I have lots of cut bodies here if you want to tackle removing a charger bridge and reinstalling it. It's not a job for the faint of heart.

Your No.4 has throat erosion from cordite which is causing the keyholing. It's not the muzzle. I'd take a wild *** guess you're using boat tail projectiles. Switch to flat base and that should cure it.

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Almost forgot. Go to Sarco and get one of the RFI SMLE barrels while they still have them. They are excellent. I've installed a ton of them with nary a complaint.