Case head expansion?

[Linefinder]

I was at the range today working up a pdog load for my 6mm Remington.

Long story short.....even though I was working with "mid-level" loads and nowhere near book max, one obviously was.

I just measured my case head expansion, and it was .007" over the typical fired case. The primer pocket won't hold a primer anymore, in any orientation. One of the letters on the case head is no longer legible due to the ejector pin embossing it.

Nothing seemed broken, and the rifle shot fine for 85 rounds after that.

I remember reading somewhere in the past that case head expansion of .00xx" indicates dangerously high pressure levels, but I don't recall the number and can't seem to locate my source that contains it.

.007" expansion was what I had. How bad was this?

Thanks

Mike

 

[Asa_Yam]

According to http://www.shootingsoftware.com/ftp/dbramwell%20july%2019%2004.pdf, case head expansion is not a reliable means of determining pressure. Although the article is hosted on a site that sells pressure measurement hardware, the science and engineering behind it is sound. Bramwell,the author) has this to say in conclusion point #4:

PRE ,Pressure Ring Expansion - Asa) and CHE ,Case Head Expansion) always do always give an answer, but I can give you a random number table that is almost as good. That has the added advantage of not requiring a micrometer. Since firearms are typically very conservatively designed, your chances of blowing one up are slim, as long as you stick to reasonable powder choices. The illusion is that the methods work. The fact is that both methods can lead you to think that you are safe, when, in fact, you are punishing your firearm.

As a degreed metallurgist, I can confidently say there are reasons why two,or more) dimensionally identical pieces of brass made from the same melt don't perform identically under the same load.

 

[Cheechako]

Mike

There is no simple answer to your question but the simplest I can think of is, "no expansion of the case head is acceptable." Using that as your guide will certainly prevent most problems.

Lacking pressure testing equipment forces us to rely on less accurate methods. One way that gets complicated includes chronographing the loads and using velocity/energy data to compare with other known data from reliable testing laboratories. Estimating pressures is a tricky business. But there is a direct relationship between muzzle energy and breech pressure, IF YOU KNOW WHAT YOU ARE DOING.

Since stuff like this is usually beyond the abilities of the average handloader, he must rely on simpler methods. One way is to fire a few factory loads and measure expansion at the solid web, not the case walls, and use this as your upper limit of expansion with your handloads.

Things to watch, other than case head expansion, include a sticky bolt lift, higher than expected velocities, and flattened primers,not always reliable). I'm surprised that you didn't see any of these signals before reaching the pressures that caused such severe case head expansion. They usually mean that you are already too hot and need to back off.

I'm a firm believer in modest loads regardless. If I need more velocity/power, I go to a bigger case or a lighter bullet.

Good luck.

Ray

 

[WayneShaw]

You don't say what rifle you have, but with that much expansion, I suspect it is a factory gun, and a generous chamber. If any case is unsupported, I think some expansion will take place. With a good BR chamber, the case is held so well, that expansion shows up with the "click" of the bolt. And even then, it is more a function of the sizing die not doing it's job, than a high pressure load.

Just a thought.

 

[Linefinder]

Thanks for the info!

I wasn't getting any pressure signs prior to this round because I was working with 37 grains of Varget, which is 4.5 grains below book max according to Hodgdon, and 4 below according to Hornady.

What I feel sure happened was I had a charge "bridge" in my Uniflow that I didn't notice. I was shooting,3) five shot groups at 100 yards, then,2) final groups at 200.

My first 2 shots of the first group went into the same hole. The third went 1.5" lower, even though the sight picture looked right as the rifle fired. I examined the case, and saw nothing unusual with it. I finished firing out that group, then the,2) other 100 yard groups. I then backed the target out to 200.

Upon my first shot at 200, I immediately,I mean really quickly) saw the hole appear about 2.5" above expected POI. I looked down in time to see a nice cloud of smoke wisping out around my bolt shroud. Lift was sticky, to say the least, and I thought for a minute I'd need a hammer and piece of wood to get it open.

After I'd ejected the round, I found the flattened, cratered primer in the mag well. Every classic sign of overpressure was evident on the case.

I've had great results for the past 3 years with Varget, but about 1 charge out of 150 or so will bridge in my Unflow. Until this time, it's always been very obvious. I always look into the cases after charging to verify powder drop and adequate fill level, but since my normal charge weight is only about 2/3rds capacity of the case, it's very easy to not spot what apparently is a serious overcharge. While I've always been aware of this possibility, I've thus far been successful at avoiding that. But this one got by me, and it has seriously seized my attention.

Just wondering......anyone have any experience with a Harrell measure and Varget? Think that would end the bridging problem?

Thanks

Mike

 

[Linefinder]

Wayne,

It's a Rem 700, with a new Krieger barrel that I just got back from the smith last week. The chamber appears to be tighter than the chamber on the original barrel....my old fired brass won't chamber unless F/L resized, which I don't care to do.

I was firing brass that had been once-fired in this chamber. I neck size only with a Redding S-Bushing Neck die.

The rifle suffered no damage,that I can tell), and I went on and found what will become my "standard" pdog load for it....75 VMax over 39 Varget, which is still considerably under book max.

But....between .223 Rem and 6mm Rem, I launch thousands of rounds a year at pdogs. I certainly consider this incident a wake-up call, but the truth is, I'm a very careful handloader. Shy of hand weighing each charge,not practical in pdog qtys), I don't know of any method to ensure this situation doesn't occur again.

