LFP Comment Thread

[LRRPF52]

I put a grid overlay onto a copy/paste of the picture into my little graphics program. Now this is 600 wide by 975 tall grid, I'll call them pixels, they're not inches or mm, and I had to make a couple assumptions:
1. Measuring the band-gap (grooves) entails going from the outer edge of a band-cut, across both chamfers to the other outer edge.
2. The two bullets look to have the same linear length from ogive to start of the respective boat tail, so I'm assuming that.
Meaning the difference in bearing surface, if any, would be the delta of the band-gap sums.
For the 120 TTSX, I measured a total of the band-gaps (grooves) of 120 pixels, 40 pixels for each gap.
For the MKZ, I measured 100 pixels, well: 1 at 51 and the 2d at 47 so I rounded up slightly, conservatively.

Meaning to me the MKZ may have a bit more bearing surface. Of course the friction is also a function of the hardness + any lubricity of the two metal surfaces, which could be different in the 1 vs the other.
Both are solids not lead/copper so probably uniform hardness properties radially through a bullet's diameter.
So --- hard to say IME - maybe the MKZ is a bit longer of a bearing surface,
But Rick you had the real things not just a picture, so what I'm seeing could be less precise.

O, one more point - rather a question, since the MKZ gaps are further to the rear of the bullet, does that cause more initial friction for the first few microseconds for bullet travel vs the TTSX bands which allow some yielding "sooner" in bullet flight? I wonder but don't know.

Interesting.

That's where I'm leaning. This doesn't mean it's a bad bullet. Barnes and other solid bullet manufacturers used to use no relief bands at all. Some still don't have them. The start pressure considerations for a bullet without relief bands close to the ogive are going to be different than one like the TSX where the first relief band is within ~.080" of the ogive, effectively forming a short ring that is driven into the leade, followed by the space of the band where there is no additional resistance until the second ring is encountered.

The Barnes engineers said it took them years of experimentation working to try to reduce the start pressure with the older product line, when the answer was under their noses all along with cast bullet shaping. Even with the relief bands, start pressure is noticeably higher with a solid, and especially increased when you kiss the lands with long COL.

They said they've seen pressure spikes as much as 10ksi with the same load just with touching the lands.

The only time I've ever blown a primer with a hand load was with the GMX in .260 Remington. It blew the snot out of that primer, and I ceased shooting and pulled that load. Everything appeared to be in a reasonable load range, but nope, it was way overpressure.

These bullets need pressure-testing in a standardized pressure test breech by someone competent, who has calibrated the system meticulously. Barnes said it took them years of calibration and re-calibration, which they are always chasing. When they see people slap an RSI on their chamber and take a few readings, they know that person is not getting any valid readings unless they are building an initial library and calibrating the gauges over a long period of time.

That's why when I see the figure of 31.5gr of CFE being thrown out there to "safely" load under this bullet, I want to see some test data to support it, because 31.5gr of CFE under a 120gr TSX is way over the top for me, I won't even go near that and haven't in my load development with it. Things start getting crazy after 30.3gr of CFE223 under a 120gr TSX at 2.230" COL.

Notice the increase in speed:

120gr TSX, CFE223, 2.230" COL, Hornady 1x brass, CCI 450, 18" Lilja 3groove, 1/8 twist
30.0 2435 2424
30.3 2463 2497
30.6 2563

When you see 66-100fps jump with .3gr of charge weight, you know you're in a really bad place, and that's with a bullet that has the relief band right behind the origin of the ogive at the shank.

Another thing that may be in the MKZ's favor is that it appears to be softer with an advertised 1600fps expansion threshold, but even still, a solid resists being driven into the lands much more than a cup and core bullet. I've never seen a spike that high with a cup and core with CFE223, even when I pushed my initial pressure ladder with the 123gr AMAX waaaaaay past MAP just to see where it would depart. I was able to get the 123gr AMAX going so fast from a 16" barrel that it exceeded 24" velocities, but that doesn't mean it's anywhere near safe to produce that load.

If I had the inclination of other people who have just jumped into this discipline, I would rush off to the internet and yell, "I just got 2700fps from a 16" Grendel with 123gr AMAX! No pressure signs!" Insane velocity is your first pressure sign.