Ruger NMV 45 Colt cylinder chamber diameter?

[Tall John]

 

What should be the cylinder chamber exit diameter Ruger NMV 45 Colt pistol.

 

I bought the gun used, but noticed when I took it to the range that I was having some difficulty both loading and extracting the cases after shooting from the cylinder. This is with OEM loaded ammunition from choice ammunition 200 grain RNFP bullet I also noticed the same thing with six of my dummy rounds that I’m preparing to reload.  Both the Choice OEM ammunition, and my dummies drop in and out of a case gauge with no problem and they also cycle and plunk test in my 1873 with no problem so I don’t think I have a crimp issue.  When loading, I’m having to push the round into the last 1/8” on about half of the chambers on my NMV.

 

When I use a pin gauge on the cylinder chambers, a 0.450” pin slips right through, a 0.451” pin will slip through with some pressure, but a 0.452 “ pin stops solidly where the case ends in the cylinder. Is this normal? Do I just need to use a hone on the chambers to polish them or have them reamed?

 

See the pictures below…

 

Note that My unloading issues, seem to be the case rim scraping on the frame when trying to extract the empty case.  I do not have have the ability to test the cylinder for its concentricity to see if is out of round.  
 

thoughts?

.450”

 

.451”

 

0.452”

[Blackwater 53393]

Was the gun used with .45 Cowboy Special loads before you obtained it??
 

If so, thoroughly clean the chambers and try it again. 

[The Original Lumpy Gritz]

Take a brass .50 cal bore brush in a drill motor and clean out each chamber.

Choice ain't the best 

 

[Larsen E. Pettifogger, SASS #32933]

https://www.slixprings.com/proddetail.php?prod=SliXprings-Scraper

[Larsen E. Pettifogger, SASS #32933]

Oh, .451" is about perfect.

 

Original specs for the .45 Colt was for a .454" bore diameter.  The .45 ACP was .451.  After WWII the industry standardized the .451 diameter for both.

Edited December 31, 2025 by Larsen E. Pettifogger, SASS #32933

[Eyesa Horg]

5 minutes ago, Larsen E. Pettifogger, SASS #32933 said:

https://www.slixprings.com/proddetail.php?prod=SliXprings-Scraper

+1

These work excellent.

[Tall John]

4 hours ago, Larsen E. Pettifogger, SASS #32933 said:

https://www.slixprings.com/proddetail.php?prod=SliXprings-Scraper

Great idea!  I didn’t realize they had one for the 45 Colt.  I have the 38/357 and it works great!

 

thanks for the reminder!

[Tye Riverman, SASS #41433]

I was not aware there was such a thing.  Just ordered a couple for .357 and .45.  Thanks for the heads up.

 

Tye

[Colorado Coffinmaker]

Additionally >

 

Ruger is Famous or Infamous if you will, for UNDERSIZE cylinder throats.  With the chambers and throats thoroughly cleaned, you will need to check the throats.  For lead bullets, and a .451 bore, you're gonna want .452 throats.  The throats in that cylinder may well need reamed. 

[The Original Lumpy Gritz]

What you are checking is not the chamber, that is where the cartridge sits.

Where the bullet travels through to the barrel is called the throat. 

Ruger has a history of undersized throats.

I had to open up the throats on both of my OMV .44's. I used a dedicated throat reamer for this, from Brownells. 

[Larsen E. Pettifogger, SASS #32933]

A .451 throat on a .45 is not undersized.

[Colorado Coffinmaker]

 

Yes . . it is.  Nanny Nanny Poo Poo 😇

[The Original Lumpy Gritz]

32 minutes ago, Colorado Coffinmaker said:

 

Yes . . it is.  Nanny Nanny Poo Poo 😇

And we're off! 

[Eyesa Horg]

Wouldn't a .451 throat swage a .452 bullet down to .451 ?

[The Original Lumpy Gritz]

2 hours ago, Eyesa Horg said:

Wouldn't a .451 throat swage a .452 bullet down to .451 ?

Easily 

[Tall John]

UPDATE:  got the SlixScraper today.  I barely fit the cylinder chambers due to the buildup of crud.  Didn’t solve the problem so I gave each chamber a 15 second session with my 45 cal 400grit and 800grit flexhones and the sticking problem is gone completely except for one chamber.  
 

i have to push the round in the last 1/8th inch and then the round doesn’t want to immediately come out..UNLESS…I rotate the cylinder one fulk turn. (???). I know it isn’t my ammo ‘ aid that same bullet now drops in and out of the other five chambers.  It almost seems like the case rum is binding against the frame.

