RSI Pressure Trace, First Experiences and Results

[Brian Pfleuger]

Well, as many of you will know, I won an RSI Pressure Trace II on eBay.

These last few weeks have been an interesting time while I figured out how to get everything together and working.

The system I acquired was unused but rather on the old side. Unfortunately, the supplied glue, which is supposed to be unlabeled Loctite 401, a special kind of Super Glue, was dried up. I had a tube of Cyanoacrylate-base glue, so I tried to use that.

My first project gun is my beloved Ruger M77 MkII in .204Ruger.

Let me tell you, trying to super glue a very small, roughly 3/8"x 1", smooth piece of plastic onto a clean, glass-smooth piece of curved steel, exactly where and oriented how you want it is no easy task. My first impression of the PTII is that I really wish there was an easier, more reliable way to install the gages. It's not their fault, I can't imagine how they'd do anything to make it any easier but I wish they could.

In the end, I got the gage on there and was reasonably happy with it. It was square to the receiver and where I wanted it. I waited the requisite 24 hours and went to try it out...

Short story here... no results. I couldn't get any readings. I had read that it is theoretically possible to remove and re-install the gages, though it is heavily discouraged in the RSI literature. Anyway, I tried, and failed miserably. I destroyed the gage and I can't really imagine how it could even be possible to get one off intact. I recommend that you don't try.

Anyhow, after extensive discussion via email with Jim at RSI, I decided that the glue that I used was probably not sufficient for the task. I needed more gages for other guns anyway, so I ordered 3 more gages and the proper glue.

With the proper glue in hand, and placing the gage in a slightly different place, I tried again. Still, this gage installation is no easy task. What a pain. As I found out later on a different gun, it's a bit easier if you have two people.

Anyhow, new gage installed and first results posted below...

 

[Brian Pfleuger]

I have to say, I've been a real pain in the butt to Jim at RSI and he's been very patient with me. My first attempts at using this device are with an already relatively low pressure cartridge, with even lower pressure Trail Boss loads and with a barrel, because of the cartridge, that has unusually thick metal.

Here are my first traces, with a 100% charge of Trail Boss under a 32gr V-Max bullet.

You'll notice the weird, choppy bits from around 0.75 ms and on. The only theory we can imagine on those is that they correspond almost perfectly with OBT barrel times, meaning that the pressure wave would be at the chamber, causing those spikes. That's just a theory though, and those spikes are an oddity. I was also letting the gun "free-recoil", since it has none to speak of with these loads, and the excessive movement may have caused fluctuations on the wires.

Jim at RSI really, really doesn't like QuickLoad, BTW, saying "In my estimation Quickload's reliability is one of the greatest cons being perpetrated on shooters."

Still, in spite of his obvious dislike with QL, he has discussed my results and expectations very cordially.

I mention this because the results of the Trail Boss loads are very much different than QuickLoad would suggest. QuickLoad thinks that this max charge would be around 32,000psi and the PT results show a max pressure of around 18,000.

This is an interesting experiment for me, because both of these products (QuickLoad and Pressure Trace) have disclaimers that effectively say "You can't trust this data, never exceed published load data!", yet both also claim, or at least imply, to be accurate... because they tell you about all this wonderful data you can have and what good would all that data be if it was wrong... right?

Anyhow, on to test day 3 in the next post....

 

[Brian Pfleuger]

Day 3 of testing....

We know have a gage on a Ruger M77 MkII in 6mm Remington as well as my .204. Today is the day to test some "standardized" loads from published data in the .204, as well as some "made up from whole cloth", QuickLoad loads for the 6mm.

Unfortunately, I still messed up the gage installation on the 6mm and got it too close to the shoulder of the chamber. It is supposed to be at least 0.25" away from changes in diameter of the barrel and I've only got it about 0.19" away.

CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.

First up, I used QuickLoad to conjure up the suggested max load for 55gr Nosler bullets in the 6mm. We're looking for some real varmint killers and we're willing to push this gun a bit. It suggested a heavy dose of RL-17. I started at a reasonable 45.0gr, which is under max loads for 70gr bullets, and loaded up in 1.0gr increments up to 52.0gr, which is a predicted 68k psi. Don't try this at home, boys and girls.

Here are the first traces (green #3 is a fake)...

Notice that "scary bit" of #1 around 1.25ms. We surmise that it is just an artifact from not holding the fore-end of the gun, since it's on a bipod. Shot #2, the gun was held more normally.

Realizing that the "true" barrel dimensions where the gage is situated are probably being effected by being too close to the shoulder, I made adjustments to the measurements, adding about 1/4 of the change at the shoulder to the actual measurement.

This change produced this trace:

QuickLoad's predictions were 42k and 45k, respectively. We have fairly reasonable agreement.

Interesting, but inconclusive... more on the 6mm to come at a later date.

 

[Brian Pfleuger]

On to my .204 data.

I conducted an experiment here again, loading 3 of my "go-to" accuracy load with CCI BR primers that normally use and 3 with regular CCI SR primers.

Here is the combined trace, with 1-3 as SR and 4-6 as BR...

I thought I had something really interesting going on here but #6 kind of screwed it up... here are the seperated traces, first the SR and second teh BR primers:

It was looking like the primers made a real difference in the curve until #6 screwed it up.... more testing is needed.

The BR primers actually seem less consistent in this small sample, and you'll notice that the barrel exit times for the SR primers (Xs on the traces) cluster very tightly around the predicted OBT exit point (red dot at bottom). This would explain why this load is so accurate, if that trend holds true.... more testing is needed.

So, that's my fun so far with the PT. It is a learning curve and not all has gone to plan. I think I'll get better at installing the gages and I think I'll send the unit in to RSI to get it refurbished with the new style connector and a larger capacitor that they now use. Jim tells me that both will improve my results....

Looking forward to more trials....

 

[SL1]

Brian,

Where are the velocity values coming from? Are they calculated from the pressure trace, or measured externally with a chronograph?

SL1

 

[ScottRiqui]

So how do you "calibrate" the PT? Because unless you know all the intimate details of the barrel's construction right at the location where you have the gage mounted, it seems like RSI's numbers would be just as "voodoo-prone" as Quickload's.

In other words, while I might believe the *shape* of the pressure curve given by the RSI program, I don't know how much stock I'd put in the actual magnitude of the pressure readings.

Still, looks like a good product, and I'm looking forward to seeing more results. Thanks, Brian!

 

[Brian Pfleuger]

Velocities come from a chronograph.

Scott, if you look at RSI's site, it will explain everything you could ever care to know about the measurements, including errors that might be introduced by different steel formulations and similar unknowns.

 

[Brian Pfleuger]

I just received a reply from Jim at RSI regarding all these graphs.

He believes that they are essentially correct. In the 6mm, he points out the significant ignition delay (flat spot early in the chart) which when coupled with a slow powder and a light bullet, is leading to a secondary ignition.

Oddly, my .204 loads with the BR primers seem to exhibit the same sort of delayed ignition, where the regular primers do not. The delay is actually much worse than the 6mm, he evens thinks it might be enough to be noticed by the shooter. He thinks, MAYBE, a 5,000psi correction might be due in the .204.