10MM Velocity Comparison-(3) Guns- 4 different barrel .

[REDLINE]

Some claim Hornady printed a mistake with those hotter loads; I'm not so sure. 15gr AA7 under a 155gr MG JHP runs at 1600 fps from my 6.6" G20L.

I was going to give Hornady a call questioning that very thing.  Maybe tomorrow.

[REDLINE]

Called Hornady regarding the Hornady 6th Edition load manual data on a 155gr XTP and 15.1gr Accurate No. 7.

I was told they don't use those numbers anymore and the claim was they have revised the data in the newest load manual based on now using piezoelectric pressure testing equipment over the copper crusher method they used to use.

So their newest load data with 155gr XTP bullets and Accurate No. 7 powder in now shown with a starting load of 11.9 grains up to a maximum of 12.7 grains.  The 6th edition load manual showed a starting load of the same 11.9 grains, but going to the 15.1 grain maximum.

I asked what pressure reading their new pressure testing equipment showed at the new maximum of 12.7 grains.  Direct quote;  That is probably going to be right at about 30,000.  I wonder what he meant by probably.

This is all quite mind boggling knowing that Accurate's own load data tells us that a load consisting of:  155 Hornady XTP, 12.7 grains of Accurate No. 7, COL of 1.250", and a Winchester LP primer, yields 37,500 psi.

So, take it all for what it's worth.  At this point I'ld say all of the above is worth waaaaaaaaaaaaay less than 2 cents.  So, I guess it's back to fiddling till you blow yourself up.

[Yondering]

One of the big differences between the two measurement systems is "time at pressure". The piezoelectric measurement system looks at the peak pressure value, regardless of the time value, and resulted in reduction of a lot of load data when it was implemented. The crusher system gives a measurement of how much the copper pellet is crushed (obviously), which involves some time value.

Pressure signs with hot AA7 loads are unusual, compared to most other powders, like Blue Dot. With Blue Dot, in my experience, flattened primers show up just a little before, or at the same time, as case bulges or "smiles" show up. With AA7, flattened primers seem to show up waaay before any case bulges. I've gotten flat primers with 12.5-13gr AA7 and a 155gr bulle, but even at that 15gr load above, I haven't seen any sign of case bulging. This is in my LW barrel which has about the same unsupported area as a stock G20 barrel.

This tells me that either 1) the fine AA7 ball powder is leaking into the primer pocket and causing the flattened primers early, and/or 2) that AA7 has a pressure spike that occurs too fast to bulge the brass, even though it may technically be over the pressure limit. 

[REDLINE]

That is interesting.

[475/480]

So, take it all for what it's worth.  At this point I'ld say all of the above is worth waaaaaaaaaaaaay less than 2 cents.  So, I guess it's back to fiddling till you blow yourself up.
[/quote]

Exactly .
IMO ,NO way can pressures given by the ammo companies be EXACT. To many variables .
I wonder how accurate the gauges are that they use now-adays. Who calibrated the FIRST gauge many years ago and what was used as a STANDARD (they had NOTHING to go by back in the day).

Sean

[Yondering]

And beyond that, just the variations in different barrels. Little things like throat length make a big difference in pressure. How was the test barrel set up? All good reasons to start low and work up with any load.

[nickE10mm]

Fascinating info here ... I've always wondered if I need to go back and mess with AA7 some more.  I'd really like to know if the primer flattening is a sign of overpressure or something totally different.  I really like AA7 but was always too scared to push it.

[475/480]

Fascinating info here ... I've always wondered if I need to go back and mess with AA7 some more.  I'd really like to know if the primer flattening is a sign of overpressure or something totally different.  I really like AA7 but was always too scared to push it.

IMHO- in a semi-auto , a primer getting flatter is a sign pressure is going up .
In a  revolver it could be a question of the headspacing or a few other variables.

Sean

[The_Shadow]

Some 10mm brass gets shorter with use which can increase the headspacing slightly thus allowing the primer some movement.   Therefore without actual pressure testing we are only guessing? 

[Yondering]

Fascinating info here ... I've always wondered if I need to go back and mess with AA7 some more.  I'd really like to know if the primer flattening is a sign of overpressure or something totally different.  I really like AA7 but was always too scared to push it.

IMHO- in a semi-auto , a primer getting flatter is a sign pressure is going up .
In a  revolver it could be a question of the headspacing or a few other variables.

Sean

There's something weird with AA7 compared to other good 10mm powders; primer flattening shows up a long ways before case smiles or other major pressure signs are found. With other powders, like Blue Dot, the relationship between primer flattening and other pressure signs is much closer. Example: in my longslide, 155gr JHP's over 15gr AA7 gives no pressure signs except flat primers, and does about 1600 fps. Reducing the charge all the way back to 12.5gr with the same components still shows flat primers, with much lower velocity.

I'm not recommending to ignore the primer flattening, just saying that AA7 acts differently, and I'm not sure why.

[The_Shadow]

The Flattest primer I ever got was with AA#9 testing it with 10mm 140 gr Barnes TAC XP seated to 1.260" these are 0.680"
12.6 grains of AA#9 (I was trying 13.0grains but had to adjust powder shoving the bullet back out 1.270")(Totally compressed @ 12.6 gr.) @ 1181 fps from the S&W 1006 5", Primer was totally flattened to the edges. It penetrated 5" of sandy dirt and the bullet failed to open, partially folded in on itself. Case expanded to 0.4280



Picture added...it was down the other day!

[REDLINE]

I hope someday we find out what Accurate No. 7 has against primers, while not showing other pressure signs.  Weird stuff.  And why isn't any other powder doing the same?  I've seen mention around here that it is maybe just that with A7 being so fine grained, possibly enough of it gets into the flash hole, causing this specific issue.  But then the strange part to me about that, is even when pushed quite hard, it doesn't seem to be causing primer pocket stretching.  It is a very interestingly curious nature going on there.

[Taterhead]

I hope someday we find out what Accurate No. 7 has against primers, while not showing other pressure signs.  Weird stuff.  And why isn't any other powder doing the same?  I've seen mention around here that it is maybe just that with A7 being so fine grained, possibly enough of it gets into the flash hole, causing this specific issue.  But then the strange part to me about that, is even when pushed quite hard, it doesn't seem to be causing primer pocket stretching.  It is a very interestingly curious nature going on there.

No. 9 does the same in my experience. I would believe the hypothesis (Yondering is the first I heard mention this) regarding the small granules filling the voided spaces inside the primer. I don't know for sure though.

[REDLINE]

No. 9 does the same in my experience.

Wow.  Haven't heard that yet.

[DM1906]

Tater and Red....

Please disregard this as a cause.  It's the nature of the specific powders, and NOT the size of the granules.  Many other powders are this fine, and more so, with no similar issues reported.  If it were a real condition, it would be more prevalent than just isolated situations.  If it were, any powder that is small enough to fall through the flash hole would suffer similar inconsistencies.  There are lots, including some of the most revered powders ever used.