By Jeff Brown.
In 2011 I was fortunate enough to be gifted a Martini action rifle barreled in .310 Cadet (Greener). From day one it proved to be a challenge to get to shoot accurately when burning smokeless powder in brass created from .32-20 cartridge cases. Everything I read both online and on paper said .32-20 brass could be sized then shortened to suit the .310 chamber providing the rim of the case was thinned down to suit the Cadet specifications. I followed the instructions and was rewarded with nonexistent accuracy and barrel leading galore. Eventually I gave up and sourced some Bertram Brass .310 Cadet cases from Australia and have been shooting with them since however in the back of my mind I have been continually irked that I have not been able to make converted .32-20 brass work when so many other folk report great results. That was until YESTERDAY (Dec 8th, 2022) but more on this soon.
First a bit of background on .310 Cadet brass. There appears little agreement on the true OAL of the cartridge case. Cartridges of the World list Cadet brass at 1.020 inches. Bertram brass comes ex factory at 1.090 inch. Original Berdan primed Cadet brass from broken down ammunition measures 1.096 inch. Wikipedia states 1.120 inch for OAL case length. That is 0.100 inch or 2.5mm difference between the longest and shortest suggestions which is A LOT. So what length should one use and how do we establish this? I'll come back to this.
BELOW: Top, converted .32-20 brass with .321/.315 bullet, case length 1.180 inch. Bottom, Bertram brass at 1.090 inch.
In 2011 I was fortunate enough to be gifted a Martini action rifle barreled in .310 Cadet (Greener). From day one it proved to be a challenge to get to shoot accurately when burning smokeless powder in brass created from .32-20 cartridge cases. Everything I read both online and on paper said .32-20 brass could be sized then shortened to suit the .310 chamber providing the rim of the case was thinned down to suit the Cadet specifications. I followed the instructions and was rewarded with nonexistent accuracy and barrel leading galore. Eventually I gave up and sourced some Bertram Brass .310 Cadet cases from Australia and have been shooting with them since however in the back of my mind I have been continually irked that I have not been able to make converted .32-20 brass work when so many other folk report great results. That was until YESTERDAY (Dec 8th, 2022) but more on this soon.
First a bit of background on .310 Cadet brass. There appears little agreement on the true OAL of the cartridge case. Cartridges of the World list Cadet brass at 1.020 inches. Bertram brass comes ex factory at 1.090 inch. Original Berdan primed Cadet brass from broken down ammunition measures 1.096 inch. Wikipedia states 1.120 inch for OAL case length. That is 0.100 inch or 2.5mm difference between the longest and shortest suggestions which is A LOT. So what length should one use and how do we establish this? I'll come back to this.
BELOW: Top, converted .32-20 brass with .321/.315 bullet, case length 1.180 inch. Bottom, Bertram brass at 1.090 inch.
Secondly, a quick look at .310 bullets. The .310 Cadet is essentially an oversized .22RF. Featuring a heel bullet design where the section of bullet held in the cartridge case is smaller than the driving bands of the projectile and is required to 'slug' up to full diameter upon firing the round is what is referred to as an 'outside lubricated cartridge' as the bullet lube is not in grease grooves as with modern bullets but applied to the exposed bullet. Like .22RF ammo the Cadet was manufactured with cup based bullets that allow for the rapidly expanding gases operating at high pressure to swell the base of the bullet up to 1) act as a gas seal and 2) bump the heel diameter up to match the bullet driving bands to deliver superb accuracy. In addition much of the factory ammo featured a crimp of some sort to hold the bullet firmly in the cartridge case and to provide a degree of resistance up firing building up pressure to properly 'bump' the bullet heel to correct size. Bullet alloy was soft by nature in the region of 6 BHN so not a great deal harder then pure lead. An original bullet pulled from loaded ammo measured .304 on the heel and .318 on the driving band requiring the heel to slug up a full 0.014 inch to match the rest of the bullet hence the soft alloy. Short of a special order bullet mould all those available for the handloader are bottom pour with flat bases handicapped by a lack of a cup to assist with obturation of the heel. This is where the fun begins.
