Preferred groove depth on a .36 RB barrel?

[rich pierce]

On a .36 or .38 with wide lands and narrow grooves (most originals) what would be your preferred groove depth?

Mine is currently about 0.012” and it needs a very thick patch it seems. A larger ball close to bore diameter with an 0.015” patch cuts patches. Wondering if I will get better 50 yard accuracy cutting the lands down so grooves are closer to 0.008”.

[Daryl]

Due to the size, I think ratios work to a point. For a .32 or .36 having a 48" twist, I don' t think I'd want shallower than .008" and anything up to about .012" would be just fine.
The balls are so small and easily moved, very tight patched balls are easily loaded without having to use a short starter, just choke up on the rod for the initial push into the bore.
If patches are being cut, I'd address the crown a bit, Rich.
My .32 had .008" rifling depth, but very wide lands, narrow grooves. I was still able to load (pure or exceptionally soft lead, mind you) a .320" ball and 10oz (.0225) denim patch, without
tearing. Once started (I always used a starter except when demonstrating one was not actually needed) they would go down with just forefinger finger and thumb on the rod.
2 images of that muzzle with different lighting.

This gun now has a Rice .36 on it with wider grooves, narrower lands. No pictures of that one, yet. VERY similar crown, of course, easy loading and no wiping needed.

[Hungry Horse]

On squirrel calibers I like eight, to ten thousandths rifling. But,  if the lands, and grooves, are of equal width, you could go as deep as twelve. I think the issue is deep rifling, and narrow grooves. You can probably make her shoot, but it will take some playing around with all the components.

  Hungry Horse

[Marcruger]

I like .012 for flat bottom grooves, but I am hardly an expert.  God Bless,   Marc

[hanshi]

For rifles such as my .36 SMR with lands/grooves about equal width and a 1-48" twist, anything from about .010" to .012" deep is more than enough.  I agree that .008" is about the shallowest I'd care to have.  Slower twists seem to be able to do well with a groove depth as shallow as .006".  But each barrel, of course, is an individual.

[Scota4570]

I think deep grooves are are undesirable.   I have two very accurate MLs, one a rifle with a 45-70 CTG barrel and a target pistol with a section of 43 Egyptian barrel.  Both are as accurate any ML I have ever shot, when using patched ball loads with patched balls.  TC used shallow grooves and those shot very well. 

I recently scrapped  a ML barrel with deep round bottom grooves.  You know the ones.  It obtained very poor accuracy.  It seemed no loadable combination would seal the grooves.  I replaced it with a plain old GM.  That barrel shoots very well. 

In a 36 you have a .350 ball, a .360 bore, and with 0.010 grooves a 380 bore.  A 0.020 patch would compress 0.005" in the grooves and 0.015" across the lands.  To me a 0.020" patch is thick for 36 cal.

If you went to 0.015 groove depth you are at 39 caliber in the grooves and that is pretty tough to get a seal.  If you tried with a very thick patch, the compression across the lands would distort the ball and load hard. 

If I could have one made to order it would have .005" grooves that are twice as wide as the lands.   A .350 ball and a  .015" patch would give .005 compression in the grooves and 0.010 across the lands.  The difference between land and groove diameters being less than the first example would make a better seal with less distortion of the ball. 

Keep in mind I swab between shots and am a target shooter.  I never fire multiple shots between swabbing.   I also expect 50 yard groups of around an inch for five shots off the bench with peep sights.  A trail walk shooter that never swabs will need a different barrel.   

[hanshi]

Fast twists with shallow grooves can often present problems with prb.  Unless the grooves are deep enough to get a good "bite" on the patch, the prb, driven at hunting velocities, can skip over the rifling, stripping and producing poor accuracy.  Deeper grooves will hold onto the prb and usually give good accuracy regardless of twist, within reason.  My .32s and .36s have all had .010" to .012" deep grooves and were very accurate with 1-48" twist.  Slower twists can do well with .006" depths.  I have two barrels with .016" deep rifling; one is a .32 and the other a .50.  Both shoot better than I can with patches of .019" or thicker.  The .54 has .006" rifling but a 1-66" twist; it shoots tiny groups, too. 

[rich pierce]

Thanks for good input. Here’s background and an update. When I got it this rifle was about .34 caliber with a burnt out powder chamber and awful pitting. It was in exceptionally bad shape and most would have scrapped it or had it rebored or put a liner in.  I wanted to save it and treat the hours like gym work at home.  It’s worth saving.  Swamped, 46.5” long, and thick, about “D” weight.

I’ve progressively recut both lands and grooves as time allowed and test shot it over some months.  When I got it to .36 it became easy and smooth to load but was cutting patches primarily b/c the rifling did not extend to the breechplug.  Consequently when the patched ball was rammed home it was a little loose in the powder chamber.  My reasoning is that caused shredding or burning of the patches right as the charge went off and hot gasses probably went around the ball before it got moving.

Last week I cut the lands back about 0.002” more and lapped it again.  It is currently .368 land to land and probably .388 groove to groove.  It’s hard for me to measure groove to groove diameter within 0.001” with a 7 groove barrel.    This past weekend I shot it with a .360 ball and 0.022” patching and 40 grains of FFFG. It’s coming along and now I shoot it as well as I can shoot my .45 GRRW barrel.  In other words at this point it’s the shooter not the barrel or load that is the weak link.

[Daryl]

[quote Rich] "This past weekend I shot it with a .360 ball and 0.022” patching and 40 grains of FFFG."

Sounds good to me - maybe 45 to 50gr. if shooting 50 - to 100 yards.