Revolver Chamber Pressure Variation

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DillHarris

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I loaded some 357 cases with H110, small magnum primers, and some 158 gr JHPs from Roze distribution (Zero brand). Anyway, load development and testing went fine and my final load shot as accurate as my shooting abilities would allow. I inspected the brass/primers at the range and all seemed well. The load is within loading data I confirmed from two sources (Hodgdon and Lyman#47).

After several distractions with one thing or another it was time to load up another set. I was looking at the brass/primers again and noticed that one primer out of every cylinderful was "flatter" than the others. I usually just load light, target loads and hadn't noticed this before. I've included a picture below. So do you think this flattening is excessive and I'm flirting with danger, or have I just not shot enough "big boy" loads and this is normal? The flattening occured even in loads with 1 grain less of powder. I checked my chamber throats with calipers (not super accurate) and one wasn't particularly smaller than the others, though there is variation. There is a ring around the dimple in the flatter primer, this doesn't feel elevated, but is that what primer backflow into the firing pin hole looks like?

Unfortunately, I have no way of telling which chamber (if it was only one) was causing the flatter primers. I'm not one of those guys who has to push the limit, but the recoil/boom of this load is very satisfying. Recoil is stress leaving the body.

Thanks for any information and help in advance. Let me know what I need to clarify, I'm just not sure what information is relevant.
 

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It looks normal for .357 Magnum loads to me. You should shoot a cylinder full of factory magnum loads for comparison. I'm sure they'll all have flattened primers.

Primers are also just one indicator of high pressure and can give false readings. Case expansion is another indicator.

Hope this helps.

Fred
 
A silly question but were all the primers from the same box? If not they could be slightly different in composition, even within brands say old or new batch and look different. Factory loads and close inspection will show a lot as mentioned above.
 
You may well have one chamber tighter than the other five, enough for the cartridge to "know" but not enough to detect with calipers. If you want to find it, mark the cylinder so you will know which chamber each round is in.
 
As Jim Watson stated or...When working up a load with AA #5 for my Ruger .357 I noted that AA #5 was very temperature sensitive. In that as I fired each round the primers showed more flattening then the previous one in the cylinder. Thinking that I might have a tighter chamber I rotated firing sequence and found that as the cylinder heated up so did the powder which increased the pressure in the next round. The hotter the cylinder the flatter the primers. I no longer use AA #5. Not heard of it with H110 though...
 
J.W. and Bushmaster gave the first things that came to my head. Only other thing I could say is H110 is very sensitive to amount of crimp. If your brass varies in length and your crimps are not consistent, you will see the same effect. I don't think you're flirting with danger as most of the primers in my spent casings with H110 in .357 look like the one on the right.

What kind of charge were you using?
 
Your primers look fine. As long as your loads are within published guidelines, and extraction isn't stiff you're ok with that load.

Another thing that could be causing it would be a slightly rougher finish on one of the chamber walls. The microsecond timing of the primer and case movement during ignition would differ, giving you a different effect on the primer appearance.

(Isn't this hobby fun? Variables beyond measure seems like.)
 
Unless the same cylinder does it every single time compared to the others, I would not worry about it. I probably wouldn't even if it was. A gunsmith could probably fix it if it worries you.
 
I wouldn't worry about it either. If you want to "gauge" the cylinder throats just drop a bullet of known diameter into the chamber and push it through the throat. A .358" cast bullet should push through without a lot of force and no obvious scrape marks.
 
Wow. Thanks for all the great replies and info. I very much appreciate it.

Next time out I'll keep better track of which case came from which chamber in the cylinder to see if it is consistently one chamber. I trimmed the brass, so I know all the cases are within .001" of each other. The crimps look consistent, but that doesn't mean there aren't differences. Extraction was not a problem, so that's good. The primers were all from the same box (same case/lot etc.). Also, the cases all resized without having to exert any extreme pressure.

The load is within published guidelines - 16.7 gr of H110 (Note: I AM AN ENTIRELY UNRELIABLE SOURCE OF INFORMATION, DO NOT USE THIS AS LOAD DATA). Lyman#47 had a maximum a little higher than that.

All great ideas. I'll test the follwing next chance I get (might be a while):

1. It is one chamber due to small differences I cannot see or measure with the tools currently at my disposal.

2. It is round to round variation due to variation in loading process (I don't think so because it was always 1 case out of 6).

3. It is due to the gun heating up with each consecutive shot.

Thanks again. Looks like the primer is normal for a higher pressure load.
 
Unless you primers flow out and leave no seam at all around the edge of the pocket and puch out looking like an old stovepipe hat when you resize them, I wouldn't worry too much.

Heck, different primers from different companies will look different do to differing hardness levels of the materials manufacturers use.(That's a lot of differentiation in one sentence....):D
 
Another thing to keep in mind. When the hammer hits the primer it shoves the cartridge forward in the chamber where upon the powder ignites and swells the case against the cylinder wall and the primer backs out ever so much. Then as the pressure recedes the case is slammed back against the recoil pad and in turn flattens the primer...Hence the reason flattened primers are not a good gauge of over pressure signs...Unless you have ruptured , cratered or primers that have soot around the edges. Or a "top hat" appearance when removed...
 
I used to have a Bangor-Punta vintage Model 27 8-3/8" that I tried using for the revolver class in IHMSA matches back in the late 1970's/early 1980's. One load I developed for the rams consisted of Speer's 180 grain flat-point .358 dia. rifle bullet (similar one here) loaded in military surplus .38 Special cases over a compressed load of H-110 (no, I won't disclose the actual charge weight :evil:). Had to use a taper crimp die, as the cannelure was inside the case mouth - it had to be to allow the cylinder to rotate.

During load development, I noticed one chamber consistently had noticeably flatter primers than any of the others. As I worked up to my "personal" maximum load, that chamber got so bad (primers would get completely flat, even extruding out into the chamfers in the case around the primer pockets) that I marked that one chamber as "bad" and quit using it altogether.

The load got the job done on the rams (flatter trajectory than my .44 Mag loads), but because of the pencil-barrel on that gun it would recoil so much that the rap on my middle finger knuckle was too painful to bear. Never did figure out what was wrong with that one chamber - it didn't look any different than the other five, and cylinder mouth measurements with the tools available (calipers) didn't show any size difference. I eventually sold the gun to a non-reloader who'd only shoot factory ammo - I figured he wouldn't get into any trouble with it.

So there's some thing (or things) in the manufacturing process that can definitely result in one chamber (or more) acting quite different from the others in your revolver. It looks like your case is a milder one than in my old gun - but maybe your softer loads aren't pushing the envelope quite so much, either...:rolleyes: Cheers!
 
I knew people would have good information backed up by experience here. There's plenty to chew on now. Thanks again folks.
 
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