BELOW: An original (disassembled) .310 Cadet factory round showing cup based bullet with heel of 0.304 inch and driving bands of 0.318 inch.
BELOW: An original (disassembled) .310 Cadet factory round showing cup based bullet with heel of 0.304 inch and driving bands of 0.318 inch.
If we work backwards from chamber dimensions and fire case measurements we begin to see where the challenges of loading modern Cadet start. My rifle's chamber at the case mouth measures 0.334 inch. Fired brass comes out of the chamber with the outside diameter of the case mouth the same; 0.334 inch. Original brass measures 0.011 in the wall at the mouth giving up a total of 0.022 inch. 0.334 minus 0.022 equals 0.312 inch. That means that when the cartridge lights up an original 0.304 inch heel would be left unsupported 0.004 either side which is why the cup was so critical in expanding to take up the slack. Failing that the bullet would not stay true to the barrel axis and accuracy would go the way of the Dodo bird. It gets worse still. Our modern bullets have no cup in the base so rely on inertia and resistance from the bullet slamming into the rifling in order to obturate the bullet heel. Now we have a bullet that must begin to travel forward prior to the heel slugging up to full size so both the nose and heel are unsupported and only the driving bands engaging the rifling. Where the driving bands engage effectively creates a pivot point allowing the bullet to cant in the bore kissing goodbye to all hopes of accuracy. So now we know the challenges let's look at how to make an accurate cartridge for our Cadet.
FIRST. Measure your chamber. Do a chamber cast using the likes of Cerrosafe or plain old garden sulphur (sulfur). There are plenty of tutorials on the web that will help direct a newby through the process. A chamber cast will allow for accurate measurements to be taken for further calculations.
BELOW: Sulphur chamber cast. Origin of rifling visible to the left.
FIRST. Measure your chamber. Do a chamber cast using the likes of Cerrosafe or plain old garden sulphur (sulfur). There are plenty of tutorials on the web that will help direct a newby through the process. A chamber cast will allow for accurate measurements to be taken for further calculations.
BELOW: Sulphur chamber cast. Origin of rifling visible to the left.
SECOND. Establish the diameter and location of start of the rifling/leade at which the bullet driving bands will begin to engage. I like to have a cartridge that engages the leade upon closing such that bullet kisses the rifling but can still be extracted without becoming stuck in the barrel. Two vital pieces of information will come from this location. The diameter of the driving bands of the bullet required and the length of the case.
THIRD. Establish the case OAL. Knowing where the bullet is going to sit in the chamber/barrel will allow a measurement to be taken from the face of the breech block to the top of the heel of the bullet where the heel transitions to driving bands.
FOURTH. Workout the optimum heel diameter. .310 bullet moulds throw projectiles with heels that vary from .304 to .312. To establish the best fit measure the wall thickness of the brass to be used at the case mouth, multiple by two and subtract from the diameter of the outside mouth of a fire case. Experience has shown that it is best not to size .310 brass but match a heel to fire formed cases and thumb seat bullets.
EXAMPLE 1: Older Bertram brass as case mouth, 0.012 x 2 = 0.024. Fired case measures 0.334 - 0.024 = 0.310. Your heel should be 0.309-.310 for BEST results. (More recent Bertam brass is 0.010 in the wall, a nuisance when you have BOTH on hand.)
EXAMPLE 2: R-P .32-20 brass reformed case mouth, 0.009 x = 0.018. Fired case measures 0.334 - 0.018 = 0.316. Your heel should be 0.315-0.316 for BEST results.
Where a larger than available heel diameter is required there are two ways to increase the diameter. 1) Powder coat the bullet that usually adds 0.002 inch to the girth or 2) hone out the heel in stages until the correct diameter is achieved.
CBE of Australia list 8 different heel style bullets for the Cadet with heels ranging in diameter from 0.295 (way too undersized) to 0.312 with most being around 0.309 inch.
DECEMBER 8th, 2022.
