Surplus magazines are just that,,used and quite often slightly bent out of shape. Sometimes easily seen,,sometimes not.
More than a few I've had come thru couldn't be inserted into the rifle at all or w/o a healthy wack to seat it. The latter shouldn't be necessary.
The side walls being bowed outward slightly from being dropped on one of it's ends is the usual problem. Careful tapping them back into line with the spring and follower removed fixes that problem.
Filing the latch surface on the mag spine is the last thing in getting the magazine to lock into place securely.
It's usually not needed as something else is usually interfering with the mag seating full depth. Most magazines (used) have already been tinkered with a bit and have been filed at the latch surface anyway so little if any needs be taken off additional there.
Another trouble spot can be the trigger guard itself which has thin rails outlining the mag opening. These rails get bent too and interfere.
If you file the latch surface on the spine to get it to lock, but prematurely,,it only adds to feeding problems as the magazine is then too low in position.
Make sure the magazine latch itself is coming full forward and isn't chipped off making it unable to secure the mag at any depth.
The best way to see what's happening on a stubborn one is to remove the forend wood and fit the magazine so you can see the mag latch and the magazine interact and get them to lock with the magazine seated at full depth.
The mag feed lip tabs get bent too or 'adjusted' by a well meaning shooter, especially if someone is using commercial soft point ammo or reloads and not standard Mk7 ammo.
An extra magazine or two is a nice thing to have. I never considered an extra a quick reload type of thing,,some will not securely hold a full charge when out of the rifle anyway.
It's just a spare to me because the one in the rifle is detachable and liable to get lost or damaged.
The quick & handy reload are those 5rd charger clips. But I don't carry those much anymore to the range either. Paper targets never seem to run away on me.