Here is an example of the failure of bombing a small coastal emplacement at Normandy:
"SAINT-MARCOUF : THE CRISBECQ BATTERY
GERMAN COASTAL DEFENSE
The only heavy battery on the eastern coast of the Cotentin Peninsula, the Crisbecq Battery, was located 2.5 kilometers from the shore on a crest overlooking all of Utah Beach. From Crisbecq, the Germans could see and defend the entire coastline from Saint-Vaast-la-Hougue to Grandcamp.
Although it was never completed, the Crisbecq battery was the keystone of this portion of the German Atlantic Wall with three, long-range 210 mm cannons and a garrison of 400 men. The Allies dropped over 800 bombs on Crisbecq between April 19 and June 6, 1944. This unrelenting aerial attack climaxed on the night of June 5 when 101 four-engine bombers unleashed 598 tons of explosives on the battery. On June 6, the surroundings were unrecognizable, but the guns were still intact.
At 6 am on D-Day, as GI’s were landing on Utah Beach, Crisbecq opened fire, sinking an American destroyer.
The battery held out for several days, despite shelling by U.S. battleships and attacks from the American Infantry in hand-to-hand trench combat.
To repel the Allied assault, the German commander of Crisbecq radioed to the Azeville battery and requested that it fire on his position. Crisbecq was finally taken at 8:20 am on June 12, after the German commandment ordered its troops to evacuate to La Pernelle, between Quettehou and Barfleur. The fierce German resistance momentarily halted the Allied Advance to the north. "
Just pulled that off of a google search on D-Day. I’m not trying to be a chump with this post, just noting that “bunkers” were built to be hidden (maybe not all of them) and to withstand such an attack even if discovered before ground operations. I mean if 101 heavy bombers in one mission the night before D-Day have little impact why should a SBR inflict damage on a bunker?