I was doing some reading into the D-Day landings and I noticed that units were always tasked with destroying certain gun emplacements, but would it not make sense to turn the guns on the rest of the inland defenders once the emplacements were cleared? Maybe I am misunderstanding, but it seems like it would be a win/win for the Allies, utilizing "free" artillery that they did not have to carry ashore. I realize that concern would be having the same emplacement used again if a German counterattack was successful, it just seems like a waste to destroy the barrels and breeches of perfectly good weapons.
World War two artillery tactics are complicated. It's not a matter of just blazing away over open sights at an enemy the gunner himself can see. Checkout this post by /u/vonadler where artillery tactics are discussed in pretty serious detail. To summarize a bit, you need complex fire control equipment and for trained cannoneers to know the ballistic properties of the guns you are firing very well indeed. The Allied troops on the bluffs would have mostly lacked this expertise with captured weapons.
Additionally, the Allies in Normandy already possessed tremendous firepower in the form of naval gunnery which could range many miles inland. Spotting targets for the guns was the main difficulty rather than any lack of firepower. Once inland, artillery firepower was already a strong suit of allied military units, and they didn't really need captured weapons because of the robust output of American industry.
Lastly, guns aren't much use without the correct ammunition to fire, and captured weapons usually are not compatible with the supply chain of the capturing forces. The Allied supply chain was designed to be as simple as possible with as few different types of ammunition and vehicles as possible. Introducing captured weapons and providing for their re-supply would not have been worth the trouble.
A few considerations:
Crew familiarity and command integration. It is simply easier at the macro level to not try to make crews use weapons they are unfamiliar with in the heat of battle with the instructions in another language. And whose performance is not as universally known as their won weapons. Especially when the great armada of naval guns were still right there.
Many simply couldnt. Many of the bigger weapons were in concrete blockhouses in fixed positions like this or this. That meant a field of fire focused almost entirely on the beach and thus of limited value once captured.
Aside from what's been mentioned, it bears noting that the Allied armies also had enormous numbers of field pieces as well. Each US Infantry Division for instance had three battalions of 105mm guns and one of 155mm guns, which was often supplemented by another battalion or two of 155mm guns from the Corps reserve. In the St Lo sector they effectively outmetalled the German heavy artillery by a factor of around 3:1, which is why the Wermacht relied so heavily on mortars for the defense of St Lo - their big guns simply couldn't compete with the volume and accuracy of the US artillery. British artillery was similarly plentiful.
The real issue in fact was ammunition - the US Army consumed so many shells that some studies in fact claim that they fired more weight of high explosive shells than the Soviets despite the latter having many more guns!
