New builds, 7.62x39mmUSA kinda like a 300/221XL

[kurtz]

A couple builds I have been working on for a while and some range results from the weekend....

The first is built from DPMS components, 20" Noveske 1:12 twist rifle length gas system chambered in 7.62x39mmUSA, JP adjustable gas block, Leupold M3....25.3 grains of A1680 under a 110 V-Max = 2675fps...25.3 grains of A1680 under a 125 TNT = 2530fps....as far as accuracy I didn't bring any targets with me to the range....centered up on the 200 yard gong 20 rounds made a nice brite spot about the size of a quarter...




The second is built from a combination of DPMS and Olympic components and began life as a 300 fireball....was recently rechambered in 7.62x39mmUSA....the original pistol length gas system under the handguard was removed / plugged and set up as a carbine length system....It's a hoot to shoot with 25 grains of A1680....a little more BOOM than the original fireball loads



The 7.62x39mmUSA compared to 300/221 and .223 Rem


The 7.62x39mmUSA compared to a 300/221 in a mag


The 7.62x39mmUSA is based on a .223 parent case..it uses standard bolt face and mags....as in my 300 fireball barrel, I hand re-chambered it with the longer reamer.....here is an old thread from 07 talking about the developmenthttp://www.quarterbore.net/forums/showthread.php?t=1481

I left a reamer and headspace gauges with John Noveske if anyone is interested in having a barrel cut.....PTG also has the print on file....Its been a little work getting there and the only powder I have tried is the A1680 but I guess thats part of the fun....

[jarhead1086]

I notice that the case length is longer than the bottom of the shoulder on the .223 case. You have to open up the case to achieve this obviously. I recently got a .300-221 die for my Dillon trimmer and now making cases is easy in one step to collapse the shoulder and trim. It does take another step to deprime since the trimmer dosen't have a stem, but I prefer having a primer in when I tumble to keep crap out of the pocket. This is much faster and easier than opening .221 Fireball like I used to do. This is a potent .30 you have in a light efficient package, very cool. Half the powder of a .308 and most the punch. Any feeding problems? Have you tried P-Mags or Beta mags?

[kurtz]

I have a small hobby lathe that I use to cut my brass down with....it has a three jaw chuck with aluminum jaws that I cut enough taper and a back-stop in to fit the .223 case....I ground a parting tool that comes in perpendicular to the shoulder just shy of the neck, it cuts clean enough that I haven't had to de-burr any cases yet....the rough cut cases are cut about .010 shorter than desired then ran through the die (I'm using a Hornady expanding button) you get the .010 back in length and end up at 1.540"...

To date they have digested 110 V-Max, 110 Sierra (hollow point varmint), 110 Hornady PSP, 125 TNT, 125 Rem. PSP out of DPMS mags with Mag-Pul followers with no stoppages....I have not tried the Beta or P mags..I do have a couple of the HK mags on the way out of curiousity that I'm going to hopefully try this weekend....

The 16" carbine cycles very positive, you actually get a little tingle in your shoulder after twenty or so rounds.....the 20" with rifle length gas system cycles a little more slugish, almost delayed to the point of short-stroking, but I'm going to tune on that a little... If I had it to to do over, I think I would try a 20" barrel with a carbine length gas system...

kurtz

[JFettig]

Very cool! I was considering the same expereriment, doing this instead of building the .300/221 I'm building now. I had designed my reamers and started making the dies but had some issues and wanted a gun done. Mine were .1" longer than .300 whisper, looks like yours are longer.


Jon

[kurtz]

Hi Jon,

The case length comes in right at .200" longer than the 300/221 depending on what you trim the 300/221 at (1.330 to 1.400).....the powder column with A1680 increased from 20grains to 25grains....the shoulder of the new round was blown out to the max that my redding 300/221 neck sizing die would accomodate without touching the outside of the shoulder and still leave a few thousandths taper in the overall length of the case....

when I re-chambered the 300 fireball there ended up being two very slight (.002 to .003) areas of transition that are noticable in the fire-formed brass...one about 3/8 of an inch forward of the base and another about .050 in front of the shoulder....the tolerance of my reamer was a little smaller than the olympic one and you end up with the two areas of transition...the one in the neck is taken care of when it gets neck sized...

[pug]

Nice job. They say great minds think alike . Very close with my 308R2. The 16" carbine is my favorite although I am contemplating cutting the 20" down to a 10" pistol.

300/221 vs. 308R2