.276 Pedersen discrepancies

#1

For a cartridge that’s so well documented, the .276 Pedersen is a bit of a puzzle to me. This milsurps.com article gives the peak pressure of the cartridge as 19 long tons per square inch, or 42,560 PSI. This is quite low, lower even than 7.62x39.

Hatcher’s Notebook gives the velocity of the .276 Pedersen at 78 feet as 2,690 ft/s, which works out to 2,740 ft/s muzzle velocity.

Using CAD software and this cartridge diagram, I modeled the case volume of the cartridge, which resulted in 47.4 grains case capacity. Weighing one of my pulled bullets, I got a bullet weight of around 126 grains.

I plugged the dimensions of the cartridge, the bullet weight, and barrel length of 24" into an online Powley computer to see if I could replicate the original performance. I was unable to reach the velocity stated in Hatcher’s Notebook without reaching a pressure of at least 53,000 PSI. The pressure stated in the milsurps.com article gives a muzzle velocity scarcely higher than 2,500 ft/s, far short of Hatcher’s value.

So what’s going on here? Does anyone have any insights?

#2

Sounds like theoretical calculations are not always verified by actual test results.

I would believe the results in Hatcher, and especially anything listed in Hackley, Woodin & Scranton.

#3

The “long tons per square inch” seems to hint at a measurement done in the UK. It is not possible to simply compute British base pressures into the radial copper crusher figures used elsewhere. This is the reason you arrive at very low pressure figures.

I agree that the Hatcher figures can be considered reliable. American Rifleman of May 1942 cites data from disassembled cartridges. The load was 31.7 gr of DuPont No. 25, Lot 1324. Bullet mass 125 gr. They have no pressure figures, but say No. 25 is very similar to IMR 4198.

#4

John,

I have no doubt that the Hatcher figures are reliable, my initial thought was that this was indicative of two different loads for the Garand and Pedersen rifle, or a difference in measuring method.

JPeelen, yes, the peak pressure figures are for British-produced .276 ammunition. I do not know much about how pressure was measured in the UK, could you elaborate on why this is likely the source of the discrepancy?

I can confirm the powder charge weight figures. When I pulled the bullet on one of my PD-42 rounds that had a cracked neck, the powder charge was just over 31 grains. The computer gives a powder charge of 35.4 grains IMR 4895/4320/4064, which agrees with that figure, assuming PD-42 was loaded with something similar to IMR 4198 (4198 has about a 15-20% lower loading density than 4895).

#5

Surely others are more entitled to correctly describe the UK method. But basically, the barrel is not drilled at all, neither in the chamber (CIP; also requires a hole in the case) nor at the case mouth (NATO). This has the big advantage that an ordinary rifle barrel can be used.
It is mounted in a fixture which in place of the bolt has a sort of piston. The case is well oiled before firing. On firing, the gas pressure drives the case backwards and via the piston it compresses a crusher.

The dynamics and involved masses of this base pressure method are totally different from the radial method. How important dynamics are, can be seen from the fact that even radial copper crushers and quartz gauges (CUP and PSI figures) have no fixed pressure relation even if everything else is the same.

#6

Thank you for your answer, JPeelen.

Does this mean one should tentatively assume the .276 Pedersen produced higher pressures than is normally listed, if tested by modern methods? In retrospect that would make sense; it’s not clear why the Army would want to reduce pressure from the levels .30-06 was already producing. Bolt thrust is the only concern I can think of.

#7

The design of the Pederson rifle required wax coated rounds the coating was developed by Pederson himself. Because of this the .276 cartridge was not loaded to the same pressure level as the 30-06 and even with this there were failures to extract if the weapon was not kept squeaky clean. Squeaky clean is something that cannot be obtained in combat as anyone with experience can attest to!
Gourd

#8

Gourd, I’ve heard that said a lot about the cartridge and rifle, but I’ve never seen documentation on it. Do you have any? I’d love to see it (even if it would only deepen the mystery).

Given that 7.62mm NATO works fine from other delayed-blowback guns, I can’t think of any reason why the .276 wouldn’t work at similar pressures in the Pedersen rifle. G3s have flutes of course, but that’s what the lubrication on the cartridge cases was for!

