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

[kurtz]

Hi pug,

Glad to see your projects are moving along as well....I find myself shooting my carbine more also, it has performed well right from the start.....the rifle length system is pretty sensitive to which load I'm using, the gas system on it seems to like the hotter loads ( above 2500 fps)....thinking about opening the port up a little to get some use out of the adjustable block...

went to the range today and tested a few loads:

22 grains H110 125 TNT = 2540 fps
22 grains H110 125 Nosler BT = 2550 fps
22 grains 4227 125 TNT = 2440 fps
25.2 grains 1680 125 TNT = 2525 fps

Also tried one of the HK 416 mags, pretty 5.56 specific..

Picked up some winchester 296 on the way home to give a try...my pet load right now is the 25 grains of 1680, has worked well from the start...

my target velocity with the 125 nosler was 2500 fps and I hit it today with the H110 but the cycling was a little weak in the 20" barrel, guess it's a little too fast burning or need a bigger port....

good to hear from you again, I have been out of the loop for the last few months...keep us up to date on your builds...

kurtz

[pug]

I think my rifle port was at .097" the last time I checked. Found the same thing that these capacity cartridges are a little sluggish ported in the rifle position. I couldn't use any powder faster than 1680 in the 20 or it would borderline function but your 16 might like the W296 with the 110s and 125s. If your cartridges tend to roll inwards as you fill the mag try some Thermolds. They have shallower ribs like the Grendels and the rounds will stack straight down.

I am going to turn my attention away from the guns now and concentrate on ammo components. People are going crazy and I need to pick up all of the LC brass while I can get it and the M193 projectiles cheap. There is still a supply of 308 penetrators but i had better move quick or they will be long gone also.

[kurtz]

my rifle port right now is .093...borderline with everything except the 1680...the carbine if I remember right is .078 and works great....

I know what you mean about components...ya wanna hear something funny, I built my first super-sonic 300-221 around the 125 TNT because at the time, a little over a year ago, you could find them for around $50.00 per 500..now they are nearing $100.00 per 500, just a year later...

The gun store that I went to today is not known for having much in the way of reloading components but they had 2000 small rifle and 2000 large rifle CCI at $15.00 a box...I bought them all...Gander Mountain Sporting Goods got in a load of small and large rifle primers earlier this week and is retailing them at $38 bucks a box, unreal...

thanks for the info on the gas port and the mags....good luck with the component harvest....

kurtz

[kurtz]

went to the range today and tested a few loads in the 20" 7.62x39mmUSA

20 grains lil gun 110 V-MAX = 2720 fps
20 grains lil gun 125 TNT = 2515 fps
22 grains lil gun 110 V-MAX = 2736 fps
22 grains lil gun 125 TNT = 2555 fps
25 grains A1680 110 Sierra hollow point = 2650 fps

the 110 Sierra hollow points and the 125 TNT are producing under 1" at 100 yards with A1680 out of the 20" 1:12 noveske....a full case load with the A1680 has a very low standard deviation, so far less than 15 fps...

kurtz