Brent, I have a queston concerning the RSI system. Do you use factory ammo to check and calibrate your system? If so what was the difference in the true reading from the corrected value? I just installed a strain gauge on my .300ultra. The chamber has a tight neck so I can't fire factory ammo in the rifle.
Pull your bullets and turn the necks, seat to the same depth over the existing charge to fire the factory ammo.
Do the careful measuring required, barrel OD, ID at gage placement, case wall thickness and you'll be as close as you possibly can to actual pressure. The more testing you actually do the more with factory ammo the more you will feel you have the right system, not much more I can say except that it is uncannely close to what you'd expect from the factory stuff. All my 300 Ultra ammo tested about 63,000 psi FWIW. Anything I can help with let me know.
I have used the RSI for a few years and it is as accurate as any. Has some useful features on storing load developement. Do'nt know if it works on a mac or not. If you use the exbal on a Palm you might want to check out the JBM website for use on your MAC when you have an internet conection.
Brent, I ran the initial test with 200gr Accubonds. With 83.5gr IMR 7828 I'm seeing 3100fps over my Oehler. This load gives me 1/2 MOA out to 300 yards, I've shot under 5" 3 shot groups at 600 yards off a bypod shooting prone. My Nosler manual doesn't list the Accubonds, though it lists 3100fps as max with 200gr Partitions. The RSI is showing pressures in the 56k range, traces overlay each other. I had the throat ground .050" shorter than standard on the reamer, not sure how this would effect factory ammo.
.050 shorter on the throat should not make too much difference in pressure.
What is the max OAL you can seat the 200 AB out to? Where does it contact the lands?
With my factory chamber it touches at 3.835, I think, so if yours contacts at 3.785 or so it wouldn't be much difference.
What is your barrel's inside diameter under the gage?
What is the fired case diameter under the gage?
You could be down in the high 50 Kpsi range but I suspect it's in the low 60 Kpsi range, possibly due to a thinner than actual chamber wall being figured in the calculations.
It seems to me that I remember Burt suggesting in the PT program documentation that he recommends adding .003" to the fired case diameter directly under the gage location to get the actual bbl ID. The reasoning is the brass springs back slightly when it cools and the chamber will be slightly larger than the case you measure, thus the barrel's wall thickness is a tad thinner.
Using the Oehler M43 for some time before getting the PT unit I never had to add any amount to the case diameter though, so I never did with PT either. I think the case does cool and shrink slightly, but it's on the order of .0005, to .001 maybe, it's not much though.
If you use the case diameter at the gage location as the actual barrel ID and don't add .003 to it, you will in effect be telling the program that the chamber wall is thicker. Now if the chamber wall is thicker but, the gage reads the same micro-strain, it will indicate pressure is slightly higher now. To get the same micro-strain on a thicker pressure vessel, the pressure must be higher, this is how that works. You can go in and manipulate the barrel ID on an existing test and see how it affects peak pressure. In doing so, you are simply telling the program that this barrel was actually thinner and to modify calculations to reflect the new chamber wall thickness.
It's been quite a while since I've played with the program much so I'd have to run through it on my own first to remember exactly how this is done, and if it applies to all tests with the same barrel or just the one you happen to be playing around with. After some extensive testing with PT in the begining stages there was some language added/modified in the pop-up windows that made it more easily understood what was being altered and to keep one from accidentally modifying a test without realizing they had.
I worked with Burt for quite a long time finding bugs with the program and getting many key features added and changed to benifit the guy using it, and I can tell you this is the most I've ever seen anyone work to perfect a program and make it the best it could ever be. The guy was relentless in wanting it to be the best on the market, and doing anything and everything fast to make it happen. This guy is simply amazing! If you guys only knew what he went through to make this product what it is today you'd $hit! Sure he wants to sell it for profit but, it goes far deeper than that and is a serious passion of his to offer nothing but the best of the best leading edge products at a fair price, all with outstanding customer service and support. Burt and Jim are just outstanding people and we're pretty lucky to have these two putting their heads together and offering these awesome products, at bargin prices I might add.
Seriously, don't hesitate to call them either if you have any questions, they certainly can help walk you through any problem you might ever encounter. They are just as willing as they are able too.
I'll help where I can too, for sure, just give me a shout.
