No shell

[ashoemaker]

my 410 M550 will not let new shell in chamber

[zippy13]

Could be a number of things...Rusty chamber, have you given the gun a good cleaning?
Are you trying to put 3-inch shells in a 2-1/2-inch chamber?
Are the shells factory or reloads?
Are you following the manufacture's recommendations? (loading errors happen more often than you'd think.)

[Salmoneye]

An "M550" Western Field?

[Edward429451]

Try different ammo before anything else.