A1/A2 receiver extension has a shoulder on the it to lock it to the back of the receiver, while on a Carbine receiver extension, have to use the castle nut to lock the tube to the back of the receiver so it does not thread back out isntead.

If your butt stock has the U slot to lock the barrel extension in place so it does not spin back out, then castle nut become aesthetics, but would still it on the tube for a gap filler at least.

As for plastic receiver tension and plastic receiver,  rosin powder between the threads would increase the tension of thread to thread, that even with the castle nut only snugged gently to not strip the tube threads, would still keep the tube from rotating.

As for something like 242 loctite, would weed away from such use on plastic parts,

Not Recommended For
Use on plastic parts, particularly thermoplastic materials where stress cracking of the plastic could result

Stock will work in that sense, but since I use my shorties for phone booth type fire fights with the butt stock more upward on my shoulder, not digging the angle of the pistol grip of that stock.

On shorties since I reduce the LOP on the stock to keep the weapon short for working around corners, I use less angled pistol grips since it puts the grip in the correct angle for the way your going to use it instead.

Hence the steeper angle of say standard grip works well for prone and standard use when you have a full lenght A2 type stock, but as your shorten the LOP to keep the rig shorter, this has your elbow more outward, and requires the pistol grip to be more upright for a better grip isntead.

Think something like this,

Now you lost me, since 27" barrel is screaming long range rig to start with, and one that your not going to free hand shoot much as well.

Hence barrel at least heavy match to keep flex to a minimal as it heats up, and then you need to back weight the hell out of the stock to get the rig to balance next with that much barrel weight up forward.  So here, I tend to favor the Magpul PRS stocks, since they do allow for the needed adjustments, and have enough placed that I can add in weight to  back weight balance the rig to balance it. Also, this allows for the needed pistol grip to be added,  to get the correct  palm plant and pitch angle the shooter needs as well.
Weight wise, would never use anything nylon on the rig, since the rig is built dam near to the edge of F class weight limits, to keep felt recoil to nothing for faster follower up repeat shots.

Simply, front end weight rig off the bench is a PITA to shoot, since it does not want to hold on target to begin with.  By weight balancing the rig with the needed stock weighting, it will hold on target without problems instead.

So on that note, if you are building her a light weight rifle, then do so with a thin 16" barrel since you are trying to keep the weight down.  Its going to have more felt recoil due to the light weight, but its the trade off of her being able to free hand shoot it.  
If your building her a bench/bipod rig, then don't spare the weight, since she will be more than fine humping the rifle as needed from location to location isntead. Hence here, we want the rigidity of parts, so no plastic lower receiver or receiver extension isntead.

Cute build, but the long pencil barrel really negates the longer barrel isntead.  Hence will gain some speed with the longer barrel, but since it will heat up faster as well, will open up the groups faster instead.  So isntead of a pencil barrel, better to just  long flute the hell out of a standard barrel to loose some weigh that way, while the none fluted areas help to strengthen the barrel isntead.

As for the French Chassepot bayonet ,  standard bayonet on a standard 20" will cause group increases, and lets just say that the long pencil barrel on the rig is not going to hold up long is anyone tries to it the use the French Chassepot bayonet on it (short of bending the barrel so you can use it to shoot around corners afterwards), and bayonet  in place just going to open up group sizes as well .

And what the Hell, girls rifle painted blue-not pink, and not one "Hello Kitty" sticker on it.




And the pistol to go with it.