best chamber dimensions for cast bullets: 40-65

  • Last Post 23 June 2020
watson posted this 21 June 2020

Hi Folks

First post here. A friend of mine has built himself a fantastic break-action rifle from scratch and we're looking into the options of chambering it in 40-65. Since that chambering is not a SAAMI approved one, there are dozens and dozens of reamer designs for it. After reviewing several chamber prints, it looks like we may as well design our own reamer. Although I’ve designed several wildcats for the AR platforms, I’m very new to cast bullets and I realize there are a lot of nuances to cast bullets as it relates to chamber spec's that will enhance or degrade accuracy. This rifle is to see a moose hunt later this year and the chamber spec’s are being driven by that – middle of the road bullet weight, #2 alloy, flat meplat, gas checked, smokeless powder, 25-30k psi. Following are the trending thoughts I have and I welcome any input you're willing to offer that will help us specify the best dimensions/approach to consider:


- His bore slugs out at exactly 0.408"


- we’re considering a chamber designed to use re-sized 45-70 casings versus one that uses the one source of 40-65 brass (Starline) (pros/cons of either option anyone???)


- the bullet we're warming up to is the Accurate Molds #41-340JG, in #2 alloy, which is a 0.410” dia., gas-checked, 340 gr RNFP, and we would marry it to a Gator 0.416 rifle caliber, copper GC., The large, flat meplat is looking superb for driving a large permanent wound cavity and #2 alloy seems about right to keep it functional as such


- swaging said bullet/GC down to 0.4085”-0.409”


- cut the freebore diameter to 0.409” (or would it be best to keep closer/at 0.408” groove diameter???)


- cut the freebore length to have the seated bullet just into the lands


- cut the mouth-to-freebore transition at 30deg and with a small “ADS” radius (anti-dounut-slicing :-) ) at the small diameter edge


- cut the leade angle at 1.5deg (is that optimum for cast?)


- cut the chamber neck diameter 0.0015” over loaded cartridge neck diameter (is that too tight for cast bullets?)


- cut the chamber length so that the resized 45-70 brass needs to be trimmed at least 0.050” (preventing short casings)


- neck size only with a Lyman M die tuned for bullet our diameter and FLS/anneal when needed


- find a 40-65 die set that has been designed around 45-70 casings, has a straight neck section (hopefully), buy them, size a few brands of 45-70 brass and determine the body dimensions of the reamer based off that. Anyone have any insight as to which dies that would be?

Any insight anyone is willing to offer is appreciated.



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beltfed posted this 21 June 2020


Here is an example of shooting my 40-65 HiWall/16 twist at 100yds, with

my Dual diameter bullets, of 9+1 ww/lino. and a case full of Swiss 1.5 bp. in cases

made from WW 45-70.  I also use Rem 45-70 brass, but the WW has more case capacity than the RP.



That said, chamber suggestions. per my successful rifle: Cut with Clymer reamer:

Chamber stop, 45 degrees, Freebore 0.150" long, Leade recommend 1.5 degrees per side.

Definitely you want a Cylindrical Neck, with a "distinct" very slight shoulder. Neck should be

about 0.525-o.550" long and diameter to allow for a snug slip fit of a 0.409" bullet.

I use the Redding die set.

The freebore chamber will allow you to use GG bullets up to 400 gr or so, and if you want to

try PP bullets, I suggest you go with Dual diameter such as in the PIC below, except you will want

Large Flat meplat nose for hunting, as you said


That sai

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watson posted this 22 June 2020

That is mighty impressive. That indicates a VERY consistent system and I wouldn't have thought a PP seal could be that consistent, especially considering the engraving forces involved. Amazing. You certainly have a recipe for success there. And now you've caused the question machine to engage... Is getting a PP to be that consistent a result of an arduous and meticulous preparation? Are you neck only sizing?


Dual diameter... that is interesting. Is that as in a bore-rider sense?


That's great to hear the Redding dies size properly on 45-70 cases. Do they keep your parallel neck and the pseudo-shoulder geometries when sized? Is the slight shoulder “just enough” to use as an indexing point for case-alignment/accuracy? Would you happen to know what the body diameter is about 0.015” above the rim and again about 0.130" up from the rim on a FLS 45-70 case with your Redding dies? On second thought, I’d pay you to size up a virgin or once-fired 45-70 case in your dies and send it to me, if you’re willing.


On the 45 degree mouth/freebore transition angle... I was under the impression that a shallower angle was "better" for cast/lubed bullets due to the more gradual engraving it affords the soft alloy (relative to copper) and thereby less of a leading risk in that area? 45 degrees is the standard for most modern bottleneck rounds that were designed around copper jackets. Does the PP negate that in your case? One of the more common, rack-grade, but competitive (supposedly) 40-65 chamberings, Browning, actually has a 12.5 degree transition. I can understand a few degrees of range being having the desired or similar impact, but 12.5 degrees to 45 degrees is an apples and oranges difference. That says to me that angle isn't that critical, or people are exaggerating about how accurate their Browning chambered 40-65's are with cast/lubed bullets, or there is some other factor(s) that play into the criticality of that angle. I just checked the 45-70 SAAMI print and it too shows 12.75 degrees. Hmmm… so much to learn.


My apologies for the onslaught of questions. You kicked my clutch in gear I guess.




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beltfed posted this 23 June 2020


I will try to comment on your questions.

Consistency. First of all, I will say that I do try to be consistent in my loading for my 40-65, and

for that matter all other ctges I load for.  In this case, I will be more specific. The "seal" in my Black Powder

loads is using a tight fitting (0.413" dia x 0.060" thick) LDPE(Low Density Polyethylene wad on top the powder charge.

I have used the LDPE wads in both GG (grease groove) and PP bullets. It is essentially a gas check..

I designed this bullet to fit my 40-65 chamber AND Fire Formed brass. I Do Not neck size or full length size once my brass is fireformed. After the cases are cleaned,I only run a Lyman "M" type expanding plug 0.411/0.413" into the case mouths to make sure case necks are round and retain that slight belling to allow seating my bullets- the PP bullets w/o tearing the paper. That slight belling also aids in eliminating gas blowback around the outside of the neck.

DDEPP (DualDiameterEllipticalPaperPatch) bullets:  I came up with this design such that:

The Base Band diameter and length is established such that it "patches to fireformed case freebore I.D. and is about 0.001-0.002" larger than groove diameter  and to the length of bullet seated in the case -about 0.125" plus about 0.150" freebore.  The body diameter of the bullet "patches to bore/land diameter" so that the bullet is overall Concentric to the bore and Fire Formed case I.D. So, Yes, the patched body is a bore rider, while the patched  base band seals well in the grooves. I have "snow shot" bullets recovered that show this very well, the rifling marks are distinct only on the base band-the lands cut thru the paper such that it

leaves the bullet typically within 10=15 feet from the muzzle.

My custom "Browning like" chamber is cut with about a 45 degree case stop at end of neck, though the Browning chamber is indeed a 12.5 degree off the neck. What I referred to earlier is that After the freebore, the Throat or properly Leade is at 1.5 degrees per side.

Actually I have owned and shot quite a bit with the Browning BPCR 40-65, and a friend has a Browning 40-65 and uses this same exact DDEPP bullet to good effect- Very Accurate. I won a number of matches with my Brg 40-65.  I would say, they are very competitive with most other rifles when loaded with the Correct Length bullet. I got to go. Wife waiting to go shopping.

I would consider you could do no wrong to chamber that rifle with a "Browning Spec" 40-65 reamer.







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