I shot my BSA 12/15 in 30-20 on sunday, as usual:
- 16.7grs of BP
- Factory-new R-P brass (from pulled factory cartridges)
- Card wad
- 151gr GG bullet, sized to .309 - the only change (before, I was shooting as-cast)

As I was pressed for time, I only fired 10 shots - as usual, prone, unsupported, with sling.  blowtubing between shots (1 single blow).  And this is what happened:
1st shot: centered, but high in the 9-ring
2nd and 3rd:  nicely in the 10-ring - me happy!
4th:  6 at 5 o'clock - ???  Let-off was good, no signs of a bad cartridge.
6-10:  all clustering in the same position as the 4th shot (6-7 ring at 5 o'clock).  Accuracy remained identical the last 7 shots , so I doubt this would be a fouling issue.  I even wiped the last 3 shots, and they also remained in that very same group.

To summarize, I had a pretty serious POI shift (3 MOA left, 2 MOA down)  from the 4th shot without any loss of accuracy, and I am at a loss as to why - maybe someone else can give me a plausible explanation?  Still scratching my head over that one.

Cleaning the rifle afterwards didn't show anything special - if anything, less fouling than last time.

Sam,

the only change was sized vs. unsized bullets - all the rest was identical.  Range Gremlins, now - hadn't thought of those....

Art,

Quite simple really:  Just a 32-20 with a .308 barrel - in my case a 1:15 twist.

Try it without the sling. Sling pressure can make a big difference especially on a rifle that the forearm is attached to the barrel. With all the moving around you probably did loading, etc. I would suspect the tension changed. Also don't mix sized and unsized, there weights are probably different and fit is probably different so either could cause a shift

Outside temp & humidity for this round compared to the temp & humidity for the previous?

Well,

I put another 20 shots on target yesterday evening.  After putting in the corrections required by the POI shift, she behaved this time, keeping a stable POI throughout.  Same load, same bullets, half a breath blowtubing between shots (a full breath puts too much moisture in that teeny little bore).  Grouping stayed stable throughout, even though I pulled a few shots.   

I did have a bit of vertical  (about 2 MOA) but that and the pulled shots was 'nut-behind' issue (mostly sling and sight management - P-H aperture sights).

I'm still fireforming brass, so we'll see what unsized, fireformed brass will do after I emptied the current box (20 rounds to go).

Cheers,
Gert

Blowtubing or wiping is something to test later on, when I have the fireformed brass.  I do have a Delrin rod ready and tried it - it's possible, but not easy.
And as you said, the advantage of a blowtube is not having to break position between shots.  I'd probably go to a mixed regime when wiping - blowtube a full breath, then follow up with a dry patch to get most fouling out.

Still, I have 20 rounds to go in this box for fireforming and 100 after that, so plenty of time...  And I have the next bullet already lined up: the Pope 308403.

To be frank, all the cases were fom the same box of ammo, so I doubt the primer thing.  One thing for sure - I'll start load development with small pistol primers.   The small case won't need rifle primers, certainly not with BP.  But the POI did remain after the shift. so...