Gun is restricted to 3 rounds by design. It has no mag plug, that is where the gas system is.
Best left to a trained gunsmith.
Remove the magazine tube cap. The plug is inserted inside the spring, and cap replaced. You may want to have a gunsmith do this. It will take less time for him to do it than it will for you to find the springs that got loose.
To remove the plug from a Winchester 240W Western Field 12-gauge shotgun, first ensure the firearm is unloaded and pointed in a safe direction. Open the action and locate the plug, which is typically found in the magazine tube. Use a suitable tool, such as a screwdriver, to unscrew or push on the retaining cap or plug, then carefully pull the plug out of the magazine tube. Always refer to the owner's manual for specific instructions related to your shotgun model.
The only way to know when a Winchester 1300 shotgun was made is to take it to a gun store or an appraiser. The first 1300 shotguns were made in 1981.
for safety sake take it to a gunsmith
You can't, not without machining expertise, or gunsmithing
Not anymore but I hear you can take a 1300 barrel and have gas ports added and that should work.
You need the services of a gunsmith
Take the cap off and point the gun doun and shake it and it should come out mine did
You can get the manual from this page with some basic info. http://www.winchesterguns.com/services/index.asp
The "plug" is just a stick slipped inside the magazine to limit the amount of space, but there's a big long spring in magazine too, so be VERY careful when you take the cap off the muzzle end of the magazine tube or stuff's going to go flying. (and save the stick, you may want to put it back some day)
Not sure how old this request is. I shoot this shotgun, can help you dis-assemble. Bill 318-268-1514 Shreveport, LA