no
End use, fit and how much money you have
.223 Calibre although .223 Will fit on a 5.65 mm, a 5.65 mm Will not fit in a .223 calibre chamber.
Magazine, not clip. Also, yes, it appears that the R-15 can take STANAG magazines (which are the magazines that the AR-15 rifle takes).
No. If it does not have 5.56 on the barrel then use .223 ammo.
The second electron shell can hold a maximum of 8 electrons.
Made in Bayonne, France, those are called bayonets.
No
No they won't fit. If you force it, it will ruin the rifle.
No, it does not.
The question is vague, as shell can mean valence shell or inner shells that can also fit atoms via 'sharing' for transition metals, however, the number of atoms that can fit would determine on the number of electrons and vacant spots available for sharing on the outermost electronic shell. (Assume the question in mind is for covalent bonding; does not apply to ionic bonding.)
IF the 13 year old is a novice shooter, a bolt action .22 is a good starter gun. Relatively inexpensive, very light recoil, it requires a positive action to loadsuccessive shots, and by simply opening the bolt, the rifle is taken out of action. However, my 13 yr old granddaughter has shot with me for several years, is a competitive rifle shooter, and uses a AR-15 in caliber .223. It is important that the rifle fit the shooter physically- beyond that it will depend on the build and experience of the shooter.
Two electrons can fit into the first orbital shell of any atom.