Which brings me back to wondering if Varget has less of a tendency to bridge in a Harrell measure.,Or any other brand with which some of you may have experience).

Thanks

Mike

 

[Cheechako]

Mike

I always use a chronograph while developing loads and it's too bad you didn't have one set up during your episode. If that hot load printed high on the target it probably meant that it was indeed flying much faster than the others. With a chrono reading and a little math you could have made a fair guess as to the pressure. But in retrospect maybe it's a good thing you didn't. The number probably would have scared the $hit out of you and ruined your day,more so).

Ray

 

[Linefinder]

Yeah, I wish I'd had the chrono with me, but truth is, I don't really pay much attention to velocity until I've found accuracy...like I said, I don't often play with max loads,or even close to it, for that matter), so pressure problems are not really high on my list of worries.

After I find the accuracy I'm looking for,usually I find that to occur with a "mid level" charge weight, then I'll clock 50 of the chosen load. From that info, I work up a "comeups" table for the rifle and load to be used on pdogs. I've found with any chambering, if I can accurately land a VMax on a pdog, it'll do it's job just fine, regardless of what speed it's moving when it collides with him.

I'm firmly convinced in my mind that this occurance was due to a "bridging" incident that caught me asleep at the wheel.

Mike

 

[memilanuk]

A while back I called Sierra about some loading info for the 90gr SMK's, and inquired why their suggested load data was so... weak compared to what most people seemed to be using regularly. Their response was that they worked up the load in the cases used, measuring afte each firing until they saw 0.001" case head expansion, and called that 'max'. They also claimed that was 'industry standard practice', which is surprising considering some of the loads I find in Nosler's load manual are far and away hotter than what I find in Hornady or Sierra.

FWIW, I recently had some cases give up on me and start losing the primers. These were Winchester .243 Win brass, and started w/ the typical Winchester undersized case head,0.464"). By the time they'd expanded to the normal .243/.260/.308 case head diameter,0.470") they started losing the primers on the way out of the gun. These were warmish loads, but no discernable pressure signs,other than case head expansion) upon firing.

YMMV,

Monte

 

[Linefinder]

Thanks for the info.

I've been thinking about this. I shoot,2) .223 Rems and the 6mm Rem at pdogs. When I first started the pdog shooting, the 6mm Rem was my first, and only, "varmint" rifle.

Nowadays, 98% of the dog shooting I do occurs with the .223's. I shoot Varget in one, and H322 in the other. The fill level of Varget in these cases is high enough that a very slight charge varience is quickly noticed. IOW, the powder qty in these cases lends itself to the "throw and stuff" loading procedure necessitated by pdog qtys of rounds. And that is the same procedure I've used with the 6mm for quite some time.

But now, with the addition of the .223's and the volume of fire from the 6mm being relatively "small", I think I'm going to modify my loading procedures for the 6mm. I believe I'll throw the charge light, then trickle the remainder. After all, it's not like I'm loading 600 of these at a time.

But......I still have one nagging question. How well does the Harrell measure throw extruded powders like Varget?

Or, should I just post another thread with this question?

Thanks

Mike

 

[PNUC]

Mike,

I use a Harrell Custom 90 measure for my rifles, but I've never run Varget through it. I have thrown VV N-160 which is similar in consistency to Varget, and the measure does indeed bridge occasionally. This is not much concern to me since I weigh every charge with this powder,match ammo.)

I've run a fair amount of N-133 and N-135 through this measure too, and I don't remember any bridging, but they are finer consistency. Ditto H322.

Good luck,
Jeff

 

[Linefinder]

Thanks, Jeff. I appreciate the Harrell info.

Mike

 

[Clarence]

line:
I must ask the question as to why you choose Varget for the 244 case??? me thinks that Varget is a bit on the fast side for that case volumn.

And there are instances of pressure excursions as Ackley found but was unable to reproduce on call.

Clarence

 

[Linefinder]

I'm using Varget because it produces the most accurate load I've tried in 6mm Rem, when you take into account the conditions I'm usually shooting under.

I shoot pdogs,seriously) 12 months a year. From 100 degree blazing sunshine to 20 F in a bonechilling wind. Varget seems to be less affected than some other powders I've tried. It also seems to powder foul less, and I may go many shots without cleaning the bore.

As to pure unadulterated accuracy, I've had my best luck in this chambering with both RL19 &,believe it or not), RL22. But, unfortunately, I only get it with 95 grain NBT's, and since pdogs and cattle co-exist out here, I can't lob those at pdogs.

I've tried 7 different powders in this chambering with the 75 grain VMaxes. So far, Varget has treated me best....at least, consistenly.

Mike

 

[Clarence]

Linefinder
Don't know where you are but there is available some Russian powder that would just fill the bill for the 244 especially with the range of bullets that you use, Contact bjonessights@yahoo.com for information on the powder.
If you can get 200 rounds,accurately) without cleaning that is enough.
Where are you shooting pdogs in 20 degrees???? we don't find them much below 50.
Clarence

 

[Linefinder]

Eastern plains of Colorado. I live in Colorado Springs.

Some of the most intense pdog shooting I've done has been around the 20-30F mark on a sunny day after 2-3 days of intense cold, or snow.

If you can catch a sunny day with a slight warming trend after a bad front moves through, the pdogs come out in droves. Granted, they don't stay up as long as they do during warm weather, but, they concentrate on their feeding with such focus, they ignore pretty much anything else.

It's days like that that you have to be careful not to torch your barrel.

Mike