An interesting question of chamber pressure.....

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This question popped up in my in-box, and I thought it worth sharing:


Hi,

Someone on another blog suggested you might be able to answer this question.

I was at the range recently and I ran into an old timer who had an M1
and reloads his rounds (I have only a little over half a century under
my belt so far :-)

He claimed, I seem to recall, that chamber pressures on his M1 were of
the order of 48,000 PSI rather than the usual 60,000+ on a bolt-action
30-06.

Today, it seemed to me that this should not be the case, since the
bolt on the M1 does not unlock until the combustion gases bleed down
the gas port and operate the piston. So, there should be little
difference between the pressure that develops in an M1 compared with a
bolt-action 30-06 with a comparable length barrel. At least that is my
first thought.

Can you comment?

Perhaps he uses less powder in his hand-loads.


An interesting question. The SAAMI specification for chamber pressure in the venerable 30-06 round is roughly 50,000 Copper Units of Pressure, which does not exactly relate to PSI (which is much higher). It's taken by measuring the deformation of a pure copper pellet which is contained in a pressure vessel attached to a tap port on the chamber of a very special pressure measuring 'gun'.

The question itself has some flaws, chief of which is this: One cannot measure chamber pressure without such a rig. One can only guess at the pressure, and no matter how good the guess the factors involved are myriad. Bore diameter, bullet diameter, chamber dimensions, bullet weight, bullet jacket composition, powder load, powder type, cartridge casing thickness, primer type, free-bore ahead of the chamber, temperature at firing.... each adds to the mix, and any single one can make huge changes in chamber pressure.

The reasonable answer is to plan on staying well under the maximum chamber pressure, leaving a wide margin for error in the interest of safety. Most firearms are engineered with such a wide margin, being capable of significantly more pressure than the cartridge would normally have. This is the thinking behind 'proof' loads, which have been used since the inception of firearms as a means of proving the integrity of the barrel. Typically a double powder loading back in the day when the charge was poured into the muzzle, a bullet rammed home and some intrepid soul pulling a string tied to the trigger.

As for the M1 Vs. Bolt Action question, I would have to disagree with the gentleman who thinks them inherently different. I can't see why pressure would be any different, given all other factors being a match. Right up until the bullet passes the gas port, and then all bets are off as pressure is bled away to operate the action of the wonderful M1 design.

IF all other factors were equal.... and that is the point. There are so many factors, each vitally important, that the rifle action itself is of little consequence by comparison right up until the op-rod shoves the bolt back and cycles the action.

More to the point when considering the difference between the M1 Garand and a Bolt action rifle of the same caliber is the Pressure Curve of the load. In other words, given a safe and reasonable chamber pressure... what is the pressure in the bore when the bullet finally passes the gas tap port? That is the pressure which will operate the action, and given a load which has too high a pressure at that point the action may be damaged.

I hope this answers the question.... and I have written it out as best I know how. I am certain there are folks more expert than I amongst the readers here, and I would welcome thoughts and opinions on the matter.

Lord knows how I became the expert..... I'm just a fat old school teacher from Pennsylvania (laughing to and at myself).