RSI Pressure Trace II vs. Savage barrel nut

**memilanuk** · 11-22-2013, 01:49 AM

So... been wanting one of these toys for a while, and decided the last time some OT came my way that it was time.

Now I have one, and after making a mess of getting the first strain gauge on (and then removing it, and putting a fresh one on) I managed to get it to the range this weekend. Everything seems to be working (a miracle!) and I'm pretty tickled with it.

The problem is... apparently Savage barrel nuts kind of interfere with the optimal placement of the strain gauges on the barrel, specifically with shorter cartridges. According to the manual in a perfect world the gauge should be mounted over the center of the chamber, and there are several required measurements such as chamber diameter at that location, and the case wall thickness at that point. In my world with barrel nuts on almost all my Savage rifles... that ain't happening.

The manual goes on to state that 'satisfactory results' can be obtained by mounting the gauge over the case mouth (not really specific on what 'satisfactory' means...)... but by my estimation with a .308 Win the case mouth is probably just shy of the front end of the barrel nut, meaning that the closest the strain gauge can be mounted with leaving some room for rotation is approx. 1/4 from the case mouth. I already adjusted the numbers a bit because I know the neck dimensions of this particular barrel pretty well (the reamer is sitting in a drawer downstairs), which got the pressure from the mid 20k to the low 40k range. I probably need to adjust it some more - if the sensor really is that far out in front of the case mouth, then I would need to just use the bore diameter (which one, freebore, land or groove?) and then '0' for the case wall thickness? The load I was using today (B185 OTM over 44.0gn N150 in a Lapua Palma case, CCI 450 primers running ~2670fps) is sort of mild as far as competitive loads go, but I would imagine its still running more than 42-43k psi.

I don't expect it to be dead-nuts-exact; even with a 'reference' load there's going to be some +/-. But I'd at least like it closer than it is now. For even shorter cartridges such as .30 BR, 6 Dasher, .223 Rem, 6.5 Creedmoor, etc. I'm not going to be out in the freebore/throat, but clear out in the lands! Anybody else out there with a Savage and a PTII unit care to share how they deal with this?

Thanks,

Monte

**darkker** · 11-23-2013, 01:56 AM

Well, if B185 OTM means: Berger 185gr bullet, at that speed 40k is very small in your rear view. I would be shocked if you aren't a ways over 60k.

**memilanuk** · 11-23-2013, 10:38 AM

Eh... maybe yes, maybe no.

These guns are chambered with a fairly long freebore (~0.170"), which does quite a bit for keeping pressure down. 'Standard' loads for the US FTR team were in the 2770 +/- 20fps range, and more than a few individuals were running a tick or so over 2800 with no noticeable pressure signs. I realize that brass can hold considerably more than the normal 55-60k before showing pressure... but I would guess (emphasis on 'guess') that we're still well below the 70-80k range where you start seeing ejector wipe, flattened primers, etc.

Back to the original question though...

**earl39** · 11-23-2013, 12:21 PM

Monte, What if you modified your nut by cutting the smooth part off the front of it. That part being non-threaded to start with would not weaken the nut much and holding power would remain the same while giving you between a 1/4 and 1/2 inch of barrel accessibility that was previously hidden at least on a small shank barrel. I believe i would put a full size nut back on after testing but it is just a thought.

**sharpshooter** · 11-23-2013, 10:38 PM

I was going to suggest the same thing. Basically the nut needs no more than 1/4" of threads on it.

**memilanuk** · 11-24-2013, 02:31 AM

Hmmm... well, I'll definitely have to try that on future setups. I've already used up my frustration quota for a little while getting this strain gauge mounted... dang cyanoacrylic glue acts almost like a lubricant against the metal at first, while almost immediately sticking to my fingers. Needless to say, the first one was removed (not reusable) and done over. So this one may stay put for a little while.

Savage6x284 · 12-02-2013, 11:20 AM

I have the original PT unit (not the PTII) but since all of my Savage rifles have been rebarreled without a nut I can place the SG in the optimal position. I just don't see any other way to get accurate readings with a short cartridge nutted barrel.
Are you placing the SG on the underside of the barrel?
PT combined with QuickLOAD makes for an awesome combination.

**memilanuk** · 12-03-2013, 12:37 AM

Strain gauge is mounted on the top... wasn't really eager to start removing material from the stock to provide clearance for it on the underside of the barrel.

dcloco · 12-03-2013, 02:54 AM

I have one of these strain gauges as well, but the software for mine only works with Windows 98. What are you using????

Built a couple of custom rifles on Savages (338 RUM, 6.5x55 AI, 22/6mm AI, 223 AI, 222 Magnum AI, & LONG 222 Magnum AI) that I would DEARLY love to know what pressures I am at.