View Full Version : 2 groove barrel and paper patch.

07-21-2007, 08:21 PM

How do 2 groove barrels handle paper patched bullets?

I want to try patching a .30 cal bullet for my .303 Pygmy with the .317 groove diameter 2 groove barrel. I figure .310 bullet and 2 wraps will give me a .318 patched bullet.

As the bore land area is 5/8 of the surface area and the grooves only 3/8 I wonder if paper patched bullets will function.

07-21-2007, 09:07 PM
That sounds like a challenge to me! I'd sure like to try to make them work. Let us know how it turns out.

07-21-2007, 09:33 PM
here is a site for PP, he only shoots Blackbut the principal should apply

grumpy one
07-22-2007, 12:28 AM
Jeff, if this was a normal 303 I'd be very doubtful of success. One of the limitations on how fast you can accelerate a bullet is the maximum shear or bearing stress of the bullet material: you can reach a point where the rotational acceleration cannot be achieved by the part of the bullet acting as a sliding key running down the rifling. Both the bearing and shear areas are reduced by the existence of only two grooves. Now on top of that you propose to further reduce the bearing area (but not the shear area) by .008 on diameter (.004 radially) by paper patching. If the bore is 0.303", and the bullet is 0.310", you only have .0035" radially (half of .310 - .303) times the length of the bullet's driving bands, times two ( because there are two rifling grooves) as bearing area. If you used a sized and lubricated bullet 0.318" diameter, the radial bearing would be increased to half of .318 - 303, or .0075 - more than twice as much bearing area for the same bullet length.

Because your cartridge will be a pygmy, and you are probably only expecting air-gun velocities, you might get away with this or you might not. On a full-on 303 British, I think it would be likely to be unsuccessful.

45 2.1
07-22-2007, 09:59 AM
It will work fine, i've done it in various two groove barrels. You will need the bore and throat diameters of your barrel. Cast a very soft boolit and size it to your bore diameter. PP it to slightly under the throat diameter. Length of patch (and boolit type/bearing surface) needs to be regulated so it will mark or engrave in the throat with no jump to the rifling.

07-22-2007, 06:42 PM
Thanks guys. Sounds like a 'suck it and see' situation.

Bore is spot on at .303 but groove is quite the generous .317 with throat .318 best I can measure.

Getting velocities of 1500fps with a 221gr bullet without any problems so a .30 patched bullet of 180gr or so won't be a problem. Small doesn't necessarily mean slow. Well.......I don't think 1500fps is slow. Anything that breaks the sound barrier is good with me!

Will report once I dig up some resources.

45 2.1
07-22-2007, 07:17 PM
Size your boolit to 0.303" and use patching paper that is about 0.0035 to 0.004" thick. That will give you the correct size boolit to load, providing you can load a 0.317" boolit. Remove any crimp and measure the inside diameter of a fired case to be sure. Lube the patch on the boolit when its dry with 50% beeswax/50% vaseline mix with your fingers.