Well, as long as the shot angle, wind call, rangefinder number etc are all spot on and your rest is ideal, knock em out!
But when a setup like this fails, and in time it will, don't ask yourself those nagging questions like "what if I had".
Instead, do the rare and unexpected. Share the experience and caution others that varmint calibers are not exactly ideal in the field on big game.
How many times do we have to see this?
Why in the hell would you not use a more suitable setup?
I'm asking that to anybody. Not just you
I'm not a proponent of using small calibers for big game at long range. However... any small caliber bullet moving at supersonic speeds will do as much or more damage as a broadhead from a bow at close range given the same projectile path through the quarry. To me, the bigger question is, how reliably can the hunter hit at distance? There's a point where very small differences in range result in a very large vertical dispersion. Laser rangefinders have made a huge difference, along with increased inherent accuracy in modern rifles and ammunition and more consistently reliable bullet performance across a wide range of velocities. Also, "varmint calibers" is a bit of an ambiguous term; is an 88 gr .223 controlled expansion bullet designed for big game moving at 2,200 ft/sec at a specific distance a varmint cartridge? Is it any less effective than an 85 gr .243 bullet designed for big game at 300 yards from a .243 Winchester? (No.)
We have learned a lot about the capabilities of specific bullet designs moving at or above a specific velocity against big game in the past decade or two, proven a lot, and disproven a lot. We aren't Robert Ruark buying a .220 Swift with 40 gr varmint bullets, hopping on a plane to East Africa, and then trying to shoot a warthog up the backside... and then being disappointed in the lack of bullet performance instead of recognizing that we (he) chose the wrong bullet and the wrong shooting opportunity from profound and prideful ignorance and thus writing off the idea of using such a cartridge/caliber on big game. A lot of advancement in this area is because people challenged the conventional wisdom, applied some rational thinking, and helped to advance the technology. Ruark was a great story-teller, not that great of a shot with a rifle (from his own telling), and based most of his conclusions about rifles, cartridges, etc., from his African hunting experiences with Harry Selby as his guide and mentor... and Selby was also a man of his time with significant experience with the rifles, cartridges, and bullets of
his time, not our time. Selby (and Ruark) were not fans of Roy Weatherby and his ideas around effective big game cartridges because they flew in the face of what was then the conventional wisdom. It's the old Jack O'Connor versus Elmer Keith view of what ballistic characteristics were best for hunting.
Unlike many other forum members, I believe that kinetic energy is an important factor when it comes to bullet performance and commensurate quick kills. This forum is not supposed to focus on the ethics of long range shooting, yet I believe (and I'm sure the vast majority of people on the board would agree) that at some distance with a particular hunter and rifle/cartridge the issue of ethics comes into play... but there is widespread disagreement on that distance.
Clint Eastwood's "Dirty Harry" persona had a great saying: "A man's got to know his limitations." Each hunter must know his personal limitations, in terms of shooting ability and gear capabilities, set a threshold based upon those limitations, and adhere to that threshold. If there's a desire to extend the threshold, then each hunter should honestly assess their shooting capabilities and their gear capabilities and make the proper adjustments. I personally believe that larger caliber, more powerful cartridges provide an increased margin for shooter error... but the margin is not that significant and the drawbacks of larger caliber, more powerful cartridges may create more error than they protect against.