won't stay closed
 

My SBT opens up when fired it locks up tight with no play but opens up when shot??

That is called a ''Self Opener'', Some Guys payed extra money for that ''Feature''
With all Kidding Aside, What make of S.B.T. ARE YOU TALKING ABOUT?

Parker SBT SC grade

What position is the Lever in when you Snap the gun closed, and how long have you owned it? Did you or some one replace the ''WEAR'' Plate on the locking bolt? If the answer is yes, it may not be fitted properly.

It is usually a bolt bite issue. Or a weak/broken top lever spring. But usually the bolt.

thanks I'll contact previous owner and find out as much as I can about previous work etc

forgot to ask what is the bolt bite

My SC does that on occasion with 3 dram 1 1/8 oz loads. My shoulder can’t stand a steady diet of 3 dram shells so I am delaying getting it fixed. I load very light 3/4 or 7/8 oz. Loads and never have an issue. Score is not a big issue with me but comfort and pain is.

I assume an SBT "wear plate" is something along the same lines as the locking plate in something like a 101, Citori etc. and not shaped like the locking block in my Perazzi's?

Parker bolting from a 1930 catalog.

Attachment 123736

The fit of the replaceable "bolt plate" is critical as well as the strength of the top-lever spring.