We can't really guess- your pistol was made by Fabrique National in Herstal Belgium- but you did not tell us the model or caliber.
Nagazine tube (body) spring, follower, base plate, lock plate.
for magazines try: edssportshop.com
The best way to go about it is to remove the floor plate, then take out the spring and follower. If you're having misfeeds with that magazine, the most likely culprit would be either the follower, spring, or perhaps both, and it may be necessary to replace them.
See the link below
You are describing a detachable magazine, such as used by the M9 pistol or M16 rifle.
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100-800
gun shop, want ad, gun show
Follower, Floor Plate, Magazine Tube, Magazine Spring, Retaining plate, Locking Block Plunger.
Need complete sn and full descripiton.
Try gun shows, gun shops, Brownell's, e-gunparts.com
FN Herstal simply means Fabrique National, Herstal Belgium- a company name and city- but you are probably asking about the FiveseveN pistol made by them. It will depend on the backstop and design for the range. In an indoor range, this is usually a metal bullet trap. The civilian ammunition available for the FiveseveN is not as powerful as the restricted military ammo, and should be OK on MOST pistol ranges- but check with the range to be sure.