#9

I don’t know of any use of chamber fluting in small arms before the Tokarev M1938 rifle, tho perhaps the 1936 Simonov did also. If in fact the Pedersen .276 was loaded to a lower pressure level than the U.S. .30 M1906 it would be interesting to have that figure, particularly if it’s in CUP. Jack

#10

We simply have to accept that we currently do not know the typical pressure of the .276 Pedersen. In my view its ballistics is quite ordinary and from this I would expect its pressure on par with typical military cartridges.

The (British manufactured) Pedersen rifle was tested in Germany at the time and the curator of the Heereswaffenamt small arms collection (Otto Morawietz) never mentioned to me anything hinting at lower pressures than usual.

#11

JPeelen, that is basically where I’m at right now. It’s impossible to know exactly, without documentation, but the “.276 Pedersen had low peak pressure” idea doesn’t seem to make a lot of sense. What we know seems to suggest it operated at more or less normal pressures for the period. It has greater case web thickness than .30-06, for example.

#12

Tau
Some of the FA production cartridge lot 23 boxes say on the top side of the box “LUBRICATED PROCESS NO. 2”.

Some of this same cartridge lot 23 boxes are without this printing on the box top, but do show slightly different front labels than the Lubricated Process No. 2 version. These are without any mention of lubrication.

As Gourd says it is a ‘dry’ lube.

I’ve no other record of this but the boxes themselves note there were apparently at the least, 2 lubricants used.

Sorry not to be of more help.

#13

I was under the impression that the boxes without markings for lubrication were intended for the Garand only.

#14

Tau, these figures taken from the Vickers made Model P.A. rifle manual may be useful to you:

Regarding its lubrication the manual says: “The cartridge is coated with a thin film of transparent waterproof substance to improve its functioning and to prolong the useful life of the rifling”; “This waterproofing also preserves the cartridges in storage and prevents season cracking”.

#15

I believe this is the source for the figures from the surplusrifle forums history section. Thanks for posting it.

#16

General Hatcher, in his Book of the Garand, gives (on p. 71) the breech pressure of the .276 Pedersen cartridge as of 25 February 1926 as 47,865 pounds per square inch. This loading is with a 125 grain bullet and develops 2600 fps. Further on in this chapter (p. 103) a chart relating to the “improved type” .276 cartridge tested by the Semi-automatic Rifle Board of 1931/32 credits the cartridge as firing a 125 grain bullet at 2700 fps. In this second reference mention is made of a powder charge of 32 grains; no propellant weight was given in the 1926 data first cited. Jack

#17

Excellent find, Jack. Thanks a lot, really.

Well, you know, nitpicking is one of my many weak points: my copy says 2600 fps.

#18

Besides the British using different mechanisms ( base pressure) in cartridge Pressire determination, the stamped “19tons per sq” " is what is called “Service pressure” ( ie, the pressure which the rifle would normally be subjected to whilst in “service”. That and the fact that Base pressures are always lower than “copper crusher case body pressures” ( there is no friction or case movement to effect the energy tranfer to the crusher) will lead to widely differing Pressure levels (Pmax in pressure cycle)

Whilst I agree that the 276 P cartridge was a great step forward, especially in a non-lubricated Rifle (Garand)…it was overtaken by pragmatic Supply problems ( having a few billion M1906 and M1 cartridges in stock, and all the MGs in the M1/M1906 cartridges; a"duo" of calibres was an opening for a Supply Nightmare ( look at Japan, and to a lesser extent, Italy…and France)

But the novelty came at the wrong time, as it did for many other otherwise valid “light” cartridges…War, Politics, and Finance have a way of burying the Best cartridge designs…(unless you are a monolithic, one-Party,
one-Dictator state…any takers?).

Doc AV

#19

Jack, I don’t have my copy of Hatcher handy, thanks for posting that info!

“Back of the envelope”, we can roughly estimate the pressure of the 2,700 fps “improved” load as being in the region of 52,000 PSI. This agrees with my earlier Powley estimate. It is then safe to say, I think, that the .276 Pedersen in fact had perfectly normal peak pressure levels for a cartridge of that period.