Remington model 12 assembly instructions

Answer 1

I don't know of any assembly instructions but I suspect your trying to get the breech block back in ;)

With the front section upside down place the breech in it's location and slowly move the pump back. It will push it back but eventually when the pump is all the way back (if it has a button on the bottom behind the pump push it to let the pump go ALL the way back) it will go over the breach and catch it and bring it back into the receiver when pump is moved forward. It's of course one of those once you did it once it's simple but for someone that pumped it with the rifle broke open and it fell out, it's like oh s...t ;)

If that doesn't do it, post this on the Remington Society of America forums, there's a LOT of model 12 collectors who may describe how to do this better.