Well this isn't a conspiracy theory per se, because I don't have an alternative explanation.
I still don't think the morphology of JFK's cranial injuries are at all consistent with a bullet fired from a 6.5 Mannlicher Carcano. And I'm not the only person familiar with firearms and firearm injuries who's made this observation. Firearms expert, Massad Ayoob has said it more than once.
Briefly, the 6.5X52mm cartridge is slightly less powerful than the more familiar American 30-30, giving a nominal MV of 2200 fps with a 165 grn bullet. Besides it's relatively low power, the single most striking features of this cartridge are the unsusually high sectional density of the bullet (Ratio of diameter to length) and its thick copper jacket.
You can put a watermelon on a fence post and shoot it with the 6.5X52 cartridge / FMJ bullet combination. I've done it. The melon, won't get knocked off the fence post. It won't even jump. All you'll get is neat clean 6.5mm holes that look like they were punched with a cookie cutter. This is typical of non-expanding, slow moving bullets, with high SD's They do not transfer their energy to the target.
One of these bullets passed through the chest cavity of Governor Connally. This certainly had the potential to be a lethal shot as it was within what is considered to be the kill zone in human beings The Governor didn't die, partly due to the actions of his wife, Nellie, but mostly due the fact that the bullet punched a neat, clean 6.5mm hole in him with very little in the way of hemorrhagic shock and other collateral tissue damage. The Governer was actually propped up in bed, giving an interview very soon after the incident. This is entirely consistent with being struck from behind by a slow moving, monolithic bullet.
You can put a watermelon on a fence post and shoot it with anything in the centerfire 22 family (e.g. .223 Remington, 220 Swift etc.) and get very different results. These cartridges fire extremely fast moving bullets with low sectional densities. The watermelon will likely explode into pieces and even when it doesn't, a chunk will be knocked out of the exit hole the size of your fist.
The movement of the President's head (Frames 307-330) and the nature of the cranial damage were not consistent with being struck from behind by a slow moving, monolithic bullet. It is consistent with a much faster bullet with a lower sectional density.
Like Cofty, I've watched the documentaries which deconstruct some of the more inane theories surrounding this assasination. However when it comes to answering this question, the 'experts' resort to comparing the wounds inflicted by a 30-30 to those inflicted by a 6.5X52 because both catridges propel bullets of comparable weight at comparable velocities. (These comparisons are usually based upon photos in the book, Gunshot Wounds: Practical Aspects of Firearms, Ballistics, and Forensic Techniques. DiMaio, Vincent J.M.: Elsevier Science Publishing Co., Inc., New York, NY; 1985)
Problem is the 30-30 is a hunting cartridge, not a military cartridge and most firearms chambered in this round have tubular magazines. Consequently, this cartridge is loaded with soft nosed bullets 99.9% of the time. Apple and Orange comparisons don't answer the question