The only conclusion I can come up with (and it is my own) is that it was done to send a message. In other words it would have been a warning to agencies that Oswald was not the killer and that they should be wary of making waves for the new regime. Sort of like the old mob trick of leaving a dead fish (Godfather sleeping with the fishes sort of thing). My theory-based on nothing other than my attempt as to understand why such a useless rifle was selected.
Why send a money order to Chicago for a weapon you could walk into a Dallas gun store and buy?
The postal money order was purchased when Oswald was on the clock at the Depository.
The Mannlicher-Carcano was sent to a post office box for A. Hidell which was not authorized for Oswald's use--and no one saw Oswald pick up anything from that box.
The weapon in question was a different model than the ad showed, and the lot of weapons offered was deemed defective.
Nelson Delgado testified to Oswald's repeatedly missing the target on the Marine range and getting the red flag, or "Maggie's drawers"--and he stuck to that testimony despite the FBI spending hours badgering him.
The curtain rod package was determined by testimony of Frazier and Randle and the FBI agents' measurements to have been 27"--the length of other curtain rods in the Paine garage, but too short for the longest piece of either model of Mannlicher-Carcano when disassembled.
The rifle scope had to be reshimmed before testing and then could not be adjusted properly according to Hoover.
The bolt was difficult to operate according to those designated to test it.
The trigger pull was smooth to a point, then rough, another source of complaint by the testers.
The skill of Oswald was doubted by Carlos Hathcock in this excerpt of Craig Roberts Kill Zone
Craig Roberts, Kill Zone, CPI, 1994, pages 89-90:
According to my friend, Gunnery Sergeant Carlos Hathcock, the former senior instructor for the U.S. Marine Corps Sniper Instructor School at Quantico, Virginia, it could not be done as described by the FBI investigators. Gunny Hathcock, now retired, is the most famous American military sniper in history. In Vietnam he was credited with 93 confirmed kills--and a total of over 300 actual kills counting those unconfirmed. He now conducts police SWAT team sniper schools across the country. When I called him to ask if he had seen the Zapruder film, he chuckled and cut me off. "Let me tell you what we did at Quantico," he began. "We reconstructed the whole thing: the angle, the range, the moving target, the time limit, the obstacles, everything. I don't know how many times we tried it, but we couldn't duplicate what the Warren Commission said Oswald did. Now if I can't do it, how in the world could a guy who was a non-qual on the rifle range and later only qualified 'marksman' do it?"
Craig Roberts is coauthor with Charles Sasser of One Shot One Kill on military snipers beginning with Carlos Hathcock as Chapter One. Roberts is a member of the U.S. Marine Corps Scout/Sniper Association.
The accuracy of the weapon was not the only suspicious element to the framing of Oswald.
He had no motive, admired Kennedy, never purchased any ammunition, was not witnessed carrying the weapon into the Depository or firing it (Brennan is a complete fraud, an imbecile).
The deputies who found the weapon observed "Mauser" on it, yet later that weapon disappeared. A .303 Enfield was handed down from the seventh floor or roof, and that, too, vanished.
We are left with a weapon without a fingerprint, and only had a palm print after it was taken to Oswald's body "for reference"--and the alleged print was not seen by the FBI: the FBI could not find a trace of a palm print or of its lifting by Dallas police.
And the clip seems to have disappeared, and the strap can't be accounted for--and Marina said Lee never practiced--until the FBI worked on her: then she changed her tune, again.
An additional problem is that the first two shots would have been obscured by the live oak tree--unless divine providence was doing the sighting--
Forty years on and all we really can be certain of is that Kennedy's brain escaped the stainless cylinder.