ARComp in 223AI with heavy bullets

[queen_stick]

I'm using this powder with Lapua cases, Tulla SRM primers, and 80gr Amax.

My fire forming load held half MOA out to 300yds, which was 23.3gr, and had MV of 2875. This is a varmint rifle, so I'm really happy with the accuracy

Anyway, tonight I loaded up my fire formed cases for the first time to determine the max load. What I learned in this process is that this powder has a VERY fine line when you reach pressure. At 25.1gr I was getting 3005fps, and had no sign of pressure. Not even a slight ejector mark. However when I went up another .2 grains, at 25.3, the primer fell out of the case after stiff extraction, and there was a heavy ejector mark... Obviously too hot. Anyone else see similar results with this powder?

By the way, I found a node that looks good right at 2875, which happened to be the same velocity that shot well in fire forming. I also have a potential load right at 3000fps, but it's JUST under that fine line...

Almost forgot... This is in a 27in 7-twist Shilen.

Anyone know how temp sensitive ARComp is? And, is it normal to exceed pressure with no warning signs as you approach it?

 

[acloco]

Which bullet?

Length of barrel?

 

[queen_stick]

80gr Amax.

27in 7-twist Shilen.

 

[BoydAllen]

Years back, a friend that caused me to coin the term "chronograph idiot" had a couple of AI varmint rifles. He had them loaded up to the number that he thought appropriate, with no pressure signs, and when he ran out of the Federal primers that he had been using, switched to Winchesters, that he had a case of....without backing off and reworking. This little change blew primers out of cases, and showed the advantage of a Hall action over a Panda in how the bits and pieces were handled. The firing pins were blown back with such force that the Jewel triggers required work to restore their functionality, all this from a primer change. My point is that because of their design, AI cases do not show the progressive signs that give a prudent reloader plenty of warning that he is approaching disaster, and so the prudent reloader must resort to more sophisticated means for evaluating pressures as he inches toward the maximum. Also, ball powders are more abrupt when they loose their linearity at high pressures.

 

[queen_stick]

Thanks Boyd.

When you say "sophisticated means," are you talking about a chamber pressure gauge, or is there something else I can look for, that I'm not aware of?

 

[BoydAllen]

Head expansion was what I was thinking of. Some loaders have used blade micrometers that read to .0001 to measure in the bottom of extractor grooves, and correlate that to pressure. You make a mark across the head with a permanent marker so that you take the measurement the same way each time. This is not a guarantee, but it does give an additional indication that is independent of case shape. Smaller cases have smaller expansion limits. It is best done with an unfired case, but I have used it to track what was happening in a similar manner by measuring the belt diameter of a belted magnum, on a single case. It moved on the first load that was not hot, and was stable up through some progressively hotter test loads, and then it moved again, at a very hot level, on the same case that I was doing all testing with, loading at the range. I would play around with it with all of my reloading to get a feel. Remember that different brands of brass have different limits, and your testing should be brand specific.

 

[Hal]

Queen

It's not your powder that has a narrow pressure window.

A load that will work for fire forming will sometimes be to hot for the formed case.

Hal

 

[queen_stick]

Boyd, I no longer have an unfired case on hand, but I'll measure the case that was way over pressure, and compare to the others. I have a good mic, but it's not a blade, so I can't check in the grooves.

Hal, I worked up to the max on the formed cases. I was a full 2.0gr above my fireforming load before I saw any sign of pressure. That 'sign' happened to be pretty extreme. Just 0.2gr below that failure, the primers were nice and rounded, not even a hint of ejector mark, and extraction was normal.

 

[queen_stick]

Boyd,

You may have found my problem, and I think it's the chamber...

as precisely as I could get, I measured the cases at .200 above the head (marked with sharpie as you recommended). Per the spec shown on the 223 page (http://www.accurateshooter.com/cartridge-guides/223rem/ about half way down for the AI drawing), this should be .3767.

I still have my cases in the order I shot them last night...

