Increased Pressure over factory actions

FRANK1669 · March 14, 2007, 09:42 PM

Hello all: a little hypothetical question for you. When rechabering a rifle to a different caliber the general rule of thumb is not to exceed the pressure of the cartridge the gun was designed for. But what about when you are going to a smaller case ? My knowledge of pressure comes from working with hydraulics so I may be way off here. Hope someone can clarify this for me. If you have a smaller diameter head on a cartridge it would take a higher pressure to exert the same amount of rearward force as a cartridge with a larger diameter head and assuming the same barrel contour was kept the barrel thickness would be greater than the original. Also the inverse would seem to me to be dangerous if you kept the pressure the same and went with a larger diameter on the cartridge head it would exert more force on the bolt.

Jim Watson · March 14, 2007, 10:09 PM

True, but of limited utility.
TC could make the Contender in a small head diameter high intensity cartridge like .223 but not in .22-250 because the larger case head at the same chamber pressure would have a higher thrust against the breechface than the Contender would take. Hence the beefed up Encore.

But going down to a smaller head cartridge is not a license to boost the load in inverse proportion; the strength of the brass becomes the limiting factor.

Harry Bonar · March 17, 2007, 06:51 PM

Sir:
Jim, as usual, hits it on the head - "brass is the limiting factor."
P.O. Ackley conducted an experiment on breeching methods. He machined the face off of the 98 bolt so it contained the case entirely in the bbl.
He then took toilet paper and wraped the action. Using a load which with the norman headspacing was causing gas leakage, he then tried the new breeching system - the toilet paper showed exactly the same leakage in the paper - as Jim says, "brass is the limiting factor."
If chambering is done correctly case head thrust is ameliorated to a small degree, but like Jim says I do not want to state that small case heads do much for thrust. The case ought to grab the chamber walls and the case head ought to be supported solidly by the bolt head in the lugs.
It is amazing what happens to steel upon firing and this is part of fine accuracy.
Harry B.

T. O'Heir · March 18, 2007, 02:21 AM

"...My knowledge of pressure comes from working with hydraulics..." Totally different thing. Think in terms of gas pressures.
You're posting is a bit difficult to read. Paragraphs would help.
In any case, the diameter of a case head means nothing.

March 14, 2007, 09:42 PM	#1
FRANK1669 Senior Member Join Date: July 4, 2006 Posts: 124	Increased Pressure over factory actions Hello all: a little hypothetical question for you. When rechabering a rifle to a different caliber the general rule of thumb is not to exceed the pressure of the cartridge the gun was designed for. But what about when you are going to a smaller case ? My knowledge of pressure comes from working with hydraulics so I may be way off here. Hope someone can clarify this for me. If you have a smaller diameter head on a cartridge it would take a higher pressure to exert the same amount of rearward force as a cartridge with a larger diameter head and assuming the same barrel contour was kept the barrel thickness would be greater than the original. Also the inverse would seem to me to be dangerous if you kept the pressure the same and went with a larger diameter on the cartridge head it would exert more force on the bolt. __________________ It is better to be judged by 12 than carried by 6

March 17, 2007, 06:51 PM	#3
Harry Bonar Senior Member Join Date: December 5, 2004 Location: In the Vincent, Ohio general area. Posts: 1,804	pressure Sir: Jim, as usual, hits it on the head - "brass is the limiting factor." P.O. Ackley conducted an experiment on breeching methods. He machined the face off of the 98 bolt so it contained the case entirely in the bbl. He then took toilet paper and wraped the action. Using a load which with the norman headspacing was causing gas leakage, he then tried the new breeching system - the toilet paper showed exactly the same leakage in the paper - as Jim says, "brass is the limiting factor." If chambering is done correctly case head thrust is ameliorated to a small degree, but like Jim says I do not want to state that small case heads do much for thrust. The case ought to grab the chamber walls and the case head ought to be supported solidly by the bolt head in the lugs. It is amazing what happens to steel upon firing and this is part of fine accuracy. Harry B.

March 14, 2007, 10:09 PM	#2
Jim Watson Senior Member Join Date: October 25, 2001 Location: Alabama Posts: 18,541	True, but of limited utility. TC could make the Contender in a small head diameter high intensity cartridge like .223 but not in .22-250 because the larger case head at the same chamber pressure would have a higher thrust against the breechface than the Contender would take. Hence the beefed up Encore. But going down to a smaller head cartridge is not a license to boost the load in inverse proportion; the strength of the brass becomes the limiting factor.

March 18, 2007, 02:21 AM	#4
T. O'Heir Senior Member Join Date: February 13, 2002 Location: Canada Posts: 12,453	"...My knowledge of pressure comes from working with hydraulics..." Totally different thing. Think in terms of gas pressures. You're posting is a bit difficult to read. Paragraphs would help. In any case, the diameter of a case head means nothing.