It appears I'm going to need to postpone accuracy testing of my U.N.C.L.E. carbine (i.e., "The Acid Test Affair") due to unresolved technical problems: Unfortunately, it's exhibiting some failure to feed issues. The ammo I used last Friday, 124-grain FMJ, would angle up and contact the underside of the barrel hood instead of flowing into the chamber. At first, I suspected iffy spring tension, so I switched to a standard 8-round magazine, but no joy.
Other times, the carbine would fire and eject successfully only to fail to chamber a new round, leaving not only an empty chamber, but a fallen hammer as well. Such behavior is indicative of short cycling, or bolt over, which suggests the slide is unable to cycle completely to the rear, prohibiting reliable extraction/ejection and cocking the hammer before beginning to rebound. The culprit? Too much tension on the recoil springs.
Now, here's the funny part. I'm using unaltered P38 springs in an attempt to compensate for the increased chamber pressure caused by the extra length of the carbine barrel, thereby preventing excessive battering of the frame. Incidentally, the springs in question are the same ones which reliably cycled the Hornady Critical Defense and Cor-Bon Pow-R'-Ball ammo some months before as chronicled in "The Hunter Thompson Affair."
Over the weekend I swapped the standard P38 recoil springs with the trimmed springs I typically use in my pistol variant and returned to the range over lunch yesterday, only to discover the pistol still malfunctioned as before. My next step is to re-do the test using the springs from my Walther P5. The sweet spot has to be somewhere between the length of my P5 springs and the trimmed ones of the U.N.C.L.E. pistol variant.
Sadly, there's one more tragic aspect to this tale. The four set screws I've been using to anchor the hand guard on my carbine barrel are insufficient. Despite milling the divots into the barrel and using a thread locking compound on the set screw threads along with Brownell's Acraglas gel, the inertia generated by the slide slamming into the barrel every time it returns into battery jolts the hand guard from the barrel. On possible remedy I'm considering is to affix some nylon/phenolic contact material to the face of the recoil spring guides in an effort to absorb the shock.