The whole curtain rod thing is a big fat red herring, concocted by the conspirators as part of Oswald's sheep-dipping. Oswald knew he was the designated patsy and his handlers instructed him to take a long package into the TSBD. Whether it contained a rifle or not is irrelevant; it only needed to implicate Oswald to the rifle found in the building.
The conspirators got Oswald a job there, they gave him a whole empty floor to set up shop, they gave him a job as a false defector and portrayed him as a commie sympathizer, they acquired a MC rifle and took some shots into a swimming pool to create CE 399 (but forgot to sight in the scope), wiped all their prints off the rifle, took the MC and a handgun (which would become the other murder weapon) over to Ozzie's house and took some pics with him holding the murder weapons and some commie literature (using 2 cameras). This left the conspirators (DPD) with 2 options:
1) Leave the rifle with Oswald and let him smuggle it into the TSBD, disassembled in a paper sack.
2) Have Oswald pretend to smuggle the rifle into the building but have someone else actually do it.
If (1) is true then this is how Oswald must have carried the paper sack with rifle parts in it:
Clearly, the paper sack found at the scene was not long enough to carry the disassembled MC rifle. The conspirators would also be relying on Oswald not getting caught reassembling the rifle on the 6th floor and setting up the SN. No way. Especially since the MC was only planted to frame the patsy, not to take any shots with. The only time the MC was used was to create CE 399, and not by Oswald. So (1) is out and someone else must have smuggled in the rifle.
Whoever took the MC into the TSBD, must have had a key to the building and brought it in discretely and off hours. Then they had plenty of time to wipe off any prints but they likely used gloves to handle the rifle in the 1st place before ditching on the 6th floor under some boxes. They also set up the sniper`s nest and placed the 3 hulls in a tight group next to the window, which Fritz thought looked so contrived that he picked them up with his bare hands and later tossed them back onto the floor for a more believable in-situ photo of the crime scene.
Otherwise, Oswald never touched the rifle again after the BYPs and he didn't smuggle in the disassembled rifle in a paper sack, reassemble it on the 6th floor, ditch it, later retrieve it and take 3 shots at the POTUS, ditch it again and not get a single fingerprint on the rifle or any nitrates on his face. The conspirators would never have relied on one unpracticed shooter using a piece of crap rifle with a wonky scope. Would you?
Face it, this was a conspiracy and Oswald was the designated patsy. He never took a shot because only token shots came from the TSBD (probably from a Mauser) to incriminate patsy Oswald. He just hoped he wouldn't get double-crossed by his handlers, the DPD and Jack Ruby.