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I pick up a used Model 37A 410 shotgun. I have not tried to fire any 410 shells out of it yet but I did chamber a once fired (out of rifle) 303 british case with spent primer still intact. It closed on the 303 case just fine and extracted/ejected just fine. The problem was I could not pull the hammer back with the 303 case in the chamber. With no case in the chamber the hammer will pull back, although it is a heavy pull. I am hoping to try and fire some 410 shells out of it next weekend but if I have the same results where the hammer can't be pulled back with a shell chambered, what might the problem be?

Edit: I wanted to add that with the action broke open the hammer will allow full cocking and repeat that with action closed on empty chamber the hammer will allow full cocking. It is only when a 303 brass case was inserted in the chamber and the action closed that the hammer would not move back to cocking position.
 
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Not familiar at all with a .303 fitting a .410, but you may want to pickup some .410 snap caps and try that before live ammo.
I guess it's a thing to blow out 303 brass to make improvised brass "hulls". I will look in to snap caps but I am guessing somebody familiar with the firearm might know what the problem is.
 
Your 37A is actually a Cooey Model 84.
Winchester bought the Canadian manufacturer in the 60's and reintroduced the Model 37 a few years later, but they used the Cooey single shot and simply re-branded it as their Model 37.
The 37A and the 370 were both built in Coberg.
Production of real Model 37's ended around 1963, IIRC.
That condition with the hammer doesn't sound right.
Before you buy any ammo for the gun, I'd have a competent smith check it out.
It almost sounds like there's a broken part inside.

Dean
 
I pick up a used Model 37A 410 shotgun. I have not tried to fire any 410 shells out of it yet but I did chamber a once fired (out of rifle) 303 british case with spent primer still intact. It closed on the 303 case just fine and extracted/ejected just fine. The problem was I could not pull the hammer back with the 303 case in the chamber. With no case in the chamber the hammer will pull back, although it is a heavy pull. I am hoping to try and fire some 410 shells out of it next weekend but if I have the same results where the hammer can't be pulled back with a shell chambered, what might the problem be?

Edit: I wanted to add that with the action broke open the hammer will allow full cocking and repeat that with action closed on empty chamber the hammer will allow full cocking. It is only when a 303 brass case was inserted in the chamber and the action closed that the hammer would not move back to cocking position.
So whatever happened with this gun?
 
So whatever happened with this gun?
Nothing yet, I did fire one shell out of it and it functioned fine. It's another project put on the back burner, I have a bunch of those projects. I did pick up some items including new hulls to reload for it so I can put off trying to find brass cases that will work.
 
So the hammer just started working on its own?
The hammer works with nothing in the chamber and it works with 410 shells in the chamber. it will not work with the 303 British case in the chamber. The shotgun will close on the 303 case but I can't pull the trigger back far enough to cock it.
 
I know that you're doing what many others have done. I sold an old .410 years ago for exactly the same purpose.
I'd measure rim thickness. I'm thinkin that the gun isn't completely locking up and won't allow the action to be cocked. A protruding primer maybe, too?
 
I know that you're doing what many others have done. I sold an old .410 years ago for exactly the same purpose.
I'd measure rim thickness. I'm thinkin that the gun isn't completely locking up and won't allow the action to be cocked. A protruding primer maybe, too?
The Cooey / Winchester 37A shotgun is well- designed, in that the mechanism will not allow the hammer to be cocked, unless the barrel latch is fully engaged, insuring that the action is fully closed. You have answered your own question, by successfully firing the proper ammunition in the gun. The 303 case is obviously too large, preventing the gun from fully closing.
 
303 rims need to be thinned for .410. They are usually too thick.
I believe that is the issue. Haven't tried it yet but I was going to put sandpaper on a board with hole in it and try to thin the rim that way. I also picked up another foreign made 410 break open shotgun. I was going to try it with untouched 303 brass, maybe the tolerances will be looser on the foreign made shotgun. Thanks for reminder to try that.
 
I believe that is the issue. Haven't tried it yet but I was going to put sandpaper on a board with hole in it and try to thin the rim that way. I also picked up another foreign made 410 break open shotgun. I was going to try it with untouched 303 brass, maybe the tolerances will be looser on the foreign made shotgun. Thanks for reminder to try that.
I just tried an untouched 303 case in the foreign shotgun and it must be even tighter than the Winchester action because I couldn't get the action to close on the 303 case. I am going to keep my eye out for 410 brass hulls to come back into to stock.
 
303 rims need to be thinned for .410. They are usually too thick.
I went ahead and chucked a piece of 303 brass up in my cordless drill (squished the neck in the process) and used a flat file to shave off some of the inner rim (opposite side of the headstamp) I was able to get the Turkish shotgun to partially close on the case but not enough to set the trigger. I took a little off the headstamp side of the rim and same results in the Turkish shotgun. I dug out the Winchester shotgun and tried the 303 case in that shotgun and it worked. I was able to pull the hammer back all the way and pull the the trigger to release the hammer. I guess if I kept filing on the 303 case I could probably get it to work in the Turkish shotgun too. It took a lot of filing and the case is worse for wear. The squished neck would blow out if I wanted to try and load it up. I may try filing one more case to a lesser degree and see how much filing it takes to get it to function in the Winchester. The Turkish shotgun has a looser choke and I wanted to run 41cal lead projectiles through it. The might run through the winchester but I think it has a tighter choke.
 
I went ahead and chucked a piece of 303 brass up in my cordless drill (squished the neck in the process) and used a flat file to shave off some of the inner rim (opposite side of the headstamp) I was able to get the Turkish shotgun to partially close on the case but not enough to set the trigger. I took a little off the headstamp side of the rim and same results in the Turkish shotgun. I dug out the Winchester shotgun and tried the 303 case in that shotgun and it worked. I was able to pull the hammer back all the way and pull the the trigger to release the hammer. I guess if I kept filing on the 303 case I could probably get it to work in the Turkish shotgun too. It took a lot of filing and the case is worse for wear. The squished neck would blow out if I wanted to try and load it up. I may try filing one more case to a lesser degree and see how much filing it takes to get it to function in the Winchester. The Turkish shotgun has a looser choke and I wanted to run 41cal lead projectiles through it. The might run through the winchester but I think it has a tighter choke.
Turns out both shotguns have full choke measuring about .395
 

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