I believe you are referring to the lifter. This part is not intended to keep debris out but to literally lift the next round from the mag tube level up to the barrel level so it can be chambered. Bottom eject guns like the Ithaca M-37 and the Browning BPS do not have a lifter in the traditional sense. The lifter can be skeletonized on some guns or have a slot cut in it to help clear a jam.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball