Good afternoon!
Looking through the archives at random for this group, I find there is a lot of concern for the strength of "brass" framed receivers. The question is asked, as I did the other day, what would the proof test chamber pressure be for the .45 Colt in the Uberti Henry. Sensibly, some allowed that the SAAMI pressure limit for the .45 Colt round is the answer. I have not found any discussion of Bolt Thrust here and this measurement may make a lot more sense in regards to how much a brass framed rifle gets stressed as it discharges a round.
Parker Ackley was a well known gunsmith located in Salt Lake City in the late 50's and into the 60's. He wrote several books, one of which was "Volume One: Handbook for Shooters & Reloaders". His specialty was the development of wildcat rifle calibers which he created and tested and built rifles for. He was also a contributing gunsmithing editor for "Guns and Ammo Magazine" for a number of years. In the handbook I mentioned above in a chapter entitled "Pressure" (page 138 and following), he talks about an experiment he did with an old Model 94 Winchester. Briefly, he removed the locking lugs from the action. Then he rechambered the barrel for his 30-30 Improved (less taper to casing walls than a standard 30-30). With the lugs removed from the action, he fired two standard factory .30-30 cartridges. Prior to shooting, he made sure that the chamber was clean and dry. The rounds he fired formed to the Improved chamber perfectly with no primer back out. Then he unscrewed the barrel one turn and fired two more rounds. The primers backed out equal to one barrel thread, but the cases did not back up against the bolt, which means that the brass case withstood the pressure. Two more factory cartridges were lightly oiled. The primers did not back out, but the shoulders of both rounds were blown forward the distance of one barrel thread. Thus the oiled cases did not adhere to the chamber walls but backed up against the bolt face. Last, he unscrewed the barrel two full turns and cleaned and dried the chamber. He fired two more factory rounds. The 1st casing remained tight in the chamber but the primer fell out. (He lengthened the firing pin to fire the rifle). He then oiled the case of the second round and fired it. The casing stretched and separated just above the base, indicating that it again did not grip the chamber walls and expanded back to the bolt face until it separated. He felt that this experiment proved that a clean, dry chamber will allow a cartridge case to grip the walls of the chamber and not move back against the bolt face in most cases, making the actual back-thrust pressure on the bolt face itself little or none. I think it is the back thrust of the casing on the bolt face, then to the toggle lock-up, that is the pressure we should be concerned about. The actual chamber pressure means nothing (if it is a safe load to begin with) as the barrel and receiver ring area contains all of that pressure itself.
I'm thinking that the brass framed Henry rifles that we are involved with will have little stress to the lock up area of the action itself if we keep the chamber clean and dry and the ammo that we fire in the same condition. Hot loaded .45 Colt ammo will tend to defeat the design strength of the action and destruction may come with blown chambers/receiver rings. I would bet that in cases like this, the actual movement of the casing back to the bolt face would be minimal until escaping gas from the split casing took over.
Have you guys read anything that could be added to this?
Regards,
.56/50 Iron