Primary Extraction and the Ross Rifle

[David Scott]

There was a thread recently that brought up the infamous jamming issues of the Ross Mark III rifle in WWI. This comment from that thread caught my eye:
“The Ross probably has better primary extraction that most rifles. The bolt lugs are essentially a large screw, and cycling the bolt causes the lugs to both rotate and move rearward at the same time. It has plenty of primary extraction.”
The comment is true as far as it goes, but doesn’t capture the entire issue. The primary extraction of the Ross Mark III is very typical of similar vintage bolt action rifles, see attached table.



What stands out with the Ross is the low mechanical advantage its bolt system provides when applying that primary extraction. The seven rifles in the table represent a not very random sample of just what happens to be in my gun cabinet. The primary extraction of the sample rifles range from 1.3mm to 3.8mm, with the Ross right in the middle at 2.6mm. Where the Ross stands out is in the mechanical advantage of its bolt system. I calculated mechanical advantage as the distance the bolt handle moved during the unlocking of the lugs divided by the primary extraction. An analogue for this mechanical advantage concept would be the pitch of screw threads. Screws with fine threads are easier to drive against friction than a similar coarse threaded screw. As you can see from the table the Ross rifle is lowest of the group and has less than half the mechanical advantage of some of its contemporaries. The Ross has a value of 10 whereas the Lee Enfield has about 25. I believe this is really an important part of the Ross jamming issue. All of the military rifles of WWI were shallowing some very bad, mass produced ammunition. Some rifle designs were just better at ripping those sticky casings out of their chambers.

A more complete discussion of primary extraction is available from Bloke on the Range at: https://www.youtube.com/watch?v=ZFqOK843IyU

 

[Potashminer]

Your calculations may need some refinement - your major assumption is that the distance travelled by bolt face is the same throughout the travel of the bolt handle. For the 98 Mauser and the M1917, it most definitely not the case - in the mauser's case, probably 50% of initial bolt upward travel as no rearward movement at all; for M1917, a very high leveraged initial, very small rearward movement until about 60% of bolt upward travel, then similar rate to Mauser or Springfield. M1917 and P14 have initial angle on closing similar to Springfield, the Enfields then change to much shallower angle that continues to move the bolt forward until fully closed. Mauser and Springfield have similar initial angle, then lug faces go vertical and no further advance of the cartridge for about half or more of the bolt handle movement - sort of the reverse on opening/extration...

And, my apologies for the length of the following - from a post about this on milsurp site - gives the source of the information - FYI - the Remington Model 30 is the post war "sporter" version of the M1917 action:

"The following is a letter from the Works Superintendent of the Remington Arms factory. This information comes from “The Dope Bag” appearing in the AMERICAN RIFLEMAN magazine for February 1934, p 32. I quote as follows:

A review of the correspondence between yourself, Mr. Hadley, and others Indicates that your method of headspace measurement is with a cartridge or dummy cartridge and shims of known thickness. In view of this procedure of headspace measurement, we wish to call to your attention to the locking cam arrangement of our Model 30 as compared to the locking cam arrangement of the Springfield. Figures which are given below are approximate but are sufficiently close to demonstrate the comparison.
In the Springfield the bolt lugs ride the locking cam having a lead of 1.5” per revolution, through a swing of approximately 37 degrees, after which the flat rear face of the bolt lugs contacts the flat face of the locking shoulders of the receiver.
In the Model 30 the bolt lugs ride a lead cam having a lead of 1.5” per ‘rev’ through a swing of only 20 degrees, after which the ‘cam-cut’ rear faces of the bolt lugs contact the locking cams on the locking shoulders of the receiver and move through of a swing of approximately 70 degrees on these locking cams, which have a lead of 0.14” per ‘rev’. The power of the locking cams on the Model 30, as compared to the locking cams on the Springfield may be said to be in the ratio of 1.5 to 0.14; that is the power of the locking cam on the Model 30 is 1.5/0.14 or 10.7 times as much as the power of the locking cam on the Springfield. In other words, on the Model 30 rifle to close the bolt against resistance requires a force approximately one tenth of that required by the Springfield
This proposition may be stated in a different manner as follows:
The Springfield has a lead cam of 1.5’ through a swing of 37 degrees and no locking cam.
The Model 30 has a lead cam of 1.5” lead operating through a swing of 20 degrees and then a locking cam of 0.14” lead operating through a swing of 70 degrees.
In the Springfield the bolt swings through approximately 37 degrees on a 1.5” cam to get to the limit of full forward linear travel. The remaining swing of the bolt in the closed position to its closed position is on flat surfaces of bolt lug and receiver and there is no forward travel after completion of the 37 degree swing
In the Model 30 the bolt swings through approximately 20 degrees on a 1.5’ lead cam and then through 70 degrees on a 0.14” locking cam to get to the limit of full forward linear travel.”
There is further discussion in which Mr. Brown states:
“We believe the power of this locking cam to be an advantage in the use of the gun, since it provides considerable power to close over a cartridge which may be mutilated or too long. We believe also that the locking cams on the Model 30 permit easier opening after firing.
H.A. Brown”

 

[green]

The above comments are relevant to the P14 and M17 Enfield since the Rem M30 was the same action as a M17.