Couple thoughts. I've had both (a 6" Springfield 1911 that I ran .45 supers in, and many 10mms).
-HK USP is strong, but still not as strong as an all-steel 1911.
In what way is steel 1911 stronger than an H&K USP with regard to the use of 45 Super? Pressure handling? Recoil absorption? Long-term durability?
