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Pressure signs??
bpost
Member Posts: 32,669 ✭✭✭✭
I was shooting a load ladder of .2 grain increments in the 6BR. I had some loaded at 32.7 of varget with a 70 nosler match bullet CCI mag small rifle primer. The primers were OK, extrraction was effortless in the encore.
One case showed pending head seperation [:0]. There was the telltale bright ring above the base [B)]. I have no history on this brass. The groups SEEM to show improvement but as you can imagine I am getting nervous about going up any more.
Encore rifle
Bullberry barrel 19.5"
.263 neck
bullets seated to kiss the lands.
VV-N133 is on the adjenda [;)]
Any words of wisdom out there???
One case showed pending head seperation [:0]. There was the telltale bright ring above the base [B)]. I have no history on this brass. The groups SEEM to show improvement but as you can imagine I am getting nervous about going up any more.
Encore rifle
Bullberry barrel 19.5"
.263 neck
bullets seated to kiss the lands.
VV-N133 is on the adjenda [;)]
Any words of wisdom out there???
Comments
Case head separation is a sign of a headspace problem not pressure.
Check your dies and press as well as the batch of brass.
Best.
Please explain???
If the pressure is high enough to "flow" the brass will it not eventually fail in the area the brass flows from?? I have shot all these cases at leasts once. The die is very carefully set to bump the shoulder by about 003.
Is this too much push back on the shoulder?
I guess what I should say is that pressure helps to create the situation but the root cause is a headspace problem or possibly the Contender action itself, stretching slightly, which is a headspace consideration. Either section some cases and look at the inside for the ring or use a paper clip with a short right angle bent into it and point on the tip to feel the ring on the inside.
INCIPIENT HEAD SEPARATION
I am told to watch for a bright ring above the belt on the
magnums, others say you can measure the expansion of the web.
Can you reload until you see a case crack, or is it wise to
retire the cases after a set number of firings?
The most important early warning of case failure is that bright
ring at the juncture of the case wall and case base. This is
where case stretch that is due to headspace tolerance or sizing
die dimensions/setting occurs and is referred to as incipient
head separation.
The bright ring that I refer to is pretty
recognizable in that it has a crystalline appearance and is
likely to be about 1/8" or so wide. Do not confuse this with the
abrupt ring that appears at the same juncture when a relatively
sloppy chamber is encountered. One can check to see if incipient
head separation has occurred by using a bent paper clip to feel
inside the case. If it is present, an abrupt, easy to feel ridge
will be apparent and these cases must not be used again.
If your cases have been loaded three to four times and have not
been checked for length, it is time to do so. Over length cases
can present a condition where the case mouth, upon chambering,
can jam into the chamber throat. This prevents the case neck
from expanding sufficiently to release the bullet as it should
and can result in very high pressures. Trim to about .010" less
than the SAAMI maximum.
Some rifles have the characteristic of allowing brass
to stretch/swell an amount upon each firing which can eventually
prevent chambering. This USUALLY happens in rifles with "springy"
actions; (edit - Contenders) those like the Lee Enfield that have rear locking
lugs. Neck sizers will not correct this condition. Cartridge
pressure also plays an important role and holding the pressures
down in the Lee Enfield, etc. will go a long way towards minimizing
stretch.
The answer that I have found to work well across-the-board is
controlled (partial) sizing using a full length resizing die.
Fitting your lot of cases to a given rifle will pay off in case
life increase. To do that I use a little known, but not original,
Technique:
Using a light charge of powder, seat a heavy (long) bullet into
the case just enough so that it will not fall out during normal
handling. Single load. The intention here is to cause the bullet
to be jammed into the origin of the rifling by the bolt. A tight
case neck will aid in holding the case base back against the bolt
face during this operation. When the round is fired the case
will not be pushed forward by the firing pin but rather the
shoulder will be blown forward thereby forming the case to the
chamber, resulting in zero headspace.
Then, using a match or candle, smoke the case neck and shoulder
of one fired case. Back off your sizing die several turns and
size the fired case. Turn the die down in steps until you have
sized the case (as indicated by the removed carbon) 90 - 95% of
the neck length. Be sure that you stop short of the point where
the die touches the case shoulder. Once the proper amount of
sizing is determined, lock the die at that position in the press.
