Picking a patch and ball size based off of slugging the barrel

A friend has me working on an under hammer pistol of his. He has never shot it. After slugging the barrel the grooves measure .460" and the lands measure .445". How would I go about picking a ball diameter and patch thickness to start testing with?

I don't know for sure but I would start with a .440 ball and a patch around .015 or .018 or even .020.

(ball + (patch * 2) <= grooves) && (ball < lands) := first patch size.

(ball + (patch * 2) <= grooves) && (ball < lands) := first patch size.

So... (.440 + (.008 x 2) = .456.

a .440 ball and a .008 patch?

Is it (ball + (patch * 2) </= grooves)? Meaning it should be less than or equal to groove?

Just to start, a .440 ball and 100% cotton patch made from old sheet or pillow case or tee shirt. Lube of your choice.

Onemancoyote said:

(ball + (patch * 2) <= grooves) && (ball < lands) := first patch size.

So... (.440 + (.008 x 2) = .456.

a .440 ball and a .008 patch?

Is it (ball + (patch * 2) </= grooves)? Meaning it should be less than or equal to groove?

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Take it with a grain of salt; I just made it up this morning loosely based on experience, and I guess intended to recommend 0.010 or thinner as first trial patch thickness:

(0.440) + (0.010 * 2 ) = 0.460

Theoretically 0.010 should be enough to squeeze that patch material down into the grooves to catch the rifling, but at the same time it might be too thick for the lands.

I'd be curious to know how it works out. Much of the time trial and error wins over science and math.

I tend to start with a ball 0.010" smaller in diameter than the land-to-land diameter. In this case, a 0.435" ball would require a custom mold. I would start with the 0.440" ball. The groove depth is 0.0075. To get a good seal, the patch needs to compress to 0.010" (0.0025" for windage between ball and land + 0.0075" for depth of groove). Most tickings of 0.015" published thickness will compress to 0.010". You will need a tight weave of sturdy cloth to withstand the fitting into that windage space by engraving into the ball or further compression. On a new barrel, there will be cutting of such patches. Avoid prelubed patches as the prelubed patches often deteriorate in storage and fail as a patch for such a tight fit. You will need a short starter to get that ball and patch to load.

Grenadier1758 said:

I tend to start with a ball 0.010" smaller in diameter than the land-to-land diameter. In this case, a 0.435" ball would require a custom mold. I would start with the 0.440" ball. The groove depth is 0.0075. To get a good seal, the patch needs to compress to 0.010" (0.0025" for windage between ball and land + 0.0075" for depth of groove). Most tickings of 0.015" published thickness will compress to 0.010". ......

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Thanks!! Good knowledge for me to learn. I had considered fiber compression, but didn't know how to account for it. Maybe I'll come up with a new formula based on that new information. Science and math learns from trial and error experience!!

Don’t forget that the soft lead ball doesn’t stay completely round when it’s under compression by the squeeze of the cloth patch between the ball and the lands of the rifling. The ball distorts to acquire flat spots around its circumference.
I like to insert a ~6” brass rod into the barrel the test fit the experimental size of ball and patch. Press the the PRB an inch into the barrel wait a minute for the lead to conform and get embossed with the patch weave marks. Then up end the gun to slowly allow the brass rod to slide down to touch the PRB. Now shake the barrel up and down so the brass rod taps the PRB out the muzzle. Slide hammer fashion, the PRB should land on a soft surface and don’t use excessive force pounding the PRB with the brass rod.
Examine the ball if the ball diameter and patch thickness are correct you will see patch weave marks embosses all around the balls circumference in an alternating pattern of deeply embossed and lighter embossed cloth weave marks representing the lands and grooves.

The guns accuracy is a compromise between the fit the PRB has in the gun barrel and the muzzle velocity you drive the load to. A looser PRB fit without patch weave marks embosses on the ball at the grooves may give the shooter great accuracy at a low MV but the same PRB driven to higher MV would likely show the group size much larger. A felt over powder wad may recover some of this looser PRB accuracy at slightly higher MV. But eventually if the MV is made higher than the PRB can hold to the rifling without or with the OP wad the accuracy group size will be bad. Then a tighter PRB will need to be tested, a thicker patch is should be tried next as it the least expensive option.
The PRB needs to be tight enough to prevent burning powder gas from channeling along the grooves and burning holes in the patch, here where an over size felt grease lubed OP wad will help keep the hot gas behind the PRB.
You don’t want to be launching smoldering patches onto the leaf covered forest ground.

Like this...

I've tried the slugging method but it never helped much. More better to just try a couple different ball sizes and patch thicknesses to find wat works best in your rifle. This a game of try and cuss, try again then cuss more and............ You get the idea, experiment until you find your best ball/patch combo. Then you start trying different powder charges to find your sweet spot. Just don't run out of cuss words.

Rifleman1776 said:

I've tried the slugging method but it never helped much. More better to just try a couple different ball sizes and patch thicknesses to find wat works best in your rifle. This a game of try and cuss, try again then cuss more and............ You get the idea, experiment until you find your best ball/patch combo. Then you start trying different powder charges to find your sweet spot. Just don't run out of cuss words.

Click to expand...

and be sure you are out of hearing range of the boss. mine gives me a time out when she catches me.
I hate cooking for meself!

Onemancoyote said:

A friend has me working on an under hammer pistol of his. He has never shot it. After slugging the barrel the grooves measure .460" and the lands measure .445". How would I go about picking a ball diameter and patch thickness to start testing with?

Click to expand...

Trial and error.
I have an assortment of patching material in different thicknesses on hand that I bought 15-20 years ago for the most part.
Never have a problem coming up with a good working combination.
I tend to prefer smaller balls and thicker patches. The thicker patches will carry more lube and will not tear when loading or shooting. Loads a little easier too.