Reloadron
Contributing Member
The Remington 511, 513 and the 521 all use the same magazine. Last Christmas I handed down my 511 to my grandson. I had two original factory magazines which I kept as kids tend to lose magazines so I bought a few of these after market magazines. They worked just fine. Finding original Remington magazines can be a challenge and the few I saw a few years ago were $50 and up. The 521T is one sweet rifle so enjoy it. 
Ron
Ron