I bought a browning gold years ago and the thread on the end cap was damaged. The shotgun worked ok until I fitted a extended magtube,getting a few jams.
Really needed to get the thread on the magtube squared up.
I suspected the ratty thread was causing a bad fit up to the magtube extension.
I think the cartridge rims were sticking inside the magtube,I noticed a 1/8" 3mm gap due to damaged thread.
I did a little research on the net and found out the browing gold 12 gauge magtube thread was M26 x 1.5 square thread.
After searching the net I discovered this size nut is almost impossible to buy.
A couple of friends are machinist,and were reluctant to make me a M26 x1.5 square nut.

Step 1: Get You Suppies

You will need:-
2 x M26 washers sourced off ebay.
Duck tape( but a jubilee clip would better).
Polymorph moldable plastic.
Coffee cup full of hot water.

Step 2: Pepair the Polymorph.

Put coffee cup half full of hot water and polymorph in the microwave for about 2 minuets.

Step 3: Set Up the Washers.

Tape a washer with duck tape touching the thread.
A jubilee clip woud be a better answer,but I couldn't find one.
And be careful not to damage the tube if using a jubilee clip.

Step 4: Start Moulding.

Take the polymorph from the coffee cup with fork,be careful not to burn yourself.
Sandwich the polymorph between the two washers.
Screw the end cap down to get it all square.
I gave this a blast with hot air paint stripper gun for good measure,as it was starting to harden.
Be careful notbto burn polymorph with heat gun.
This was my second attempt, my first was not square.

Step 5: Start Filling.

Give the polymorph about 30 minuets to cool down.
Fill the magtube with some paper kitchen towel,this stops allumium powder getting in your action.
Wind up your new plastic nut to the lowest point of the damaged thread.
Get a nice fine file and start filling.
Use the washer as guide to keep it square.

Step 6: Unscrew Polymorph Nut.

Vacum the magtube and remove kitchen towel.
Make sure no allumium dust as got into your browning.
Wind off the polymorph nut.

Step 7: Final Check.

Hopefully your magtube end cap will fit.
Assemble your shotgun and think how much money you have saved.