To attach the trench mag to the Gew 98, you removed the Magazine Floor Plate, Spring and Follower.
The Trench Mag is NOT a Magazine; it is a Magazine Extension. It EXTENDS the Magazine which already is BUILT INTO the Gew 98.
By itself, it holds ZERO rounds because it has no cartridge retainers in the form of Feed Lips which, in the Gew 98, are built into the receiver.
The fitment for attaching the thing is very like a regular Floorplate, just hollowed out and with a long Box below it, Follwer which is partly a hold-open (Gew 98 did not have a Hold-open like the Kar98k).... and this humungous long zigzag spring which shoves ALL TWENTY-FIVE rounds up into feed position.
You COULD mod one to fit onto a Kar. 43, but you would wreck BOTH in the process.
Please believe me on this: I OWN an original Trench Magazene and I have owned a Kar-43 in th past.
Not worth it.
Probably easier to mod an FG-42 mag...... if you can even find one. ZB-26 mag MIGHT be a possibility.
But in this Free Country of ours, NONE of these are legal.
And for THIS kind of "freedom" we fought 2-1/2 World Wars and the Cold War? Gimme a BREAK!
