I haven't had them expand that rapidly. The performance you had at 300 on the spiker is exactly the performance I have had at 150, twice. Once on a fallow hind and the other on a larger red spike. Hit high in the shoulder, bang flop found the jacket under the skin on the off side.
I have shot a red hind at 320, placement was behind the shoulder, in the crease and the bullet passed through. Smoked everything inside but took probably 30 seconds to expire, didn't go anywhere.
Shot a fallow hind in the neck at about 40 and it nearly decapitated it.
Shot a chamois in late summer (very light skinned compared to a winter bull tahr) doe at 500 through both shoulders and exited, dropped on the spot...and then rolled way down a hill.
The performance I saw more often than not was the dropped on the spot and jacket under the skin on the offside, didn't seem to matter the range if you hit that shoulder.
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