Throat / barrel dimensions

Discussion in 'Handguns' started by atllaw, Sep 18, 2019.

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  1. Sep 18, 2019 #1

    atllaw

    atllaw

    atllaw

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    I did a bore/throat diameter measurements check on my '51 & 61 Navy and '62 Police Uberti's. The throats measured .371.5, .373 and .3735 respectively. The bores (land to land) measured .359, .3605 & .359, again respectively. These measurements were made with pin gauges and I'm fairly confident in their accuracy. I slugged the bores and they measured, groove to groove, as .3765, .377 and .378. I am less sure of those measurements.

    Uberti specs say that the bore is .360 and the grove diameter is .379. Has anyone else checked the grove to grove diameter on their uberti's?

    What's the consensus on a round ball bumping up from a throat diameter of say .373 to a grove diameter of Uberti's claimed .379? Or my measured .377 +/-

    My thoughts here concern the benefits of reaming the cylinder throats out to more closely match the groove diameter.
     
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  2. Sep 18, 2019 #2

    Woodnbow

    Woodnbow

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    How do they shoot as they are now? I would not ream the chambers for the sake of matching the groove diameter unless the pistol was inaccurate as it is.
     
  3. Sep 18, 2019 #3

    Phil Coffins

    Phil Coffins

    Phil Coffins

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    If you have the correct reamer I’d cut the chambers to .374” to have them all the same size in each cylinder. The ball will obturate (sp?) to groove diameter when fired. Start with the one gun you find the least accurate then after testing and reporting back here decide if you want to do others. I did this on a 44 caliber and it did improve group size.
     
  4. Sep 18, 2019 #4

    bang

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    Some time ago I attempted to get an answer as to why cylinder bore was less than bore max from manufactures. The reply I recieved was "They are just reproduction black powder guns."
    My consensus of this leads me to believe it's a liability issue. Should someone ream the cylinder to cause the projectile to properly fit the bore and they get injured then it's on the owner. Granted one could switch cylinders but considering if someone ream one they would ream all. Put that with the possibility a person might cast projectiles not of pure lead as advised then the manufacturers choice to undersize the cylinder bore is their answer to idiot proofing.
    Just sayin.
     
  5. Sep 18, 2019 #5

    atllaw

    atllaw

    atllaw

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    Good question. I've shot them a good bit, but never really tested the accuracy on them. But, I've got my gear all set up to do so on my next range trip. I'll be looking for the most accurate powder charge for each revolver, and in doing so will get a good idea of their potential.
     
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  6. Sep 19, 2019 #6

    bang

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    My walker was undersize. It shot horrible. However it also had , more or less, zero crown. I reamed cylinder to .451 and worked the crown and it came inline. It was hitting 12" high, 12" right. Wish I could say it was just the ream but since I did the crown too I can't say. I do think the ream was most of it because the crown work was minimal.
    My pair of 1858s in the higher accuracy grade can hit inside a door knob all day with any of the 6 extra cylinders I have. Have to attribute it to a way better made barrel.
     
  7. Sep 19, 2019 #7

    SDSmlf

    SDSmlf

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    May want to check your throat dimensions again. I believe the throats are added after bore and rifling are cut. Here is a photograph of a 45 caliber Uberti. upload_2019-9-18_15-14-38.jpeg
    As far as cylinder chamber size, I have found .001-.002” over groove diameter gives best accuracy. The above pictured bore plugs at .438”. The cylinder as received from the Uberti factory measures .4495-.4500”, an undersized diameter in my opinion for the published .458 groove dimension. I will open cylinder chamber dimension to .459-.460” to optimize accuracy. Uberti seems to like the groove diameter to be .019” larger than the bore, while Pietta, interestingly, are at .012” larger than bore.

    Most recent Pietta 1860s I have obtained have .438 bores, with groove diameters at .450”. Multiple Pietta cylinders I have measured were in the .444” range, +/-.001. Reaming them to .451-.452” in general, cut group size by more than half.
    Totally disagree with .374” chamber dimension. For accuracy, suggest .380-.381 diameter to match OP’s groove depth (.376 to .379) plus .001-.002”. Just my opinion.
     
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  8. Sep 19, 2019 #8

    atllaw

    atllaw

    atllaw

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    My Bad Snuffy... :( I was referring to the "chamber throats" which more appropriately describes the area in a cartridge revolver. The measurements I listed for the throats were in fact for the the cylinder chambers
    This is, of course, the expected wisdom for cartridge revolvers. I wondered if it held true for BP revolvers or if obturation solved the undersized chamber problem.
    Yup, that's what I was thinking... if the balls don't bump up when they hit that "throat" you pictured! ;)
     
  9. Sep 20, 2019 #9

    Phil Coffins

    Phil Coffins

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    The ball will obdurate at the cylinder mouth as was proven by Doctor Mann back in 1909 to well over groove diameter. Being able to use easy to find ball size would be useful to a average shooter such as myself. Having equal size chambers in any revolver is always of value. I also like to do slight polishing of machine marks in the forcing cone of the barrel.
     
  10. Sep 20, 2019 #10

    SDSmlf

    SDSmlf

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    The .001” over bore diameter is based on actual testing with roundballs. Opened up three of six chambers on two different cylinders. Both cylinders shot two groups (one smaller than, but inside of, the larger group) with two different 1858 Piettas. Ultimately opened all six chambers on both cylinders and accuracy held.

