Hello all, I've been tinkering around with handloading for about six months now and wanted to share some lessons learned and bounce some theories off this distinguished forum.
Lesson 1
Early on in my learning, I was (overly) enticed by the idea of minimizing jump to the lands as a key variable in pursuit of precision. Now, I'm running a stock piston-driven 16" PWS Mk116, and while I knew CBTO/jump is more of a consideration for precision loads in bolt guns, I figured I'd see if it made a difference in my AR. So after working some loads for the Nosler 125gr BT using Nosler's published data (2.06" COAL) I decided I would start lengthening COAL to see if my precision improved. I felt safe in deviating from published because a longer COAL over the same charge will result in lower pressures. So I lengthened to 2.15", and then eventually to 2.20", and ran some ladders.
But what I found did not match expectations at all. Here's a portion of the load tracker I built in Excel (lot numbers on the left).
(The usual disclaimers apply - I make no claims on how this works in your rifle, always start supers at published lows and work up, etc. etc.)
These are all my 125 BT lots sorted first by COAL, then by charge (W296/CCI400/GCG converted brass are common). You can see I was figuring my jump in both the 116 and my SBR, in case I ever shot the same loads through it. It is apparent that those long COALs, even though my jump got almost as small as I could make it, resulted in large variability in MV and there's no noticeable trend as COAL increases.
But when I re-sorted the same data by MV, suddenly the light came on:
The obvious lesson I learned is that the most significant variable to precision is MV, with my rifle's best nodes around 1990, 2070, and 2130 fps. This isn't to say jump to the lands isn't a factor, just that if I haven't found an MV node first, tinkering with COAL/CBTO is only going to muddy the waters.
I'm sure this'll get a "no sh!t" from the old salts!
Lesson 2
I noted in the first sort (by COAL) that once I went out to 2.15" and longer, variability in MV went way up. Again, no surprise to the masters out there, but my gut feeling is that all that extra space in the case under the bullet makes for a less uniform burn. The powder is much looser in there and won't ignite the same way every shot. The 2.10" lot was tested yesterday and I was pleased to see my SD/ES back in the range they were when I started.
So, powder volume/COAL will have a significant effect on MV spreads.
The way ahead?
Next I'll try to duplicate the two nodes at 2070 and 2130 with 2.10" - try a 0.1gr adjustment up and down, etc. but I don't think I need to try to get any better than 1MOA, seeing as how I'm only building for self-defense and don't expect to shoot past 200-300 yards (hopefully the chance this country disintegrates to the point that a 300-yard S/D shot becomes a possibility is VERY remote!) If I were to keep increasing precision, I'm thinking the next step would be to start sorting brass and bullets by weight. So far, I'm just pulling them all right out of the box and loading.
Anyway, it's my hope if any newbies read this, it might save them some cash, materials, and trips to the range chasing the wrong variable!
Welcome any corrections, tips, or wisdom.