Gun is restricted to 3 rds by design.
No mag plug, that is where the gas system is.
No, one cannot add a mag extension either - again the reason being the gas system.
1400 is the softest shooting semi-auto shotgun that has ever been made. Very good gun, but it likes to shoot what is was designed to shoot. 2 3/4 dr equiv, 1 1/8 oz loads min will run this gun all day long. 1oz might run, I cannot recall if these were popular loadings at the time the 1400 was mfg or not.
Take down. Hard to for me share using this medium, and being honest it has been awhile. If I handled the gun, it would come to me, thinking about it I would miss a step, or not be clear.
It is not hard, just has some nuances like most firearms.
Harly Nolden here on THR most likely has the owners manual. His forum is down below. If he does not catch this thread, PM him and ask him.
Mr. Nolden is a super nice guy!
That 1400 flat works! It is not a SX1 or anything, still bone stock as it is , it flat works and the Winchokes are proven in my experiences as being great.
I used the 1400 in 20 from everything from teaching students, to skeet, 5 stand, Sporting clays, quail, doves...and loaned to folks that have used for same and includes taking deer.
Just shoot it, clean and lube it, and feed it more ammo. All it needs.