Pressure signs in rifle cases.

[leftiye]

Anything that happens at the start of the launch has amplified effect on pressure (and everything else). Try making that initial exiting from the case a little harder by having a ding on your case mouth. Blown primer time! While .002" clearance works just fine, a little dirt can make huge differences in pressures in that scenario.

[B R Shooter]

Factory chambers are made quite generous for a couple reasons. They need to be able to shoot any brand ammo of any weight bullet, and be able to chamber a reasonably dirty case in the event a round was dropped in the field or similar. This is understood. When reloading, people have the abilty to taylor the rounds for their intended purpose. If one wants to fit the case in a given chamber they can. At the same time, if one wants to seat a bullet closer to or even touching the lands, they can. That alone many times can produce better groups than following the "listed" COL. That's why we reload. Can having a bullet very close to or touching the lands cause an increase in pressure? Slightly perhaps. But I firmly believe that if you have an over pressure round simply by seating the bullet to touch the lands, you are already over the reasonable limit. With some factory chambers and military chambers, then throw in the tolerances of mass production, it's a wonder some rounds even go off.

[leebuilder]

Been reading alot. Talked to another silverback. I believe, and what i have done in the past, with pressue signs back off or go to another powder. Plus 1 with generous clearances and use of peowned barrels, and factory ammo (some of that eastern european stuff is HAWT). Pressure increases with tempurature too. One silverback talked at lenght about primer material and form, leading to false assumptions. We all agreed the brass flowing into extractor cuts is bad and shall be avoided/prevented.
Be safe

[littlejack]

One other thing that has been identified as a false pressure sign, but not mentioned, is a burr around the extractor groove or the plunger hole.
These will leave a bright mark on the face of the case, but no over pressure really exists.

[Larry Gibson]

littlejack

I have some experience pressure testing the 7.62x54R in my SAKO Finn M39. I have tested 8 hand loads, 2 commercial loads and 9 milsurp loads. Keep in mind C.I.P. standards for the 7.62x54R give a PMAP of 46,000 psi (transducer). Both commercial loads (Winchester and Norma) I tested in my M39 using the Oehler M43 produced very close to 46,000 psi(M43). The milsurp loads produced from 39,000 - 45,000 psi(M43). My hand loads duplicating milsurp "L" and "D" ammunition using 150, 174 and 180 gr .311/.312 jacketed bullets produced 39,800 - 46,500 psi(M43). Cast bullet loads using 29 gr 4895 under the 311299 and 311291 produced 22,800 and 26,200 psi(M43).


I do not know what level of psi a MN will handle before letting go because I have not and do not intend to find out. Thus using fired primer condition as an indicator of psi is indeed "sketchy at best" considering the lower levels of psi the MN was apparently made for. By the time you get into the realm of the brass flowing into the ejector slot the psi is way, way above that intended for the MN. Some specific information regarding your exact loads (how much 4064 and neck clearance in the chamber neck?) may provide useful further information?

Larry Gibson

[littlejack]

Hey fellas:
Went out of town yesterday, just got back.
Hey Larry, glad you chimed in. I wasn't going to intentionally load till the brass flowed into the cracks, was just getting information for my mental archives.
I went out and got some chamber dimensions from a impact cast that I done a few years back.
First off, I measured one of my loaded cases at the neck, with a .312 bullet seated. Diameter is .330. The impact cast dimension at the same point is .341. That is generously over sized for that area.
Next, I measured the extended length of the chambering in the neck. I was approximately .150 longer than the case neck itself.
Next, I measured the length of the taper from the end of the neck length + .150 to the starting of the bore and grooves. It measured approximately .100 in length.
Next, I measured the cross diameter of the starting of the grooves. That was .318. Then .540 down the bore it measured .315. The .540 was the length to the end of the impact cast.
Case head dimension of the impact cast, .486 (.200 up from the base of the case according to to my Hornaday book)
Shoulder diameter of the impact case is .463.
So, this particular chamber has very generous dimensions. The diameter of the neck area and the starting of the bore/groove area leaves plenty of tolerance for gas blow by. I am quite amazed at the accuracy this rifle is shooting with the bore/groove dimensions it has. I used my Mitutoyo calipers to measure these dimensions. They were so generous, I don't think mics would make any difference. My calipers measure to within .001.
The loads I tested were with 46 and 47 grains of IMR 4064 under a 200 grain bullet. I did start lower, then worked up to that.

Nise, now I understand. As you can see, the chamber neck area in my rifle must be a lot longer that yours.