Not to be beaten I embarked again on the quest to convert .32-20 brass to .310 Cadet for use with my go to Cadet load of 4.5gr Unique lit by a CCI small pistol primer. Taking one R-P case from my .32-20 stock it was run through the Cadet FL sizing die to form to Cadet chamber dimensions (nominal). Using my trusty mini lathe I thinned the rim down to 0.045 inch from the original 0.060 inch. Removing material from the head of the case then required the primer pocket to be restored to correct depth using the Lyman took I had on hand. The cartridge case was then trimmed to previously established OAL of 1.180 inch for loading.THIRD. Establish the case OAL. Knowing where the bullet is going to sit in the chamber/barrel will allow a measurement to be taken from the face of the breech block to the top of the heel of the bullet where the heel transitions to driving bands.
FOURTH. Workout the optimum heel diameter. .310 bullet moulds throw projectiles with heels that vary from .304 to .312. To establish the best fit measure the wall thickness of the brass to be used at the case mouth, multiple by two and subtract from the diameter of the outside mouth of a fire case. Experience has shown that it is best not to size .310 brass but match a heel to fire formed cases and thumb seat bullets.
EXAMPLE 1: Older Bertram brass as case mouth, 0.012 x 2 = 0.024. Fired case measures 0.334 - 0.024 = 0.310. Your heel should be 0.309-.310 for BEST results. (More recent Bertam brass is 0.010 in the wall, a nuisance when you have BOTH on hand.)
EXAMPLE 2: R-P .32-20 brass reformed case mouth, 0.009 x = 0.018. Fired case measures 0.334 - 0.018 = 0.316. Your heel should be 0.315-0.316 for BEST results.
Where a larger than available heel diameter is required there are two ways to increase the diameter. 1) Powder coat the bullet that usually adds 0.002 inch to the girth or 2) hone out the heel in stages until the correct diameter is achieved.
CBE of Australia list 8 different heel style bullets for the Cadet with heels ranging in diameter from 0.295 (way too undersized) to 0.312 with most being around 0.309 inch.
DECEMBER 8th, 2022.
My CBE 323-125 mould casts in 40-1 alloy a bullet measuring 0.323 on the driving bands and 0.314-0.315 on the altered heel. A chamber cast had established driving bands of 0.321 were required and as luck would have it my Cadet seating die when used like a Lee push through die sizes exactly to 0.321 inch. Alox/wax treated bullets were sized (driving bands only) and seated in the altered case over the prescribed dose of 4.5gr Unqiue.
Shootings at 50m/55y and loading the same cartridge case each time produced one of the best groups the little Martini has ever turned in. The first shot lubed with alox/wax was predictably out of the group and I called one pulled shot. The balance of 5 rounds when into less than one inch with 4 of the 5 in 5/8 inch!!!
BELOW: 50m/55y group.
BELOW: 50m/55y group.
FINALLY!!! A smokeless powder load using .32-20 brass converted to .310 Cadet that actually shoots accurately, doesn't lead the barrel and doesn't deposit bullets sideways on the target.
Strangely enough powder coated bullets that shoot very well out of the Bertram brass did not do so in the converted brass. They exhibited the same inaccuracy as experienced in the past and this remains a head scratcher. Fortunately the alox/wax coated bullets shot better than one might ever expect them to.
CONCLUSION.
Case length is absolutely critical in Cadet accuracy as is matching the diameter of the heel to the brass being used. Flat base bullets need full support at the time of ignition in order to stay true to the axis of the bore or zero accuracy will be achieved. Use soft alloy (40-1 lead/tin at the hardest) that allows the heel to slug up to full diameter at the relatively gentle pressures the Cadet runs at. Fit, fit, fit.
Strangely enough powder coated bullets that shoot very well out of the Bertram brass did not do so in the converted brass. They exhibited the same inaccuracy as experienced in the past and this remains a head scratcher. Fortunately the alox/wax coated bullets shot better than one might ever expect them to.
CONCLUSION.
Case length is absolutely critical in Cadet accuracy as is matching the diameter of the heel to the brass being used. Flat base bullets need full support at the time of ignition in order to stay true to the axis of the bore or zero accuracy will be achieved. Use soft alloy (40-1 lead/tin at the hardest) that allows the heel to slug up to full diameter at the relatively gentle pressures the Cadet runs at. Fit, fit, fit.