thing back at him would give him warning as well. A person can react in as little as a quarter second, and a Supplement 4 corsair could move a third its length from a standing start in one second. That's not necessarily enough to get away without damage, but it's certainly enough to spoil any thoughts of aiming at a specific point.)
The problem comes after that: one can hardly say that the KEAP round or HEAT missile are going to do more damage than a laser or missile at that range. The good ol' standard 250 MW pulse laser has a hefty penetration of 79, 1000 cm steel, enough to punch through the ship's hull, through the ship, and out the ship's hull on the other side at that range.* The good ol' standard space missile is certainly not going to do worse than its planet-based cousin. Your cannon gets one hit that might maybe have done a bit of damage to the maneuver drive, more likely takes out a weapon or vents a bit of hydrogen, assuming the gamemaster resorts to the Book 5 damage table - during which time he's hitting his drives for all that's left to them. Then, as you say, he does not take it lying down: he rains missiles or lasers down on you from far beyond the effective range of your now-useless cannon.
It is, in other words, a rather expensive but nicely dramatic method of flippin' the hawk off before he kills you.
*
(Most of its space combat performance occurs at what would be considered extreme range for that weapon, where the penetration drops to a "mere" 63: 250 cm of steel. It is arguably correct to allow the fellow with the laser to have a -5 bonus on the damage roll if your roleplaying situation has him firing from inside 166 kilometers.)
