Evaluating throat/bore

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Evaluating throat/bore

Post by orpheoet on 4/9/2017, 7:38 pm

I like revolvers even though they are somewhat of an anachronism in our sport. It seems that it's time I learn how to evaluate this relationship between throat diameter and bore diameter. Specifically how best to measure and what relationship am I looking for. I have a 14-3 for instance thats pretty well used and outshoots a 14-1 with much cleaner rifling. I suspect its something to do with throat/bore or maybe I just need a different diameter bullet...I know of no better source of information than this forum and its members.
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Re: Evaluating throat/bore

Post by AllAces on 4/9/2017, 8:34 pm

What load are you shooting: bullet, weight, diameter, powder, grains, crimp.  There are some classic loads for the revolver for both wadcutter and semiwadcutter.  Work up the most accurate load possible. then find a really experienced revolver smith to check out the revolver.
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Re: Evaluating throat/bore

Post by orpheoet on 4/9/2017, 8:57 pm

In .38 I'm using the Zero swaged 158 gr 0.357" bullet taper crimped 0.365" OAL 1.445" 3.3gr WST Winchester small primer. That load got me 6 points twice in DR with the 14-3. The 14-1 doesn't seem to like that load. I just received a 25-2 and have been reading all sorts of stuff about tight throats, oversized throats, so as I'm trying to get a good load for it I thought I'd try and figure this all out. Do you think it's beyond the abilities of a layman? It could well be....
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Re: Evaluating throat/bore

Post by JIMPGOV on 4/9/2017, 9:08 pm

14-1 SAME LOAD, BUT TRY ROUND NOSE. JP

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Re: Evaluating throat/bore

Post by John Dervis on 4/9/2017, 9:48 pm

I don't have direct experience with .38 caliber but I can tell you about the knowledge I've recently acquired with a .45 cal Ruger Blackhawk.
The Rugers are known for undersized cylinder throats and there are a ton of bulletin board threads on this subject.  These undersized throats will constrict a bullet down to less than the rifling before it gets to the forcing cone and ultimately to the rifling.  From what I read, copper jacketed bullets are affected less that lead bullets.  The jacketed bullets seem to have some "spring back" to them so they will resume their original size.  Lead bullets seem to stay the reduced size once they have been swagged down.  The progression that makes the most sense to me is that each step should be reduced slightly until the bullet gets to the rifling.  In the case of the .45 the throat is best at .4525 and the cone should be slightly less that with the bore being .450.  
You can precisely measure your chamber throats with pin gauges but you can start by using lead bullets.  Drive one through each chamber and measure them.  That should give you a good idea if that is your problem.  
If you find you need to open up the throats, that is done with a tool called a Cylinder Throat Reamer.  You can buy one for $140.00 or you can rent them from the company I linked below.  There are also some guys on the internet that will do the reaming for you if you aren't handy. (It's really easy though)

Good luck.
John 

https://4drentals.com/products/reamer-rentals/cylinder-throater

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Re: Evaluating throat/bore

Post by orpheoet on 4/10/2017, 6:46 am

It's been suggested that I slug all throats and barrel to get an accurate picture. Seems like good advice!
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Re: Evaluating throat/bore

Post by jglenn21 on 4/11/2017, 6:55 am

Pin gages are easy to use and exact. You can buy individual sizes from MSC.


.3570, .3575,
3580 would be the most common used for a 14.. every stock 14 I've worked on had .357 cylinder bores. Many would have one or two slightly tighter than others. If they have been reamed then most likely .358.

Cylinder alignment also is important to accuracy as well as cylinder end gap.  Cylinder end shake also needs to be checked.

Lots of good books our there on the smith revolver. Kuhnhausen's book is very good as usual.

Round nose bullets do tend to shoot better at the long  line


Last edited by jglenn21 on 4/11/2017, 7:52 am; edited 1 time in total
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Re: Evaluating throat/bore

Post by orpheoet on 4/11/2017, 6:59 am

jglenn21 wrote:Pin gages are easy to use and exact. You can buy individual sizes from MSC.


.2570, .3575,
3580 would be the most common used for a 14.. every stock 14 I've worked on had .357 cylinder bores. Many would have one or two slightly tighter than others. If they have been reamed then most likely .358.

Cylinder alignment also is important to accuracy as well as cylinder end gap. 