[Larry Gibson]

littlejack

The loads I tested were with 46 and 47 grains of IMR 4064 under a 200 grain bullet. I did start lower, then worked up to that.

That load would be about right for duplicating the "D" ball ammunition with 174 or 180 gr bullets. Might be a bit stiff with 200 gr bullets. What 200 gr jacketed bullets are you using?

Are you FL sizing the cases? If so are you headspacing on the rim or the adjusting the dies to headspace off the shoulder?

Larry Gibson

[Cap'n Morgan]

I sometimes have used case head expansion as a poor man's pressure gauge: http://shootersnotes.com/articles/wh...ures-too-high/ Done correctly it is a safe and reliable method, but I would not recommend it for anything but modern high pressure cartridges.

[1johnlb]

For those that are unsure of the Mosin's strength. Here's one shot with a full case of 2400, although I don't recommend trying it yourself.

Mosin Nagant Torture Test: Part 1 - YouTube


www.youtube.com/watch?v=gfiXFyIbOZw

Be sure not to miss part 2

https://www.google.com/url?sa=t&sour...F_D_Nu5hV9LnLQ


Here's a case head of a 308 shot with what I believe was a double charge of 2400 in a Savage 110

[Larry Gibson]

Interesting. Yup, strong actions. The use of steel only cases, even with their reloads, certainly helped. Would it have been different with brass cases? Probably. I, however, prefer not to have to beat the bolt open. Much easier if I stay with in the operational parameters of the cartridge and rifle. I don't envision needing more than 3190 fps with a Hornady 123 gr jacketed bullet, 2900+ fps with a 150 gr jacketed bullet or 2700 fps with a 174 - 180 gr bullets bullet in either my Russian M28/30 sniper or my Finn M39. That can be done by staying under 46,000 psi and those loads don't lock up the bolt.

Larry Gibson

[littlejack]

Larry:
The bullets that I have been testing lately are of unknown make. I looked to find a example of them on the net, but nothing was posted. They are a 200 grain jacketed 8mm bullet with a round nose and flat point. I bought these from another member here on the forum a couple years ago. They did not shoot well in my 8mm-06 AI, so I sized them to .3127. They shoot exceptionally good in my Mosin. The slugged diameter of my Mosin is .313. These particular bullets have a crimp groove, and, what looks to be a shallow square groove to lessen friction/pressure. I really don't know.
In the beginning, I was neck sizing. Then I switched to partial sizing. Now, I use a .080 washer under the sizing die that is set to partial size. When the case gets hard to chamber, I just remove the washer and partial size one time. Then, replace the washer and repeat the sequence.

1johnlb:
That is oner impressive Torture Test.
I looked in my P.O. Ackley book the other day to see if he had tried to blow a Mosin. I did not find any section where
he did. His other tests
with the other military rifles was quite impressive and very informative. Very surprising that the Japanese rifles held
up to, and as good as most, and better than some of the other rifles.
There was also a test done where they took an 30-06 chambered barrel, reamed the neck to take a 8mm bullet,
loaded and fired it down the 30-06 barrel. The results were that the pressure did not spike as they thought it would,
and every thing stayed intact. They did say, (I don't think most people realize) that the bullet is sized down as soon
as it 's whole length is in the bore. Then, pressure would drop.
Jack

[Patrick56]

They tried to blow up the MN in Finland and they were not able to do it. The only result was a bolt that could not be opened. Headspace is a common issue with the MN. As an young conscript gunsmith, we checked the headspace by simply using a no go and a go gauges and a big box of bolt lugs. If you hold the MN bolt removed from the rifle with the safety towards you, turn the safety one click clockwise and you can remove the lugs without releasing the striker spring tension.

[Larry Gibson]

littlejack

Sounds like you've a good handle on case sizing. I'd venture your 4046 load is pushing well into the high 50K psi range which is perhaps where the pressure signs are coming from.

Many of us could have told the guys doing the destruction test the bolt would not come back out of the receiver. There is too much steel in the large rib on the bolt. Almost always the MN fails with a burst barrel (as the older dude thought would happen) or the bolt head (locking lugs) lock up solidly. Once the bolt head locks up solidly the action is toast as the lugs have set back the locking surfaces. Too bad they didn't try a bore obstruction with a standard service "L" load, that's how we destroyed a MN (intentionally btw) and the bolt body still was in place in the receiver.

Larry Gibson

[littlejack]

Larry, thank you for your testing results, it was interesting, and very informative. Also, thank you to all the other members that posted on this thread that had information to share.
Jack