Rifling depth and smokeless PP

[Nobade]

OK folks, here's something I haven't seen discussed here. Why are some guns easier to get to work with paper patch bullets than others? For example, I had instant success with my 30-30, 45-70, 358, etc. I have finally gotten the 375 to shoot reasonably well, but it took some work. But the 8MM (24/47) will not shoot for beans no matter what I feed it. My theory here is rifling depth. All the calibers I have success with have .004" deep rifling, or .008" total. .350X.358, .450X.458, .300X.308, etc. But the 375 has .0045 deep rifling, and the 8mm has .0055 deep rifling (.312X.323) Maybe this is keeping the paper from being properly cut? I can load the 8mm with a .312" bullet, wrapped in thick paper to .324", and it won't shoot. I can load it with a .319" bullet wrapped with thin paper to .325" and it won't shoot. I can wrap the .319" bullet to .330" with thick paper and it won't shoot. This is the only reason I can see why, as the barrel is mint and it piles regular grease groove boolits into tiny little groups. And no, it doesn't seem to matter what pressure I load it to. 20 KSI up to 55 KSI, nothing shoots with PP. Anybody else here finding this to be the case with deep groove barrels?

[303Guy]

Now there's an observation! Could that be why my worn bore cavelry carbine shot good with PP the first time? It's got pretty shallow rifling.

Something I've seen with my two-groove is the swaging down of the boolit causes base distortion. Similarly, the mint botr rifle makes pretty little trailing 'feathers' where the rifling has swaged into the boolit. I'm now using a rebated boolit so those 'feathers' don't hang over the base.

The one on the right shows what I mean. The L/H one is my new design.

http://i388.photobucket.com/albums/oo327/303Guy/MVC-875F.jpgThe base on the left looks distorted! It's just the shadows.

Here is a patch 'tail' ring with little bits of groove patch surviving the bore trip. Could that sort of thing upset muzzle exit stability?

http://i388.photobucket.com/albums/oo327/303Guy/BSAMpatchfragment.jpg

[bcp477]

And yet my old M48 with pristine barrel......0.312" / 0.3228" by actual measurement.....shoots 0.314" bullets, twice-wrapped to 0.3236" (then run through a 0.323" sizer......to finally measure 0.3231" or so, after "snap back").....perfectly. I get 1.5" or smaller groups (at 100 yards) with absolute consistency...and often more like 1", when I am shooting well.

Sorry to disagree with your hypothesis, but I think that you simply haven't hit on the right combination as of yet. There are LOTS of possibilities, as I am sure you are aware. Being that every rifle is different, some are simply more finicky than others.

[geargnasher]

I've had good success with .003", .0035", and .004" rifling so far. I think the secret to success is getting the boolit just a thousandth or two over BORE and patching to just a thousandth or two over GROOVE. Many people here told me that when I first started, and it has always worked well for me. Paper strength, seating to engrave the paper, patch technique, and boolit design have a lot to do with it.

If your patch makes a good cloud of "confetti" at the muzzle (meaning little squares or strips where the rifling has cut it cleanly) and it doesn't lead, you're on the right track. Sometimes with shallow or dull rifling I've had to go to a weaker paper, as weak a plain notebook paper, to get the patch to disintegrate soon enough. If the patch has to unwind it can affect accuracy of lighter, longer, boolits. I had that problem with 6.5mm boolits in .270 Winchester, even when approaching 2800 fps, a switch to 25% cotton from 100% cotton vellum made all the difference.

Gear