32-20 Chamber Dimensions

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  • Last Post 22 February 2016
dedseven posted this 21 February 2016

Greetings to all,

I have read this forum from time to time over the last few years with great interest and now finally have a reason to post. I am a novice at the cast bullet game and am here seeking advice. I have worked in the firearms trade and have a good working knowledge of barrels and the methods used to produce and chamber them.

I am about to begin work on a project rifle that I will use for “pistol cartridge” lever action silhouette shooting. It is what I call a “gunsmith special", a Winchester '92 in 25-20 with a rotten bore and missing stocks. I will rebore and rechamber the rifle to 32-20. I will be purchasing a chamber reamer for this project and would like to get some guidance from this forums members regarding optimum chamber dimensions for use with cast bullets and smokeless powder. Combining a standard chamber (I have a SAAMI spec reamer) with current production Starline 32-20 brass results in well over .040” gap between the case mouth and the end of the chamber neck. It is my understanding that this is not desirable when shooting cast bullets. I would also like to discuss the dimensions of the throat area of the reamer. It seems that most chamber reamers cut for cartridges that were originally black powder rounds feature a relatively shallow straight taper from the chamber neck diameter down to bore (land) diameter. Chambers for more recent cartridges typically have a more abrupt (90 degree included angle) step-down from the chamber neck to a throat section which is at or just a few thousandths of an inch over groove diameter, followed by a gentle leade angle which engraves the bullet as it passes. To summarize, if you were going to specify a custom reamer in 32-20 for use with Starline brass, smokeless powder and cast bullets, what would it measure and why?

I did perform keyword searches of this forum in an attempt to find relevant threads without much success. If this topic is well covered elsewhere please accept my apologies and if possible, provide a link. Thank you and good shooting.

I almost forgot to mention, any advise on rifling twist rates would be welcome also.

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rmrix posted this 22 February 2016

Sounds like a fun project! I am smitten with 25-20 for Lever action PC silhouette. 100 meters does not take much to knock the rams over.  I have two 1892's but I know you want to know about the 32-20.

The 32-20 is a great round for the 100 meter as you know. Are you the one chambering the 1892? Are you going to line the barrel? Fitting a new barrel is much more work (again as you know) and the barrel liners from TJ are good. I do not have experience with the other source of liners so can not comment.

As for reamers, the SAMMI works and that gap, while not great likely will not be a problem with well fitting bullets that run 12 or more BHN (air cooled WW) if smokeless is used and driven at sensible velocity.

However, if you have not committed to a reamer yet check the outfits like Dave Manson and others and look on line at their chamber prints. I bet they have something you like.

If not, this is what I had made; lever rifle chambers often work well using max-case length +0.010", then 45 degree case stop, then start a 3 degree per-side into the rifling. You can add a very short groove diameter cylinder (+0.002") section before the rifling starts maybe 0.025 or 0.040” long. But often, depending the bullet design ahead of the crimp, this may or may not be desirable. In any case, you do not want much cylinder ahead of the case mouth.

Something to Keep in Mind, NRA matches do not allow a re-barreled rifle but a Lined Barrel is OK so doing a reline could keep an old rifle's value better. Not to mention lining is a better choice to keep the Winchester marking with the rifle.

Again, I do think the standard SAMMI reamer will work fine even with the gap.

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onondaga posted this 22 February 2016

http://www.castbulletassoc.org/view_user.php?id=9651>dedseven

I can pick out what would be most important to me as a cast shooter. The gentile leade angle, perhaps, 3 degrees,  works well with cast.

Anything touching before the bullet gets a secure ride in the bore can change bullet path and the gentlest leade is far less discouraging to a bullet than an abrupt leade.

Your bullet size should be your last worry with a gentile leade also, unless you are already completely decided on using a bore rider type nosed bullet. They over-complicate the beauty of simplicity to me. I only see them as having less of a grip in the bore.

The simple Lee pistol bullet, the TL314-90-SWC casts large enough for any 32-20 and when sized to fit the throat with an ink verified slide fit, the bullet shoots excellently in a 32-20.  I like that bullet and size it .3125” for a single shot 7.62X39 Remington Spartan shooting an 1160 fps Squirrel hunting load that shoots 1/2” at 50 yards. They shoot great when you fit them well in that caliber too. It is a very popular bullet for 32-20 and worth a serious try.

