NRA Safety Notice re No 4 7.62mm Conversions

[Edward Horton]

Theoretical calculations for bolt thrust are not the same as real world figures using pressure measuring equipment and strain gauges. You have dry bolt thrust and wet bolt thrust, which amounts to a 50% to 60% increase for wet or oiled cases.

From the British “Textbook of Small Arms”.

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[jmoore]

Never mind.

[slamfire1]

Theoretical calculations for bolt thrust are not the same as real world figures using pressure measuring equipment and strain gauges. You have dry bolt thrust and wet bolt thrust, which amounts to a 50% to 60% increase for wet or oiled cases.

So, how did they design the things? How do they design things now days?

What amount of cartridge friction do designers assume in designing their actions?

Pick a number from 0 %, which means frictionless case, to 100 %, which means the case takes the entire load.

What % cartridge friction should a designer use in designing the size of the locking lugs and the receiver recesses?

[Edward Horton]

So, how did they design the things? How do they design things now days?

What amount of cartridge friction do designers assume in designing their actions?

Pick a number from 0 %, which means frictionless case, to 100 %, which means the case takes the entire load.

What % cartridge friction should a designer use in designing the size of the locking lugs and the receiver recesses?

slamfire1

The firearms manufactures don't guess at it or argue about bolt thrust in forums they know from actual firearms testing.

The designers want 40% to 50% LESS bolt thrust than a oiled or greased cartridge delivers. (do the math from the above warning) That is why you see all the warnings to NOT lube your ammo, you can end up decreasing the life expedience of your rifle by as much as 50% by lubing your ammo.

On top of this the American SAAMI and the European CIP use "DRY" proof cartridges to test firearms.

If six oiled .303 proof rounds increased the head space of the control .303 Enfield during testing by .010, how many normal standard pressure oiled cartridges would it take to increase the head space by .010, 100, 200, 300???

Below is a Powley pocket computer, it is from the early 1960s and is used to compute the best powder charge weight and powder type using DuPont IMR series of powders.

What it tells me is the theoretical "best load" for a .303 British and a 150 grain bullet is IMR-4064 powder and for the 180 grain bullet IMR-4350 is best.

This information is nothing more than a theoretical WAG (wild A$$ guess) based on loading density and case capacity. The real answers come through testing with a chronograph and checking standard deviation and your group size.

The "proof" is in the actual testing.



You can compute and guesstimate a new jet engines thrust but you will NOT get the actual thrust figures until the engine is fired up and tested in a test cell.

[Dimitri]

What % cartridge friction should a designer use in designing the size of the locking lugs and the receiver recesses?

0% as you cannot guaranty the friction co-efficient without testing and the moment you go from a chrome to plain bore in war time or from a plain to stainless you change that co-efficient so its considered good practice to apply the entire load.

Dimitri

[ireload2]

>>>Theoretical calculations for bolt thrust are not the same as real world figures using pressure measuring equipment and strain gauges. You have dry bolt thrust and wet bolt thrust, which amounts to a 50% to 60% increase for wet or oiled cases.<<<

No that is not correct logic if the calculations are made treating the case as having zero friction. You need to reverse your train of thought.
When calculations are made that treat the case as though it has zero friction the bolt thrust is greater than even an oiled case.

The answers are all right here.

[ireload2]

>>>its considered good practice to apply the entire load.<<<

In ASME pressure vessel manufacturing the pressure vessel is hydrotested to 1.5 times its designed operating pressure.

[Dimitri]

I was referring to the action of friction on the bolt through the possibility that it will reduce a percentage of thrust. A safety factor of 1.5 is quite common, cars, and aerospace applications are built to that.Aircraft must be able to be loaded 125% without failer of any components however at 150% rated loading it is allowed to catastrophically fail and that is ok.

But for a starting load a frictionless case situation is better then assuming friction a d the friction co-efficient changing.

Dimitri

[Thunderbox]

A lot of the assumptions in this discussion are highly speculative in any case, as no-one has ever precisely observed/measured what actually happens when a case - oiled or not - undergoes the firing sequence in the chamber.

The British arsenals, which fired millions of rounds in testing of one sort or another, came to the conclusion that, depending upon the type of propellant/primer combination, the case first expanded to the chamber wall at an initial circumferential point (the shiny "ring" on a .303" case), from where it rolled forwards and backwards according to a complex relationship with the propellant burn & gas expansion. The observation included that (a) case expansion due to gas pressure was not uniform for much of the firing sequence (b) most oil/water between case and chamber was simply expelled forwards or backwards by the differential expansion of the case. Point (b) becomes fairly obvious, if you shoot some of the waxed MkVIIIz ammo - the case go in sticky and come out dry....

The pointlessness of the whole "oiled case" scare is there in black and white in the ToSA quote above. The intended service load for MkVII was 19T using lubricated or oilcloth-wiped cases (which has been a standard battle preparation for ammunition since Snider days), and "dry" ammo was the non-standard situation.

[Amatikulu]

The topic of this thread is "NRA Safety Notice re. No 4 Rifle Conversions to 7.62mm"

We seem to be getting off topic and moving back into a subject that has been already flogged to death without any resolution.

If the members who are posting on this thread are trying to develop factual information to send to the NRA with the hope of changing their minds, you might like to read the thread again and evaluate if your contribution to this thread is achieving that purpose.

Then again, maybe that's not your intent