Brent, the Accubonds are into the lands at 3.785". I had to recheck it, I thought I had written the value in my notes, but due to having to use the rifle as a repeater I load them to fit the magazine (3.690" O.A.L.)so I didn't record it. Kinda surprises me, the rifle has a few hundred rounds thru it, figured the throat might have moved. I did find one mistake, I had entered the actual size of the case, forgot to add the .003" to the measurement even though I read the instructions a few times. I changed it and had the program recalculate the data, pressure is still under 58k. Like you I figure 60k would be reasonable, the load doesn't show and sign of pressure, even while shooting it in hot weather. With my .300 Weatherby, shooting IMR 7828, I had to back off 1.5gr durring the summer heat, my max load was still below the IMR data.
I just installed a strain gauge on a .270WSM to investigate. This rifle shows pressure (extractor marks on the brass) at low load densities. I haven't shot it enough to get a read on primer pocket expansion, so the RSI data should prove interesting!
All I can think of if you have taken accurate measurements around the chamber, and case wall thickness is the amount of jump to the rifling is reducing pressure some 5 Kpsi, which is normal.
For peace of mind I'd try a box or two of factory ammo to see where it registers peak psi, do what you have to in order to prep it to fire in your chamber, little more work but I think I'd go there if put in this position myself.
I have MV and pressure data with Remington factory loaded 180gr Partitions and 180 Sciroccos in the 26" factory barrel of mine to compare if you do indeed get to that point.
3100 fps is reasonable to think it about maximum load for the 200gr Accubond, and adding .003 to the case diameter should have just reduced the indicated pressure.
Tell me how you determine the case's precise location under the gage exactly, in order to get the actual point to measure the case diameter at?
With an empty case in the chamber and bolt closed I insert a cleaning rod down the bore and twist it a bit to make sure it works its in as far as it will go centered up in the dish shape of the flash hole, hold it there and place a piece of tape on the rod "perfectly flush" with the muzzle.
Now I remove the case and insert it over the cleaning rod as it was in the chamber but now line the tape back up along side the bbl to determine the correct location of the gage to be glued on. The bbl and case both get marked at this point and both measurements are taken and wrote down in my log book.
One thing I do differently than most people that I know of who use these sttrain gage systems, and probably Burt or Jim too, is move the gage location forward more than it would normally be. Instead of centering the gage between the bottom of the shoulder and the base of the case, which is very close to the front edge of the receiver on the WSM and 308 length cases, I center the gage between the bottom of the shoulder and the "front of the receiver".
My reasoning for doing this is that the reinforcement that the front receiver ring gives to the chamber has a lesser impact on pressure readings for the stated barrel wall thickness. The bbl is turned down and threaded at that location, so it's smaller than the OD you enter into the program and calculate things from, then you have the fact that the receiver ring that's much larger than most barrel diameters at this location too. The lack of a tight fitting barrel to receiver thread joint would, in my mind, act the same as if the barrel diameter got smaller there, and if the thread joint were nice and tight it may act the same as if the barrel was larger in diameter, being reinforced by the larger receiver diameter... hope that makes sense.
The point I'm trying to make is that the barrel diameter at the location very "near" the receiver face is "less" of a known factor due to its questionable "effective" outside diameter. If I position the gage between the bottom of the shoulder and the receiver face I have an even "width" of "known" barrel wall thickness on "both" sides of the gage, so to me this makes way more sense.
Here you have the bottom of the shoulder and an increasingly thicker wall bbl wall thickness helping to reinforce the muzzle end of the chamber, and the receiver ring at the breech end either reinforcing it or with a sloppy barrel thread making for a "weaker" than expected pressure vessel.
The expansion real close to the receiver ring could easily be measured and compared to where I place my gages by using two gages, one at each location then switching between the two while firing enough of the same load for an average on both gages one after the other. Then one would at least have quantified the difference between them and have seen psi go up, or down near the receiver face, which would also be interesting to know.
By placing a gage on the receiver ring itself I have measured 45 Kpsi with an actual 65 Kpsi load, both being tested simultaneously with PT and the M43. So, there is quite a measured expansion on the ring itself, but it proved not linier and unpredictable to a point so it was not useful in the end.
In all reality, I think the placement of the gage foward or back 1/4" or so doesn't make a "real" big difference if the barrel wall thickness is still accurate for the actual location. If I tested it and found a 5 Kpsi difference it would shock me, really. It's probably in the order of 1-3 Kpsi at most, if that, but it would be interesting to test it though.