Re: give me your thoughts
I have no experience with pronghorn... But, I've shot two whitetails with a 223. First one was a 1 1/2 year old buck and was shot about 80 yards though the ribs. The 55 gr. soft point destroyed the lungs and exited. He made a short dash and collapsed as I was getting the encore lined up on him again. The second was a small doe a few days later.
The first shot hit a branch and put a quarter sized hole in her ear. That was at about 50 yards, she ran out to 120 and I dropped back on the last rib for a hard 1/4ing away shot, I knew she was hit but that the intended head shot went bad. At the second shot she humped up and ran back past me and down over the hill. I gave her a few minutes and followed up trailing her in the fresh snow, very little blood. Saw her bedded about 200 yards down the hill and finished her with a shot to the head, she was bedded with a log between us or I would have went for a safer body shot. My confidence was shaken badly by this point.
The reason the second 1/4ing shot was not very effective was that it hit her in the paunch, which was PACKED with browse and stopped the little pill before it carried into the vitals. A heavier round would have went into the lungs from that angle.
My long winded point... a 223 can be a very effective killer. It also leaves very little room for error. If she is comfortable with the 300, I'd stick with that.