Understanding round vs flat rifling

[KILTED COWBOY]

Not understanding why flat rifling. I know the physical differences, but why 2 types of rifling?
Was one easier to do back in the day and has become tradition?

[rich pierce]

Do you mean square grooves versus round grooves?

Rifling starts with a reamed bore and then grooves are cut into it.  This results in the tops of the lands being flat.  The cutter for making the grooves is either flat-topped or rounded.  Thus the grooves are either square bottomed or slightly U-shaped. It seems that the vast majority of period rifled barrels were rifled with square-topped cutters, resulting in square-bottom grooves. Of course folks like to tinker to discover a magic formula so the main variables are relative the widths of lands and grooves, depth of grooves, rate of twist, and shape of grooves.

Folks who like rounded grooves say they make the barrel easier to clean.  Few claim the rounded grooves are more accurate than square-groove rifled barrels, but some say there is no difference. 

[KILTED COWBOY]

Thanks Rich, that helps. I always try to understand not just the how something is done, but the why it was done that way.

[EC121]

       Because the round bottom grooves are usually deeper, I have found that it takes a thicker patch to seal the round bottom grooves.  Even with the rounded crown, that thicker patch causes me to use a smaller ball for easier loading.  For me cleaning is about the same.
      For all we know the modern sales of round bottom grooves started when one of the barrel maker's square cutters began to wear, and he just kept cutting instead of replacing or sharpening it.  Then it he used it for a sales point, and everyone had to have one.
       

[Eric Krewson]

I have both and have found my square bottom rifling is not near a picky about what I stuff down the barrel as my round bottom rifling.

[Daryl]

Rounded grooves are normally felt to not grip the patched ball as hard as square cut rifling with it's almost 90 degree corners, thus rounded grooves are cut more deeply as a rule.

Some who rifle barrels cut REALLY deep grooves. As Rich noted, rounded rifling is not a period correct method of rifling. Oft times, period rifles had very narrow grooves, had the

appearance of an octagonal, inside bore shape.

Shallow rounded rifling was used by some high-classed gun makers in the last quarter of the 19th century. These British gun makers were making what was called "Invisible Rifling", with

up to 5 very shallow 'rounded' grooves that formed the twist. Others (Lancaster I think) went so far as to have an oval bore, which was cut in a spiral, ie: each 'side' of the bore was actually

 the rifling groove.  It also was referred to as invisible rifling.  These were advertised to shoot as well with shot, as a smoothbore, yet shoot conical bullets with the accuracy of a rifled bore.

This rarely worked as well as advertised, however when shooting large and dangerous game as 25 yards, pin-point accuracy was not needed.  These 'rifled' bores worked fairly well, but

usually had oversized, bare bullets, not cloth patched round balls.  At some point, I would really like to test approximately 16 bore rifle, with slow twist, about 90" and very shallow, rounded

rifling, but no one make such a barrel.  I think it would make a smasher of a big game rifle, it would also be accurate and as easy loading.

[hanshi]

I can't speak to the history of the two types but, just my opinion, I think square cut is best for ultimate accuracy.  I've two radius cut barrels and they make great all around shooters.

[James Wilson Everett]

Guys,

"Back in the day" the form of the rifle grooves was most probably the result of the last person to freshen the barrel, and not the original barrel grooves.  I feel certain that rifles were freshened often, maybe even more that one time each year.  The resulting groove shape we see in originals today may be quite different than the original barrel just off the rifling machine. 

Also, when hand rifling a barrel using an 18th c type machine it is much easier to use a round cutter rather than a square cutter.  The round cutter will tend to center itself into a round groove even with the loose fit-up of a wood rifling machine cam.  I have tried square rifling cutters in the past and always end up with grooves that are quite a bit wider than the cutter.  With a round cutter this does not happen, the groove shape is just about the same shape as the cutter.

This does not apply to the modern precision machined barrels that we see today.
 
Does this make sense?

Jim

[Eric Kettenburg]

Some who rifle barrels cut REALLY deep grooves. As Rich noted, rounded rifling is not a period correct method of rifling.

I agree with a lot above, but Daryl I have to disagree with you on this statement.  I have had many, many German rifles with deep round groove barrels that looked almost identical to what Colerain now sells.  Personally, I don't like that style of rifling, as it does seem to require a very thick patch and tight fit to prevent burn through, but it sure was used quite a lot in Germany.  And lots of people today are using Colerain barrels with exceptional accuracy, so insofar as actual use, it seems to be a personal thing.

I really like the style of groove you can get from Rice or Bob Hoyt and some others.  It's like a cross between square and round - the groove looks like a typical depth square groove, with squared sides, but the bottom of the groove has a shallow radius and overall the depth of the groove is not as deep as a full depth semi circular "round" groove.  I've always found these to work really well for me and for people Ive dealt with.