Safety Warning: Duplex loads done wrong can destroy your gun and even injure or kill you or others. Don't do this at home.
I purchased a few cases of WC867 from Jeff Bartlett for the sole purpose of duplexing loads in all my rifles, .223, 7.62x39, 7mm-08, 45-70 to name a few. Some may think this is about cost savings or penny pinching and even though these loads will allow me to shoot more, the process is all about duplexing.
This post is about 7.62x39 running in an AR upper from Bear Creek Arsenal, it has a 16" barrel and a 1:10 twist with a carbine impingement system gas tube and standard block. I would my end result to be good ejection of all brass, strong lock up and have the bold lock open on the last round.
Accuracy and performance are key issues at this time.
Today, I finished my workups on the 7.62x39 Russian but not as one would think. I knew that there would be an issue with getting the WC867 to burn well in such a small case and that you really need something to 'kick start' the powder as well as some sort of containment or resistance to allow for the gas to build up before the bullet gets pushed away from the casing mouth.
I started by doing a LOT of reading, here, other web sites, reloading manuals and face to face discussions with old snorts that have been duplexing BP for decades and almost universally, IMR4198 was suggested for the kicker.
OK... That's great! Now, how does one get better containment for the powders to burn in such a small case? A friend whom shoots the Quigley just smiled as she held up a bag of 500gr boolits for her 45-70 trap door, for me to see. Being rather slow, I didn't get it right away, she let me off the hook by telling me to, "Use a heavy boolit, heavier than listed for loads." She usually shoots 400gr but the powder she uses is a bit lame until you put a heavy boolit on top of the duplex and the extra inertia you have to overcome increases the pressures allowing for a better burn of the powders.
Now, I know I have heard that before, probably here somewhere but having a 120lb gurl point it out to me was more memorable to say the least. I went in search of a mold that could get me to the place I needed to be but I could find nothing in stock that would give me the same profile as the standard x39 shape then a LYMAN 311334 single cavity pointed mold came up for sale here and I was fortunate enough to grab it up!
200gr, pointed with a nose section small enough to kiss the lands without engraving. Saweet! Only one problem, no gas checks! A quick inspection of the financial reserves showed a zero balance and SWMBO told me that I would just have to make due!
Make due? OK, I'll do that! Chucked the mold up in the four jaw and milled out the silly-little-gas-check-band! THERE! Now I got something!
As you can see, the gas check band is gone, the base is flat and makes a pretty nice base, if I do say so my self! Very satisfied with my work so far, I moved on to the powder/s.
Starting with the analog of WC867 with no booster, I stumbled upon my first problem. Turns out that the case is a bit too small for 25gr of powder and this boolit seats inside the case so far, I had to reduce the powder to 21gr and even then, I had to slightly compress the load.
No big surprise that the shot was anemic and the bore was flooded with unburnt powder, the speed was 830FPS and the bolt didn't cycle. I knew that would be the case but it had to be done. With that out of the way, I started on adding the kicker 1gr at a time and reducing the 867 equally so that I had room for the bullet.
I ended up with 18.5gr WC867 over 4gr IMR4198, Winchester Large Rifle Primers, mixed brass and loaded into a BCA upper with a 16" barrel with a 1:10 twist. All shots fed fine, loaded straight, ejected slightly behind the gun and only a foot or so away. The bolt locked back on an empty magazine and a clean patch showed no unburnt powder in the bore!
The chronograph recorded 10 shots with a median speed of 1250fps with a 22fps variance. I threw the chrony out on the grass in-between thunder storms so no recorded data this time, no sky screens, cable or tablet.
The grouping was 3" with one flyer, which was me dropping the grip off the sandbag. I think a 3 MOA AR shooting a 200gr boolit at 1250 fps and 600 ft lbs of energy is OK in my book.
THOUGHTS: I liked how the recoil is a push and not a smack! The noise is much less than the 123gr Wolf and with 1/2 the energy at 100yds, that is not much of a surprise but this is a solid candidate for a suppressed round, 175 more energy at 100yds than the 300BLK and a cost amortized at $0.010 a round makes for a lot of fun shooting for very little money.
I am still working on my other duplex loads and I hope to return to this once more with a 160gr boolit just to see if I can get the speed and energy up a bit. I think I may try Silhouette as the kicker if I do.
Cheers!