 

ill take it to our club match this weekend and ask a few more experienced cowboys to look at it.

[watab kid]

451 standard - now i get why 452 boolit  is the norm , i knew it because i was told it long ago i jusat never knew why it was so this makes sense to me 

[The Original Lumpy Gritz]

7 hours ago, Tall John said:

UPDATE:  got the SlixScraper today.  I barely fit the cylinder chambers due to the buildup of crud.  Didn’t solve the problem so I gave each chamber a 15 second session with my 45 cal 400grit and 800grit flexhones and the sticking problem is gone completely except for one chamber.  
 

i have to push the round in the last 1/8th inch and then the round doesn’t want to immediately come out..UNLESS…I rotate the cylinder one fulk turn. (???). I know it isn’t my ammo ‘ aid that same bullet now drops in and out of the other five chambers.  It almost seems like the case rum is binding against the frame.

 

ill take it to our club match this weekend and ask a few more experienced cowboys to look at it.

Have you tried factory ammo for fitment? 

[Tall John]

I’ve tried Choice Ammunition and the American Cowboy C45S and had the same issue as my reloads.   Although the C45S aren’t as bad presumably due to the shorter case length.

 

Originally all of the chambers were very sticky with one that was really difficult to eject the unfired or fired round from.  After honing i still have the one chamber that wont eject while the cylinder is in the firearm.

 

note that when I remove the cylinder the rounds drop in and drop out of all six chambers without issue.

Edited Friday at 04:28 PM by Tall John
Added extra info

[Eyesa Horg]

22 minutes ago, Tall John said:

note that when I remove the cylinder the rounds drop in and drop out of all six chambers without issue.

That sounds more like a gate clearance issue then. Taking more material off the chamber is only going to cause more blowby. JMHO

My Rugers all come out ok, but has to perfectly aligned. My cylinders rotate both directions so that's not much of an issue for me however.

[Larsen E. Pettifogger, SASS #32933]

45 minutes ago, Tall John said:

when I remove the cylinder the rounds drop in and drop out of all six chambers without issue.

Then your issue is not with the chambers.  There is something interfering with the casings on their way out of the chambers.  They are likely rubbing on the loading gate or the frame cut out.

 

Close clearances and problems ejecting was why Ruger put the little plunger in where the racthet teeth go in the frame.  Is the plunger present?  Is it working?

Edited Friday at 05:07 PM by Larsen E. Pettifogger, SASS #32933

[The Original Lumpy Gritz]

As others have said. You have a case rim clearance issue.

NOT a chamber issue. 

Has this been the problem from the start?

You sure you didn't switch cylinders between guns?

[Tall John]

it is interesting that the problem is now limited to ONE chamber.

 

I bought this gun used,  yes it has been a problem since acquisition,  I currently only have 1 45 Colt NMV so I didn’t switch the cylinders.  No chamber issues any longer since I honed them, just this clearance issue on one of the six chambers remains.

 

 

[The Original Lumpy Gritz]

8 hours ago, Tall John said:

it is interesting that the problem is now limited to ONE chamber.

 

I bought this gun used,  yes it has been a problem since acquisition,  I currently only have 1 45 Colt NMV so I didn’t switch the cylinders.  No chamber issues any longer since I honed them, just this clearance issue on one of the six chambers remains.

 

 

Can you post pictures showing the interference? 

[El Sobrante Kid]

12 hours ago, Tall John said:

it is interesting that the problem is now limited to ONE chamber.

 

I bought this gun used,  yes it has been a problem since acquisition,  I currently only have 1 45 Colt NMV so I didn’t switch the cylinders.  No chamber issues any longer since I honed them, just this clearance issue on one of the six chambers remains.

 

 

 

It is not very likely, but it is a possibility, that the one chamber was cut off-center. A metal chip in the clamping fixture would cause that particular issue. Since you have a set of pin gauges, and hopefully a feeler gauge, insert the pin gauge into the chamber, and measure the minimum gap for each of the chambers between the pin and the frame. If the cylinder was cut off-center, the gap will be the tightest at your problem chamber, and the largest at the chamber on the opposite side.