    As far as roundballs bumping up, not buying it. Take an undersized ball (.444 for example), and pound it through a bore of .438” with either a grove depth of .450” or .458” with a brass rod and you will find with neither barrel the roundball obturating to the bottom of the rifling grooves. Tried it and roundball diameter didn’t really grow. If it were up to me, I would have groove depth .004” max (.008” on the diameter) over bore depth. Would like to see what .003” groove depth did. Then bore chambers .001” over groove depth. But I am not making the barrels.

    My guns, my experiments. You may find different, but doubt laws of physics very much between North Carolina and Georgia. Maybe if you were out West or in Canada....
     
  11. Sep 20, 2019 #11

    Phil Coffins

    Phil Coffins

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    I recommend reading “The bullets flight for muzzle to target” by Mann. The force delivered by a hammer blow as compared to the ignition of black powder is covered well. The math shows what we can’t see. But this is just sport so enjoy it as you see fit.
     
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  12. Sep 20, 2019 #12

    nkbj

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    Lead almost squirts under pressure.
    The cylindrical parts of these bullets are smaller than chamber diameter to slip in when the gun is dirty. The chambers are less than groove diameter. And the bullets expand to fill the grooves.
    [​IMG]
     
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  13. Sep 20, 2019 #13

    SDSmlf

    SDSmlf

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    So how much smaller diameter should the bullet be than the chamber, and how much smaller than the bore should the chamber be for best accuracy? Then same question for a roundball?

    Personally have found roundballs undersized enough to ‘slip in’ dirty undersized chambers (compared to groove diameter) to consistently be the least accurate combination, not to mention potentially dangerous.
     
  14. Sep 20, 2019 #14

    Zonie

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    Doctor Mann was dealing with bullets, not round balls.
    Due to their length and weight, bullets will expand in diameter when they are accelerated during firing and the longer they are, the more the middle of them will grow.
    Roundballs on the other hand have very little material ahead of the middle of them to resist the forces of acceleration so they will expand only very slightly. IMO, this growth will not be enough to even begin to fully engage the rifling grooves to their full depth.

    That is why sizing the chambers mouth so that it is larger than the groove diameter works so well.
     
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  15. Sep 21, 2019 #15

    nkbj

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    .001-.002 under the chamber diameter is plenty.
    However, that statement requires qualification. Some chambers in some revolvers were finished with a slightly tapered reamer and pushing an elongated piece of lead into a tapered hole poses its own problems. Another factor is the design of your bullet. A bullet with lube grooves will compress into a taper easier than one with no lube grooves because the rings on the bullet will crush with less resistance but also because you're probably going to have lubrication in the grooves. Long and the short of it is to try things out and find what works in your gun.
    Notice that those in the picture don't have lube grooves. I think grooves are usually a needless sacrifice of the already limited chamber volume but other folks find they work better than no grooves. And then again when you're shooting with bullets cast from .45 caliber pistol molds there's probably gonna be plenty of grooves. And there might even be big lube grooves, especially if it's a mold that was designed for black powder loads.

    About the chamber diameter versus the groove diameter in the barrel, if the two are pretty close then it's probably going to work OK. Pietta makes their chambers about halfway between bore and groove diameters and surprise (!) it actually works pretty good. That mold design in the pictures was figured out to work with the way Pietta makes their .36 revolvers rather than reboring the chambers on every Pietta .36 that passed through my hands. So I sharpened the pencils, sketched it up and spent the money to have a round ball mold altered to my specs. Not every idea I try works but that one did.
    :)

    Oh, and by the way (clarification), the spherical front end of those bullets shear on the chamber mouths just the way a ball oughta. When using .45 pistol molds in .44's I size the rear bands to slip into the chambers and leave the front band large to shear. That way alignment is taken care of, the chambers are sealed and loading levers don't get stressed.
     
    Last edited: Sep 21, 2019
  16. Oct 6, 2019 #16

    bigted

    bigted

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    This typically is the reason the larger balls work better accuracy wise,

    In a cylinder diameter a of .374 through .376 ... a .380 or .382 diameter ball gets the equator cut on the sharp cylinder mouth ... thereby making a .375ish ball with a long equator that will grab the rifling better AND provide mass that will resist movement enough to BUMP up in diameter with the sharp hammer hit of the real black powder hitting it in the rump as it resists with that LONG equator being forced into the forcing cone and thereby resist enough to pile up on the cone and stopping gas blowby on the ball.

    Just theory from the peanut gallery!
     
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  17. Oct 6, 2019 #17

    rodwha

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    Your theory seems to hold water as the few instances where different sizes were used the velocity typically went up despite a heavier projectile that seems to better fill the grooves.
     
  18. Oct 6, 2019 #18

    bigted

    bigted

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    I remain convinced tho ... a cylinder diameter of .oo1 to .002 OVER barrel groove is paramount to successful accuracy overall.
     
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  19. Oct 6, 2019 #19

    bang

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    Yep.
     
  20. Oct 6, 2019 #20

    M. De Land

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    I agree completely but two of the important accurizing factors that occurs with chamber reaming is uniform diameters and round holes. Neither seems assured from the factory.
    Also, when one reams chambers they do not need to be reamed to the bottom of the chamber. I feel it best to leave as much steel over the bolt cuts as possible.
     
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