In a fairly accurate, Rice round bottomed rifled gun, I've 'witnessed' the normal smaller ball, ie: .010" 'under' with a .022" denim patch being fairly accurate. When suggesting the owner try a .005" 'under' ball, there was an immediate improvement in accuracy. The rifling of this barrel is about .016" deep. We should do the math - .500" bore, + .032" for rifling = .532" to the bottom of the grooves. A .490" ball (.010"-under) + 2 wraps of .022" = .534", only .002" compression total and .001" per side. The .495" ball + .044" patch = .539" for .007 compression, or .0035" compression per side - a tighter fit and instantly better accuracy, even with the light squib load of 80gr. 2f for 1,400fps. In this barrel, 3F would probably give even better accuracy at 80gr. as the velocity would be more in tune with the rifling twist, which is slow. We tried a .509" ball with a .020" patch and it loaded fairly easily, although took more care in loading than the smaller ball. It 'should' prove to be more accurate showing more testing is in order.
To date, I've only had one round bottomed rifled barrel- and that, a test. I was unable to achieve the accuracy I've come to expect from my other barrels. This test wasn't quite fair, as the barrel was a test as I said, a 'first' attempt at deep rifling at .025" deep. I didn't like it. Rice barrels with .016" rifling seem to shoot just fine, given a 'proper' tight load, just as a square rifled barrel gives good accuracy. One I still have, a .45, has .028" square rifling - yet it shot well. It also needed a very smooth radius'd crown to load. The only way to get to the bottom of the grooves was to use a ball .009" over bore diameter and a .022" denim patch. With this, it shot exceptionally well and was fairly easy to load. The same scenario in the .seep .50 failed to elicit anything that could be called descent accuracy. Why, I don't know - it should have shot well.
Another deep grooved barrel of my acquaintance is .75 cal. and about .025" deep grooves. With only 100gr. of powder, it shoots fairly well at close range - to 50yards - and for a .75, that is close range indeed. At 100 yards and beyond, it needs a lot more powder which creates fouling problems. Deep rifling in large bores is neither wanted nor necessary. With 70" to 80" of twist, the rifling needs only be .006" to .008" at most. Today, most barrel makers over-cut rifling. The result is a requirement for heavier than necessary patches to reach the depth of the rifling. Shallower rifling would be somewhat faster to cut, easier to load and cleaner shooting.
So far, the square rifling of .010" to .012" depth works well even at extreme velocities attainable in small calibres of .38 to 40 with what seems to be a fast 48' twist. Why is it that barrel makers seem to think they need to make even deeper grooves in larger, slower moving calibres with even slower twists? The higher the speed, the higher the pressure and the more 'hold' the ball needs - if a .40 can produce well over 2,000 fps in a 48" twist with only .010" rifling, why do barrel makers still cut grooves to .016" or deeper, for slower moving balls in slower twists yet?
I think deep rifling is a wrong move for a slow moving round ball twist. They show that don't need ideep rifling. I'd like to test a .45 or .50 with 60" to 70" twist, round rifling and only .010" deep. That one I know, will shoot & shoot cleanly and clean easily. Easy cleaning is a trait of round bottom grooves. I think we need to readdress this rifling depth issue.
Due to the rounded bottoms and sides of round bottomed rifling, more depth is needed than with square rifling, but the barrel makers are overdoing this.
Perhaps everyone is still in recoil from the buttoned barrels of the past, with their .002" to .004" deep rifling. I know I did, but got over excessive depth fairly quickly. Some depth we need, like perhaps .008" for square and .012" for round, but .016" through .025" isn't needed.
One other aspect of rifling is shape; ie: width of lands compared to groove width. Narrow lands and wide grooves loads easier, cleans easier and shoots accurately. Check out a Goodioen match barrel - THAT is a good land to groove ratio - about 2:1 to 2.5:1 to one, groove to land. Couple wide grooves and narrow lands with an oversize ball and .020" patch and loading is as easy as a normal .005" under and .022" patch.
Roger - As far as swamped vs straight vs tapered octagonal - I've only experience with straight octagonal barrels. I do know what I've seen concerning swamped barrels; that, when Taylor is shooting any of his rifle which all have swamped barrels, you can only hope he whips some off the targets. Not likely to happen. He is a fellow who can shoot offhand to the capability of the rifle. Those of us shooting more accurate, straight octagonal rifles (bench proved) still come up short on the trail as we (I) can't shoot to the rifle's capability - but I'm practising now, something I've not done for some years.