Round nose bullets do tend to shoot better at the long  line
Great info, thanks. Do you know what kind of numbers I'm looking for with the 25-2 as far as ideal throat/barrel?
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Re: Evaluating throat/bore

Post by jglenn21 on 4/11/2017, 7:50 am

sorry , haven't worked on a 25
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Re: Evaluating throat/bore

Post by Wobbley on 4/11/2017, 11:21 am

B
orpheoet wrote:

Great info, thanks. Do you know what kind of numbers I'm looking for with the 25-2 as far as ideal throat/barrel?

For a 45 cal, I'd start with 0.4510, 0.4515, 0.4520, ....   but first slug your barrel.  Remember that smiths tend to have five groove barrels and these are hard to mike unless you either make an adaptor to a regular mike or buy a special mike Like this. 

 https://www.onlinemachinist.com/all-tools-tooling/testing-measuring-inspection/dimensional-measurement-tools/micrometers/0-2-1-measuring-range-0-001-graduation-ratchet-thimble-high-speed-steel-face-5-flute-v-anvil-micrometer/?gclid=CLKU5JXnnNMCFYVqfgodZrgL3A   

Then you can order your plug gauges.  

You want the cylinder throats to be groove plus 0.001.  And use bullets plus 0.001.

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Re: Evaluating throat/bore

Post by Jack H on 4/11/2017, 12:01 pm

I don't know why the great concern over the cylinder bore size exists.  From a practical point, what are you going to do about it?  I have a very early 1955 model with properly tight cylinder bores>>>>>
I have also a newer 25-2 that has the oversize bores.  I slugged each of them but do not recall the numbers. 
At 50 yards with lead, both guns shoot about the same 9 ring groups off hand.  The 1955 shines with jacketed.  That 5 shot group >>>>>  was offhand and not a fluke.  There was another one just like it on a well shot up area of the same target.  Sight adjustment between them. 
If I was going to do anything to a revolver, I'd start with a Taylor throat if I could find someone to do it.  That's what I mean about practical.
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Re: Evaluating throat/bore

Post by orpheoet on 4/11/2017, 1:19 pm

Jack H wrote:I don't know why the great concern over the cylinder bore size exists.  From a practical point, what are you going to do about it?  I have a very early 1955 model with properly tight cylinder bores>>>>>
I have also a newer 25-2 that has the oversize bores.  I slugged each of them but do not recall the numbers. 
At 50 yards with lead, both guns shoot about the same 9 ring groups off hand.  The 1955 shines with jacketed.  That 5 shot group >>>>>  was offhand and not a fluke.  There was another one just like it on a well shot up area of the same target.  Sight adjustment between them. 
If I was going to do anything to a revolver, I'd start with a Taylor throat if I could find someone to do it.  That's what I mean about practical.
I just want to know as much as I can. I may get one of my 14's worked on but as you say ....by who?
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Re: Evaluating throat/bore

Post by John Dervis on 4/11/2017, 9:35 pm

Jack H wrote:I don't know why the great concern over the cylinder bore size exists.  From a practical point, what are you going to do about it?  I have a very early 1955 model with properly tight cylinder bores>>>>>
I have also a newer 25-2 that has the oversize bores.  I slugged each of them but do not recall the numbers. 
At 50 yards with lead, both guns shoot about the same 9 ring groups off hand.  The 1955 shines with jacketed.  That 5 shot group >>>>>  was offhand and not a fluke.  There was another one just like it on a well shot up area of the same target.  Sight adjustment between them. 
If I was going to do anything to a revolver, I'd start with a Taylor throat if I could find someone to do it.  That's what I mean about practical.

In my case with the Blackhawk, I couldn't chamber anything with a lead bullet in my .45ACP cylinder.  My throats were .4500 (well, that's the pilot I was able to use with the reamer) on all three cylinders I have.  From everything I read about this, accuracy suffers when the lead bullets are constricted down and then are asked to widen back to the rifling size.  Jacketed bullets work better so perhaps that's why your gun favors them.  You can do something about the cylinder throats and that is ream them.  It's very easy to do and at the very least makes each chamber the same so everything is consistent.  If the problem lies further down the line like the forcing cone or bore itself, then more advanced tools and skills are needed.  I would then find someone that knows how to correct them.

John

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