That bullet in my Squirrel loads:

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Ken Campbell Iowa posted this 22 February 2016

regarding the 32-20 chamber for cast, i would hope to hear what ed harris might contribute .

i would speak to the very desirable 92 action, in that i would work on that before rechambering ...tightening the locking and examining the feeding mechanism for wear .

please keep us informed on this project .

ken

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RicinYakima posted this 22 February 2016

dedseven, I've been thinking about this since I read your post. 14 of them live at my house and it is far and away my favorite cartridge. Read the rules before you decide to re-barrel! Re-boring or relining may be best, best up to you. Twist: 1 in 20 will spin a 125 grain RN down to 650 f/s and work fine with anything unless you want to shoot 150's then you will need 1 in 18, except for Lyman 311440, that will work slower. Consider having Mountain Molds cut a custom one if needed.  Barrel: I have used a .300/.308 with a one and half degree included angle that worked just fine, but not for jacket bullets. Most liners are .303/.311 that are just fine with normal bullets for the 32/20. If all you could get is a .303/.313 you can make that work. SAMMI spec are worthless, as Marlin made the request in the 1990's and they used the spec's from their 32/31 Marlin cartridge from the 1890's. Dies are all too small and ruin the brass. Headspace is suppose to be .060” but brass is normally around .045". Consider cutting your chamber short and cutting the rim space with a boring tool in the lathe. Buy Redding dies for 32/20 and not the 30/20 for Contenders, and make a chamber cast for the sizer. You only need a chamber just a couple of thousands larger than the sizer. Plus you can short chamber and get what you need. PM me. I have a once used Manson 32/20 reamer that cuts about a Ackley Improved chamber, forward shoulder but small and useable with modern dies. You are welcome to try it on a stub barrel and see if it will work for you. Any other questions welcome. Best wishes, Ric  

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dedseven posted this 22 February 2016

Thank you for all the quick replies! Just to clarify, this rifle is a 25-20 with a rotten bore. I am not going to rebarrel it. I am going to rebore the barrel to 32 caliber and re-cut the the chamber using a 32-20 reamer. I will be doing all the work myself. I plan to use smokeless powder and cast bullets exclusively. I don't expect to use any bullets over about 130 grains in weight and will not use any high pressure loads. I have the option of .300” or .305” bore diameters. I can cut the groove diameter to any reasonable dimension I want. I am leaning toward the .300” bore which will result in “deep” rifling. Does anyone have any thoughts about deep versus shallow rifling with cast? Regarding headspace, I will be stuck with the existing rim cut unless I set the barrel back, which is not out of the question. I will make that decision once I get a chance to take some good measurements.

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onondaga posted this 22 February 2016

http://www.castbulletassoc.org/view_user.php?id=9651>dedseven

I believe the deeper the rifling is the more it decreases the accuracy of nose bore riding bullets cause it rips them up and the less it decreases the accuracy of bullets with fuller body contact  and no nose contact like:

http://www.accuratemolds.com/bullet_detail.php?bullet=31-155G-D.png>http://www.accuratemolds.com/bulletdetail.php?bullet=31-155G-D.png

Accurate offers custom dimensions on all their molds.

You can also design a gentile leade taper to engage a particular bullet at a particular LOA. I'd like to seat that bullet with .005” leade engagement of the top band on chambering for a target rifle but I don't know if that would cycle in your lever gun.

Gary

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Ken Campbell Iowa posted this 22 February 2016

ok, i can't resist a comment on the chamber ... first, i would jump on ric's offer of a tighter reamer ...there are many benefits , including reloading ease and better accuracy . ric's 1.5 degree included reamer throat is a very big deal ... that gentle throat and barrel condition are the two biggest things contributing to cast accuracy .

next...regarding rim width ( headspace ) ... if you use your saami reamer, if you measure the rim width of the brass you will use and find the rims are 0.040, you could cut the chamber shorter than saami ( don;t run it in so far ) and get two benefits ... 1) correct real life headspace .. maybe 0.0015 or 2 clearance .... and
2) lessen that little space in front of your case... probably a good idea .

i would stick to 0.308 and 0.300 for barrel numbers ... mostly so you can play with standard molds .. everybody gets the itch ... but if you want to go directly to best chance ... chamber your barrel, do a slug or casting image of your chamber/throat and have a custom mold made to those specs .. one killer custom mold is cheaper than 2 or 4 that don't fit .

and fit is everything . fit. everything .

there.... oh, if you don't want to buy a custom reamer or rent ric's ... the short fat 32-20 chamber can easily be bored and polished with a boring bar; then you can get exactly what you want .

just some thoughts, hope some help... i am very interested in your project ... i used to shoot an original m92 in 32-20 ... most fun gun with reduced loads on rabbits !! 3118 hollow point with unique .

oh... on the gun think about lightening the loading port cover pressure and smooth edges that can scrape fingers there .

ken

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