Hello, does the mosberg 1000 super 12 semiauto shotgun shot both 2 3/4 and 3 inch shells ? It show that it is chambered for 3".

The Winchester Model 1897 evolved from the Winchester Model 1893. The Model 1897 and 1893 were both designed by John Browning. The Model 1897 is an external hammer shotgun lacking a trigger disconnector. This means that the user can hold the trigger down while cycling the shotgun and once the action is returned to battery the gun fires. The gun itself is classified as a slide action pump shotgun. It was the first truly successful pump-action shotgun produced. Throughout the time period the Model 1897 was in production, over a million of the type were produced in various grades and barrel lengths. 16-gauge guns had a standard barrel length of 28 inches, while 12-gauge guns were furnished with 30-inch length barrels. Special length barrels could be ordered in lengths as short as 20 inches, and as long as 36 inches. Along with various grades and barrel lengths, the Model 1897 came in two different chamberings. One was the 12 gauge and the other was the 16 gauge. The shells should be of the 2-¾ inch or 2-? inch model. Any shells larger are not recommended. An average Model 1897 held 5 shotgun shells in the magazine tube. After including the one shell that could be held in the chamber, the average Model 1897 held a total of 6 shotgun shells. However, this would vary from grade to grade. When working the action of the Model 1897 the forend (fore grip) is pulled back, forcing the breech bolt to the rear which extracts and then ejects the spent shell while simultaneously cocking the external hammer by pushing it to the rear. When the forend is slid forward again, the breech bolt pushes a fresh shell into the gun's chamber and locks into place. This action of sliding the forend back and forth (pumping) is why the gun is classified as a slide action (or pump) gun.

Shotguns don't "sight in" remember. Shotguns don't "aim". They "point". That's where patterning your gun/ammo comes in. Just get a variety of shells from diff manufacturers and start shooting at cardboard targets AT THAT RANGE YOU EXPECT TO SHOOT. If the spread is less than 50% pellets on the expected target at that expected range, go tighter on the choke tube until it is. Be warned....that one perfect brand of shell you find today probably won't be working so well a couple of years from now. Be ready to repattern in the next couple of years. Nice gun, btw! Congrats!

Below is a chart of recommended shot size, gauge, choke and barrel length respectively : 2, 4 12, 16, 20 and Modified or Full and 24" to 32".

If the 870 is a Magnum model it should accept 3" shells. After making absolutely sure you have an unloaded shotgun, both chamber and magazine are empty, empty, empty, proceed with these few checks.

Assuming it may be a 2 3/4' model, have you double checked on the left side of the barrel near the receiver? There will be gauge and shell length stampings on the barrel. If you are jamming 3" shells in to a 2 3/4" shotgun you will get jams as you describe. Look at the empty shell casings if you have any and see if the shell appears to be damaged from the chambered and firing process. This could also indicate you have a improper shell length issue.
If you are shooting reloaded ammo make sure it has been properly re-sized, crimped and reloaded.
sloppy reloading of shotgun shells can cause jams as you describe. Improperly re-loaded shells can also cause leaks of bb's, (shot) in to the receiver and bore area.

Check for a loose bb. or bb's, (shot) in the receiver and chamber area. A lodged bb. in these areas will cause problems you have described.

Run your fingers along the barrel and visually check it for any slight bulge. If you detect or suspect a bulge do not shoot or load the shotgun until a gunsmith or the factory check the barrel thoroughly. If someone had loaded a 20, 16 or similar smaller guage shell in the chamber by accident and the gun was fired it will usually cause at minimum a stressed and bulged barrel. Many times a slight bulge in the barrel may not be visually obvious.

Most of the time if a 12 gauge shell gets loaded behind that one and fired you will have catastrophic failures, i.e. the gun can blow up and cause serious harm, blindness or even death in rare cases to the shooter. Shooters call this a "20/12" catastrophic failure. This type of accident is not to be taken lightly! this is why shotgun shells of different gauges are different in color normally.
Never ever, mix shotgun gauge shells in a pouch or box.

If you pump the shotgun too easily, (i.e. wimpy) it can jam up. Always pump, "rack" a shotgun with vigor. I'm not suggesting abusive action, just don't be slow or overly gentle with the pump action that ejects a shell and reloads the next one. Something related to this is if your action/slide bar is out of alignment or has become torqued out of it's proper alignment this will cause problems as you have mentioned. The action/slide bar is a long single piece of flat metal that is in alignment with the barrel and is attached to the front forearm of the shotgun. This is the flat piece of metal that actually makes the action open and close as you pump the shotgun. There is one on the left underside of the bore and forearm of the shotgun. When you pump the action you will easily observe it moving with your pumping action.

Check how the empty action feels to you. While dry-firing, (using an unloaded shotgun), does the action feel smooth or perhaps, gritty, sluggish, overly tight? If any of the later the you need to double check for debris, shot, excessive old lubrication, gumming of the action, a bent action/slide bar, etc. A normal 870 has a very smooth action.

Also, check the bore for any obvious nicks or obstructions that don't belong there. A good quality shotgun such as your 870 should operate best with very little lubrication. Excessive lubrication over time can cause gumming up of the receiver area. Also, improper lubrication products. Improper lubrication can cause problems you have described.

I am not a gun smith. I am simply an avid gun enthusiast. If you have any doubts as to anything that you observe while performing these basic checks on your shotgun, I strongly advise you to call the factory or take it to a local gun smith. Many gun shops will give your shotgun a courtesy check over and if you don't find a solution, many times they will within a few minutes. Do not be embarrassed to take the gun in to a gunsmith! Trust me, they will most likely treat you and you gun with respect and courtesy. Otherwise they won't be in business much longer.
If the gun shop fixes your problem make sure and purchase some shells and perhaps something else from them. We must support our small business firearms dealers and gun smiths

Hope this helps you!

You have a shell stop that is either bent or worn, I think there are 2 in there, would have to look. Go to Gunparts.com for the view and order parts, might find them on ebean. Hope th is helps.

Europe loads 2 1/2" and 2 3/4" shells for 16 gauge shotgun as standard and the US only loads 2 3/4" shells. If your shotgun chambers the 2 3/4" shells it should be OK. You can measure the chamber and see if it measures 2 3/4". You can also check your barrel for and bulges in it by looking down the barrel with a bore light shinning down the other end. A bulge should be able to be seen with the eye. If your gun has any bulge, do not fire it again. You will need a new barrel. But It should be OK and I would not expect their to be any bulges.

Thewre is a U shaped silver piece that is supposed to lift the shell up to the chamber so it can be pushed in. Is it missing or damaged?

Give it a good cleaning the extractor that catches the edge of the shell casing to pull it from the Chamber my not be working due to powder residue.

A ordinary 2 3/4 inch 12 guage shell equates to a European 70 mm. a 3 inch Magnum is a 76 mm.
Goof Luck.