Accuracy & round bottom grooves

[Pete G.]

So, I've got three barrels with round bottom grooves in them and all three are acceptably accurate, but it seems than none are exceptionally accurate, whereas my one barrel with square grooves is. One of the round groove rifles (36 cal) MUST have the load it likes or it will shoot a 2" group @ 25 yds; with the load it likes about 1" or so. Incidentally, this barrel makes sort of a squeaking sound when loading that the others do not. It seems to be smooth, but indubitably has some minute tooling marks in the bore that have not polished out over two years of shooting. The other barrels (54 cal & 45 cal)load smoothly and shoot into about 1" or 1-1/2" @ 25 with any reasonably tight patch/ball combination and almost any powder charge one cares to load. The square groove rifle will consistently shoot better than any of the others and is not too particular about any reasonable load either. 

My question is "Does square groove rifling normally shoot better than round, or did I just get a good square groove barrel?"

[Dan]

Pete, I don't know you have good or bad barrels either way, either style.



The image above is of Metford pattern rifling (right) and conventional on the left. They are both capable of fine accuracy.  Pope liked radiused rifling and so did some of the older target shooters such as Perry.  They shot paper patched so I don't know there's a direct comparison there for you to reference.

Dan

[bob in the woods]

This may not work for everyone, but..my experience with the round bottom grooves is that I had to use a thicker patch than in  my older green mountain square bottom grooves. With a thicker patch, my barrels shoot very well.

[Ezra]

One of the rare benifits of being a crappy shot is, round bottom/square bottom/smoothie, doesn't matter.  In fact, most can't tell the difference when I shoot!  And brother, it ain't cause I'm accurate!   


Ez

[Candle Snuffer]

IMHO a person still needs to find the right combination that will shoot most accurately from their barrel, regardless if it is square or round bottom rifling.

[ironwolf]

  Candle Snuffer is right on the mark.  A roundball doesn't know if the rifling that spun him was round or square.
  It's in the load development.  And just maybe the muzzle crown.

  Kevin

[R. Hare]

Pete,

I don't have any square rifled guns, so can't complain about my round bottomed ones!  they seem to shoot better than I can.
Could it partly be you have a better source of ignition on your single square-grooved gun?

[bob in the woods]

After checking, I believe that my swamped round bottom groove barrels have a deeper groove than the square groove Green Mountain barrels.  This makes sense, and falls in line with my findings that the round grooves liked a thicker patch.

[Daryl]

Rifling with square groove bottoms, particularly Gm and Goodoien barrels have about .010" to .012" deep rifling.  Barrels with round bottom grooved rifling will normally have .016" to .025" deep rifling.  The deeper the rifling, the heavier the patch must be.  A load that proves accurate in a barrel with .012" deep rifling, will be about .004"(thickness of patch) too loose for best results in the deeper rifling.

 With the .36 cal rifle, I'd be inclined to shoot at about 35 yards for accuracy testing, and with .40 cal or larger, move that out to 50 yards.  I've found accuracy tests at close range to be useless for determining a loads accuracy for developing loads.  The larger bores will group incredibly tight at close range, but spread even by 50 yards.  It is disheartening to shoot a 2" group at 25 yards with your rifle, only to have someone then shoot a 1" group at the same range, using his smoothbore.  I know, been there - me shooting both guns.

 The round bottomed rifling loads more nicely than square - IF both loads are tight fitting and both loads do not allow the accumulation of fouling.

 If round bottomed rifling gave as good accuracy as square bottomed rifling, due to the easier cleaning and loading, round bottomed rifling would be used for target or match barrels - they aren't.  I think the difference might be small, but small diferences are what wins matches.

(my) Rules of thumb :- .005" under ball for square rifling + .018" to .022" patch
-----------------------------.005" under ball for round rifling + .022" to .025" patch
 

[Daryl]

I'd love to have a large bore hunting rifle with this type of rifling - say 90" twist and 16 bore.  The rounded/squarish type of rifling is what I meant - second barrel.

[Dphariss]

Frankly I think they cut the grooves to deep. Sometimes by double.
The two I have also have lands that are too wide.
Have not shot them enough for accuracy testing yet. But what shooting I have done has not impressed me. Load hard and foul more than they should.
I think GM has the best land groove ratio aside from some special order barrels.
Dan

[Daryl]

The Goodioen barrel I have has very narrow lands - grooves twice the width - I like it.  For a slow twist, they don't have to be deeper than .008" in my humble opinion. Was a time when I thought the deeper the better, but that was just a re-bound form the el'cheapo button'd barrel I started with.  .008" and 70" should be just fine for a large bore - maybe .010" for smaller higher pressure rounds especially in .40's and .50's, .005" is all that's needed for 90" in a real hunting rifle ie: 16 bore or bigger.

[Candle Snuffer]

Years ago Ed Rayl made me a .40 caliber barrel and I had him cut mine with round bottom rifling.  He went to .010 depth.  This, along with the 1 in 72 twist I had him make it, has made for a fairly easy loading rifle that I can shoot all day long with an occassional wiping of the bore. 

I've had some great groups shot with this rifle and I've found it to like both light and heavy charges and perform extremely well, both on paper and on metal targets.  The 1 in 72 twist does give you a wide selection of charges you can use, even in this small bore.  I've shot from 35 grains to 70 grains in this (depending on target) and I've got no complaints and have never had one of my .018 patches blow.  By far my favorite rifle.

[Daryl]

Right-on, Marty - I'd like another .40 barrel like the one I have, 7/8" straight X 42", but with the same 72" twist and about .006" to 008" depth rounded corner, but flat bottomed grooves.  It would be a good .395" ball shooter with a .015" to .018" patch for sure - easy to load and of course, clean shooting.  The nice thing about the rounded corner, flat bottom grooves (ie: H.M. POPE) is, if cut proper width, leaves very narrow lands with wide smooth grooves.  In Pope's barrels, the centre of the groove was bore diameter. In the barrel I propose, the centre would be about .003" depth. Easy loading and clean shooting.  Even Taylor's .50 Rice barrel shoots cleanly with it's .016" deep rifling and is a dream to load with it's round grooves.  More shallow would merely mean easier loading yet and the ability to use a slightly less thick patch.  Less ball distortion or the ball from the grooves would be the end result and accuracy would be even better. 

By clean shooting, I merely mean the bore does not accumulate fouling. Any left from the previous shot is wiped down and blown out with the next.

[Candle Snuffer]

These truly are barrels one just has to experience.  Daryl, you got me thinking again as usual.  Might be time to get hold of Ed Rayl once again.   If I only knew how the economy was going to go down here???

[Daryl]

I know hwat you mean Marty - a lot of questions coming, and for a lot of months down the road.