The above should be done using the MINIMUM recommended charge for
any given powder. This technique has been used many years and it
works. DO NOT use maximum charges with this method as pressures
can become excessive. I use reject cast bullets to do the form-
ing.
The above has an additional advantage in that with 90 - 95%
resizing, the case base (pressure ring) is reduced back to normal
diameter, a feature that the neck-only sizer does not offer.
This is particularly important to those that do relative pressure
testing according to the Ken Waters' method.
I have had only one rifle that required slightly touching the
shoulder with the sizing die after each firing. It was Model 94
Winchester.
I have been an active shooter for a lot of years and have not
ever had a case head separation. Maintaining a zero headspace
clearance condition is the reason. My old Lee-Enfield No. 1,
Mark 3* has about .020" extra headspace when measured by conventional
methods but has effective headspace clearance of zero
because of the above practice. I have been shooting it since
1957 without the first problem. All my rifles are set up this
way.
http://www2.rpa.net/~bologna/advanced.htm
The firing pin's force will drive the case far enough into the chamber that the shoulder will be set back. With the shoulder set back, the chance of head separation greatly increases. With the case too far forward in the chamber when the powder ignites, the increase of pressure will press the shoulder and body walls hard against the chamber walls, but the head will be pushed back far enough to separate the brass at the thinnest point not grabbing the chamber wall. That point is about one tenth of an inch in front of the
extractor groove on normal rimless cases.
Gale McMillan - yarchives.net
Those that have little experience viewed it as being dangerous...I guess...
If the bullets are being seated to touch the lands..then either pressure limits for that gun is reached...or improper tecniques with the sizing die is the culprit.
Excellent post!
I've always loaded by 'under full-length' resizing cases because I was told that it was better for accuracy. It wasn't until a few years ago myself that I thought of headspace being the problem or even thinking a firing pin would push a cartridge forward in the case(had it pointed out to me). I was always taught to look at the primer and head for shinyness. Again, great explanation of the long headspace and high pressure issue.
I would still check the inside of the cases with a paper clip or section a couple of cases to get a good look to be sure that this is separation and not the chamber polishing the area above the head.
One of the points that gets lost in this iteration is the fact that the case stretching can occur in firearms designed like the Contenders. I've seen it before. It's not that this is a bad design at all, quite the opposite in my opinion but stretching in the frame can happen. The same can be applied to the Rem. 788 and the Schultz & Larson rear lug bolts when even normal amounts of pressure are involved, the cases stretch sometimes.
One other observation is that even a tiny amount of oil or lubricant in the chamber can cause the same or similar effect.
The other point, which I think can always bear repeating is, make your cases fit your chamber correctly. I think Highball addressed that a while ago.
The biggest problem with trying to solve these types of problems on the internet is the lack of 'hands-on' examination.
Best.
then there is the action flex issue.
as far as knowing the pressure, you are best off using the chrono to tell you when you have exceeded the MV in the books by enough to tell when you have gotten up too high in pressures. REMEMBER, shiny ejector marks, flatttttttt primers, and sticky extraction occur after some 70K PSI has been reached,..you are then more than 10K PSI too high at that point. You can typicaaly exceed book max in a factory tube, but ifin your rig is wearing a match tube with a tighter chamber, you may not make book max. Many of my match tubes won't make book max before pressure is high.
Eric,..you caught me slipping on that one. I meant to say the Headspace tool (had been working with a comparator earlier this morning) I meant the headspace tool,..it's cheap and easy to use. best part is you can buy 2 sets and use them to measure bearing surface lengths
This is in response to ECC's post to me.
[:0][B)][V]
revision of my first post for brain fart purposes
Eric,..you caught me slipping on that one. I meant to say the Headspace tool (had been working with a comparator earlier this morning) I meant the headspace tool,..it's cheap and easy to use. best part is you can buy 2 sets and use them to measure bearing surface lengths
This is in response to ECC's post to me.
[:0][B)][V]
Darn...my whole mental image of you being the perfect long rang guy out there is now shot all to hell[;)][:D] If you didn't slip up from time to time, you wouldn't be human...besides that, I learned something new, and I've got to get myself a few more reloading tools.[:)]
And, for the record, I regularly get a lesson from the guys who have the time and money to follow the circuit, so I am wayy farr from perfect[:D] Does give us something to reach for though,..huh[;)]