many years ago i bought a colt ar15  and haven't shot it alot. when i did i used the 20 round colt mags. when i bought the gun i also bought a dozen 30rnd mags. empty they seat ok but loaded they seem really hard to seat, i really have to smack the bottom of the mag. i also just bought some dpms mags [30rnd] and they are the same way. is this normal? it just dosen't seem there would be that much spring preasure to make them that hard to seat but i could be wrong.