-- the first 4 cases are .3789
-- then then there's a jump in the next 4 cases to .3793
-- then there's 2 cases at .3809
-- then the final case that fell apart, at .3850

The means my chamber is at least .009 bigger than the spec... do I need to set this thing back/re-chamber??

 

[BoydAllen]

Even if it is oversized, I do not believe that that is what you need to be looking at. Also, when I mentioned the sharpie, I was talking about making a mark across the diameter of the bottom of the case (across the center of the primer). Here are a couple of other things that I suggest you look at. Measure the diameters of your cases right at the top of the extractor groove. using a short piece of rod, measure how thick the solid head of your case is (rod, rod bottomed inside of case sticking out mouth, subtract rod from the latter). See if you can determine how far your cases stick out of your chamber. I want to know if there is an issue with the bottom floor of the inside of the case being outside of the chamber.

 

[queen_stick]

Even if it is oversized, I do not believe that that is what you need to be looking at.

good to know

Also, when I mentioned the sharpie, I was talking about making a mark across the diameter of the bottom of the case (across the center of the primer).

Go it now. thanks for clarifying.
Should I do this right now, or are we beyond that at this point?

Here are a couple of other things that I suggest you look at.
Measure the diameters of your cases right at the top of the extractor groove.

first 4 cases - .3772
next 5 cases - .3785
10th case - .3788
case that fell apart - .3865

measured these the best I could, and measured each multiple times to confirm
(I don't use precision measuring devices daily, however I'm using a Mitutoyo 293-340 outside Mic)

using a short piece of rod, measure how thick the solid head of your case is (rod, rod bottomed inside of case sticking out mouth, subtract rod from the latter). See if you can determine how far your cases stick out of your chamber. I want to know if there is an issue with the bottom floor of the inside of the case being outside of the chamber.

working on finding something that will work for this...

 

[BoydAllen]

This is less than precise (no unfired cases to measure, no before measurements for these cases) but if you think about it, when the diameter in front of the groove moved the second time, it was about to come apart. You needed to be looking at this as you tested, letting the brass cool before the next measurement and shot. The area that you measured is solid, except for the flash hole and primer pocket. Smaller cases have lower limits for safe head expansion than larger ones. Time for some research. You had real indications of pressure, you were just not set up to see them, and did not know that the AI shape, hot loads and ball powders can be a very touchy combination.

 

[queen_stick]

no luck finding a rod (my hardware selection is lacking big time, which drives me nuts every time I need something)

can I use a decapping rod from a Wilson die, with a Hornady ogive tool attached to my blade mic (for clearance for the decapping rod sticking out of the primer pocket)???

Just saw your last post, Is it still worth running through the motions without having a control case? If so, I'll grab some rod tomorrow, and cut it to length. (Or use the decapping rod if you think that will work)

 

[BoydAllen]

A cleaning rod jag that is long enough to stick out of the mouth of the case will work. We are not working to .0001 here. Just for fun Google safe cartridge case head expansion, and keep in mind that many of those that wrote on this omitted the fact that since the expansion is actually a percentage of the working diameter, that small cases should observe proportionately smaller limits.

 

[queen_stick]

Thanks again Boyd.

I'll do some reading up on the subject.

I'll try to measure the case head thickness when get home from work tonight.

How do you propose I can determine how far the case sticks out of the chamber? I can pull the barrel if needed.

 

[BoydAllen]

That would be the best way.

 

[queen_stick]

Boyd,

I was on a last-minute business trip this week, so I haven't had time for those things called "hobbies"

I'm gonna follow up on this as soon as I get a chance

 

[queen_stick]

Boyd,

I finally got some time tonight. Pulled the barrel off, and used the cleaning jag as suggested.

I measured a couple of times to double check my measurements.
Jag = 1.9175
Jag + case = 2.0965
Difference = .1790
Case protruding from the chamber (used depth gauge) = .160

.179 - .160 = .019

Is .019" above the inside of the case head too far out of the chamber?