I don't plan on shooting a lot of .45 Super out of my USP, so I'm just going to leave it in its stock configuration. Besides, I've looked into the matter, and from what I've seen based on comments made by other owners, the USP seems to handle it just fine right put of the box.

Heck, based on what I've heard, Federal actually used a USP to test proof loads and they actually put 6000 rounds of it through it before they bothered to send it in to H&K for evaluation, which was then determined to be free of damage by H&K. So yeah, if it can survive 6000 proof loads without being damaged, then I'm confident that it would hold up to .45 Super just fine sans modification.

I'd be worried that a 18lb recoil spring would render the gun unreliable with standard pressure .45 ACP and would result in inverse battering of the slide.

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Shooting Comfort is bilateral.