brass showing pressure signs at lower powder charges with subseqent firings

[jmden]

This may be a well known, talked about and explained phenomena, but I haven't found an answer yet and would appreciate some input from folks in the know.

Observations particularly while working up a load for my 300 RUM using Remington brass: While fireforming I found that I could slowly work up to 97.5 grains of Re 25 before noticing pressure signs (ejector pin marks). However, upon the second firing (necksizing each subsequent firing until won't fit in the chamber well, then bumping back the shoulder .002, the necksizing again, etc.), again working up slowly in .2 grain increments, I would only get to 96 grains of Re25 before seeing pressure sigs. Upon the third firing, again working up slowly by .2 grain increments, I found that I started seeing pressure signs around 95 grains of Re25. Subsequent firings indicated this phenomena had stabilized and pressure signs seemed to stay at about 95 grains Re25. I've surmised/theorized/guessed that it is taking 3 or 4 firings for the case to stop stretching in the chamber. (I hope I'm stating this so people can understand it.) And that some of this momentary "stretching", is taking up energy/pressure that would otherwise be concentrated more on the case head after 3 firings or so as after 3 firings or so, the brass has fully "stretched". Thoughts on these observations and my current theory?

(Load development procedure: I graph the velocity against the powder charge and look for several .2 grain increments to produce or nearly produce the same velocity, a flat spot in the graph, then concentrate further efforts there. I read in Precision Shooting awhile back that this flat spot in the graph can indicate a powder charge/bullet/barrel harmonic that is working together for best accuracy for your particular rifle. Seems to work for me. When I see pressure signs, I go below that velocity on the graph that showed pressure signs to the next lower velocity (and corresponding powder charge)flat spot on the graph and work further there. This process produced a 94.4 grain (I carefully weigh every charge)of Re25 load for me using 200g Swift A-frames recently. COL of 3.667 dictated by magazine. Now, I have a pretty stock Rem 700 LSS in 300 RUM (stock 26" barrel) that I've had the trigger adjusted on and foated the barrel, so it's not like some of the incredible rifles we see on this site, but at this point, I'm OK with 1" groups at 200 yards--slowly working out farther from there.)

 

[Brent]

Jon,

With my 30-338 Lapua Imp I worked up and settled on 90gr RL25 with the 210 JLK seated .005 into the lands, 3.820 or there abouts. MV was about 3200 with a 215M.

I'm shooting 88gr and a 210M, with MV about 3165 now. I've loaded the brass 7 times now, FL sized each time, bump less than .002 and had to drop a bushing size in order to retain the .001 neck tension after the 5th firing, as necks were harder and just didn't pull down enough with the .331 bushing any longer. My headspace though, it has always remained the exact same thing since the first firing, fired, and sized.

Mines a tight neck, turned for .002 total clearance, and so far I've managed to get away without anealing yet. I need to after the 3rd or 4th firing on them though, just put it off and kept loading.

My Retumbo load had to be cut back too, it was way too hot to keep loading at the charge I started with as well. About 96gr is where I maxed out before at about 3250 fps. Now I'm at 90gr, and about 3180 at the same pressure...

So far, my thoughts on it lead me to wonder if the throat roughening up after a couple hundred rounds results in a loss of efficency. I can't trace it to anything else. I've been monitoring chamber pressure with the Oehler M43 and Southwest Products PressureTrace, so the comparisons are based off the before and after PSI numbers as well as their corresponding MV averages. Powder lot #'s, primers, OAL and everything were identical, I even waited for temperature to approach the same as the tests done last fall. Even then, the lower pressure loads were fired in warmer temps. I had wondered if the cold (+20 deg) was reducing the bore diameter enough that that was some of the cause, but it was not. Originally the pressures were "way" off the scale, 75-80,000 psi where they normally were about 68-70k psi. Half, well almost half of my psi problem was a result of excessive fouling because I shot it until I had over 100 rounds through it to see when accuracy would fall off. Never the less, I still had to drop the load, pressure just did not come down all the way like I'd hoped.

When I get new brass, I'll see what effect it has on this all over again, anealing these will be interesting also.

I kind of do the same thing as you on the load development end, but instead of the MV "flat spot", I look for velocity points where the POI shifts up then down or the opposite, the flat sopt there. Hopefully the MV isn't erratic there when I'm done. Optimally I'm looking for a range to load that my POI is stable after shifting high, and I have enough capacity (pressure range) left to load higher until the POI shift moves down or flattens off before moving up again. Keeping the loads ES in mind, MV shift points, POI shift direction, and temperature effect on the loads MV, you pretty well know where to load for a certain temp range and if one end of the range will stack errors and go vertical much faster than the other that may cancel some of them out if you fell into that range.

Unfortunately a couple of my good loads don't produce a low POI shift with an increase in MV due to harmonics, just flattened out and went lower if you backed off MV, which just coumpounds the problem. I've got to recheck everything again with it in the new stock now.

 

[jmden]

Brent,

Your mention of barrel fouling got me thinking. I spent 2 hours with Sweets the other day before it came out clear and not blue. I've always thought I cleaned well, but perhaps not? I've used Sweets every 20 rounds or so, after using Shooters Choice in my standard cleaning regimen which I got from Sinclairs book, but have never used the Sweets until it came out COMPLETELY clear. I visit Kesselrings Gun Shop regularly just south of here and one of the guys there told me this technique for Sweets: pour it down the barrel (using a cleaning bore guide of course) until it just begins to run out the barrel, turn the gun over so that the Sweets coats the other side of the barrel and then let it sit for 10 minutes or so. (I go ahead and turn the gun side to side and tilt the muzzle up and down in my cleaning station and watch from the muzzle end to make sure that I'm coating the whole barrel evenly.) Dry patch out and then continue the process until the Sweet comes out clear and not blue. Well...2 hours later, it came out clear. Apparently, I was not getting the copper out as well as I'd thought. Since I'd been shooting Swift A-frames almost exclusively (I believe they have a 100% copper jacket--not sure if this might cause more copper fouling than some of the other manufacturer's jacket alloys--comments?) I wonder if they might cause more copper fouling. If my bore was quite copper fouled...perhaps this might be the cause of what I've observed? Maybe it's not my brass? Or, maybe it's the throat issue as you mentioned?

 

[Brent]

Jon,

I did the same thing the other day when I cleaned mine, just soaked it good with patches though. I could see some very tiny streaks at the groove's edges. It took three rounds with Sweets to take care of it. I'm not sure what effect that tiny bit has on accuracy.

I shoot the 400 A-Frame in the 416 WBY, it does copper foul in my Ruger bbl, although I haven't noticed any difference between them and the Barnes X I wouldn't expect to. I haven't shot anything but them and the solid copper bullets to even check.