Surprisingly, not very much! HS Arms tested their .45 HS-45 pistol by firing a under-loaded round that did not exit the barrel then firing a factory round.

“precisely measuring of the pistol parts showed only a small up bulging of the barrel, resulting from the extremely high pressure loads.”.

Bullets getting stuck in barrels is not uncommon when shooting .22 Long and .22 Short rounds in a rifle due to their very low velocity and energy. I did think that in a pistol the pressure would cause more than marginal bulging.

Read the article here.