One of my officer had a M870 with a slug barrel and it would not throw buckshot well enough to qualify with.
Express ars I reliable junk. I've seen them fail right out of the box. Get the 870 police model if your life depends on it.
Every single trainee I have ever had (hundreds of professional warfighters) have fired one full magazine out of a pump 12 gauge with a pistol grip and put the stock back on. Every single one
Some years ago when the flood of used 870 police turn in shotguns were on the market I bought two. One had a 18" barrel,the other a 21" barrel. Brass bead front sight. I figure if they worked for the officers who'd been carrying them,then they should work for me. Price for one was a whopping $195 and the other was $225 so in essence I got two tested 870 pump action shotguns. And both came with the two shot extended mag tubes. Got bored one night and stripped the wood on one and sanded the wood down. Sealed the wood with sanding sealer. Then lightly sanded the wood and about 6 coats tru oil. Like working with my hands and both the buttstock and forend came out nice. Found two small limb saver slip on butt pads. The origainal butt pads had ossified into rocks. On went the limb savers. Tests with both 4/0 buckshot and 1 ounce slugs showed me that the limb saver butt pads were worth the money. Frank
Standard buttstock & an 18” Barrel will absolutely do what you need. If corners are too tight in your house for this setup to work, you should be wielding a handgun anyway.