RE: winchester model 50 shotgun
Mauser 06...Many older guns have thin barrel walls...That is one reason why you should not shoot steel shot in it...The steel might deform the barrel and create a bulge in the region of the choke...
As for the loose buttstock, remove the buttplate... There MAY be a screw slot there that you can tighten with a large screwdriver and fix your problem...If not, take it to a gunsmith and have him figure out how to tighten it before you shoot it much more...Shooting it as is might damage it or possibly even crack the stock...