RPMs, 1903A3, and Larry Gibson's tests

[jonk]

Having read through some of Larry's test results and having a 1903A3 with a 1:10 twist, I might extrapolate the following.

With a 1:10 twist barrel, he got optimum group size (albeit with a 308) and a Lyman 311291 mould at an RPM of around 148,000. (From his graphs). Some quick math suggests that would be about 2055 fps. Yes?

Now extrapolating some load data for the 30-06, a 180 grain bullet being driven by 45 gr of IMR 4895 reaches 2555 fps, as per IMR's website. Granted, barrel length, exact bullet weight, and bullet profile will affect this figure, but it's good for a ballpark starting point. In other words, it is exactly 500 fps faster than we need to achieve maximum stabilization and greatest accuracy- again, as per Larry's tests.

Handloads.com's reduced load calculator suggests 35 grains to acheive this. How's the math? What do you think? I'd think that perhaps my problems with this experiment earlier were I wasn't driving the bullet FAST enough with 25 grains of 4895 to fully stabilize.

Thanks!

Jon

[Larry Gibson]

Not sure I understand the comparison of jacketed and cast bullets here?

Larry Gibson

[jonk]

Larry,

I mean to shoot the 311291 myself. I was using your test with the 308 with regards to twist rate and group size and the chrono data for the 30-06 from IMR's website to calculate a suitable powder charge to make a guestimate at where to start to reach optimum group size for a 30-06 using 4895 and the 291 bullet.

[Larry Gibson]

Ok, that makes sense. I'll suggest you try starting at 27 gr of 4895 and work up through 40 gr in 1 gr increments with 5 shot groups. That should give you (it does in all my '06s including my '03s) a velocity range of around 1700 fps up through 2500+ fps with your '06. I also use a 3/4 gr dacron filler with those loads. Without the filler you might need a gr+ more powder to equal those velocities. You can then graph out the velocity vs group size and the RPM vs group size.

This is a good experiment and one which will demonstrate what the RPM threshold is all about. I have repeatedly asked the experts who disagree to conduct this simple experiemnt yet they refuse. That is just a suggestion though. You can run your test anyway you want to. The results will be the same.

Please post your results and keep us appraised of your progress. Thanks for your efforts here.

Larry Gibson

[jonk]

Well I did 25 gr already. It did fair, but group size just wasn't what it could or should have been. 4-5" at 100y. Fair, but jacketed shoot 2" honestly out of the gun. My only concern was even with proper bullet fit, gas check, and 50/50 lube I got some leading using #2 alloy. I might have to melt the 50/50 out and use Felix for this.

[Larry Gibson]

Jonk

I've had the same problems with 4895 using less than 27 gr with 311291 and similar weight bullets. The chronograph shows the ignition is poor with a very high ES. I actuall get the best accuracy with that bullet in the 28-30 gr range with 4895. Also if you aren't using the dacron filler then ignition will be even worse with low charges. I use Javelina, a 50/50 lube, without any leading up through 2600 fps. I've also tested Lars BAC, XLOX 2500+ and carmauba Red lubes (advertised on this forum) and they work very well up through 2600+ fps also. Your leading may just be from the lack of obturation due to the low end load. Your 2" groups with the '03A3 2 groove barrel are actually quite good regardless of some others opinions. You should be able to dupilicate those J bullet groups with 311291. Keep at it, you're headed in the right direction.

Larry Gibson

[jonk]

Wonderful news- in point of fact my 50/50 is Javelina brand. :-)

It's not a 2 groove barrel so far as it goes. Will continue with a bit of Dacron fluff as well.

[Larry Gibson]

Wonderful news- in point of fact my 50/50 is Javelina brand. :-)

It's not a 2 groove barrel so far as it goes. Will continue with a bit of Dacron fluff as well.

Oops, my bad...but so much the better! I've actually got better consistant accuracy at top end with 4 groove barrels out of my '03s than with the 2 grooves.

Larry Gibson

[runfiverun]

the 28 gr with filler is @the same as 32 without only more consistent and more accurate.
ignition issue.....