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#1747407 - 12/11/10 09:27 PM 7.62 X 40
KAG Offline
Die Hard Member II

Registered: 01/14/10
Posts: 1187
Loc: Kansas
I have not even received my new 25/223 and I am already planning the next one. I copied this from another site and it looks very good to me. So it will probably be next of I do not succumb to the 25 Grendel I secretly am drooling after.

http://www.ar15.com/forums/topic.html?b=3&f=121&t=382761


The 7.62x40mm is a .223 based wild-cat offering for the service rifle family of weapons and was created by Kurt Buchert of Lake Charles, Louisiana.

First thoughts began in September of 2005 as the result of many sleepless Hurricane Rita nights huddled around lanterns thinking and talking about the use of the 300-221 as a medium-sized game hunting cartridge in a service rifle platform. The thought or goal was to wild-cat an improved .30 caliber cartridge that would take advantage of as much of the .223 parent case capacity with a 125 grain bullet and still load and function in standard service rifle magazines and mechanism components. The selection of the .30 caliber projectile in this weight class was based on the calculated velocity window of 2,400 to 2,500 fps and decades of industry wide reloading, hunting, and military experience. The compiled information showed that the 110 to 125 grain projectiles performed well at these intermediate velocities while delivering modest recoil and a high degree of controllability and accuracy in manual and gas operated weapons.

Following the intermediate cartridge concept of the 7.92x33mm kurz, 7.62x39mm Russian, 7.92x40mm CETME, and 7.62x40mm CETME from the 40’s and 50’s the 7.62x40mm is a culmination of firearms industry, wild-cat, benchrest, and personal experience and information. The chamber dimensions went through several revisions in an effort to produce the best all-around compromise of feeding, case capacity, velocity, accuracy, and chamber release. The intersection of these efforts has come to what individual owners and industry leaders spoken with have called a “makes-sense” cartridge for intermediate power in a service rifle platform.

Cartridge Data
Case length: 39.76mm / 1.565”
Case rim: .378"
Case rim thickness: 0.045"
Case base: .373"
Case taper: 0.011” total
Case shoulder angle: 30°
Case Capacity: appx 30 grains of de-mineralized H20, varies slightly by brass manufacturer.
Maximum loaded overall length (OAL) in AR15 magazines: appx 2.250", varies by manufacturer of magazine.
Usable case capacity with a 125gr flat-base bullet seated magazine length is appx 26.5 grains of Accurate 1680 powder, varies slightly by brass manufacturer.
Rifling twist rate varies from 1:8 to 1:14

Average Performance Data

16” Barrel
110 Hornady V-MAX =2,575 fps
110 Sierra Varmint = 2,550 fps
125 Speer TNT = 2,450 fps
125 Nosler BTBT = 2,450 fps
123 Lapua FMJ = 2,475

20” Barrel
110 Hornady V-MAX =2,700 fps
110 Sierra Varmint = 2,675 fps
125 Speer TNT = 2,600 fps
125 Nosler BTBT = 2,575 fps
123 Lapua FMJ = 2,625 fps



Cartridge and System Information

The 7.62x40mm is derived from the 5.56x45mm (.223 Rem.) parent case. The low cost and availability of mil-surplus and commercial brass was one of the core reasons for the development of this cartridge.

Primarily designed to use 7.62mm (.308) projectiles in the 110 to 125gr range, but has been loaded and tested up to 220gr in sub-sonic applications. The range of bullet weights and designs in the .308 family make this a very flexible cartridge.

The user selectable ballistic performance makes it an excellent choice for varmint control, medium sized game hunting, or personal defense. As illustrated below in a comparison of the .223 and 7.62x40mm.

.223 load of a 55gr FMJBT at 3,000 fps will produce an average of:
1,100 ftlbs of energy at the muzzle
885 at 100 yards
710 at 200 yards
565 at 300 yards with a 5.4” high flight path

7.62x40mm load of a 125gr BTBT at 2,500 fps will produce an average of:
1,730 ftlbs of energy at the muzzle
1,430 at 100 yards
1,175 at 200 yards
960 at 300 yards with a 7.6” high flight path

The 7.62x40mm makes the most of standard rifle components, usually only requiring a barrel change while still offering the end user the ability to customize the rifle with their choice of buffer and gas systems. You can still tune the rifle for timing and felt recoil which makes the whole system more user friendly and reduces wear and tear on the rifle, does not require any proprietary parts to function.

For current 300-221 / 300 Whisper users it is a natural progression if looking for more velocity. Simply have the barrel re-chambered, cut and form new brass, adjust your reloading dies for the longer round, reload and head to the range.

The information in this document is not inclusive of all that has gone into the 7.62x40mm cartridge, the thousands of rounds reloaded and fired, experiments with magazines, chamber revisions and results, or the performance of barrel and gas system length…..

This is meant to serve as core information, a conversation starter, and a holding area for those who wish to share their experience with the 40...

line-up of 300-221, 7.62x39mmRussian, 7.62x40mm, and 5.56x45mm



Edited by KAG (12/11/10 09:35 PM)

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#1760213 - 12/21/10 07:28 PM Re: 7.62 X 40 [Re: KAG]
dwightlooi Offline
New Member

Registered: 12/19/10
Posts: 1
Loc: california, USA
The main question I have is why?

First of all, let me say that your velocity predictions are a little optimistic. At best, I think you'll get the claimed 16" performance of 110gr @ 2575fps from a 20", maybe even a few fps short. And, that's maximum pressure loads not the typical factory load. 110 @ 2575 from a 16" is basically 6.8 SPC velocity and weights -- and that is a pretty well developed cartridge with a bigger case capacity.

The 300 Whisper (300/221) is not without a few quirks. The most significant of which being its 221 Fireball based case rated for about 10,000 psi less than the 5.56x45. Also, its base to tip length of 34.5mm is little on the short side when used with the lighter 308 bullets (the 110 Vmax or the 125 Ballistic Tip).

But, why 7.62x40 mm? Why not 7.62x37.5 mm with neck length of 7.7mm from shoulder to the case mouth using the 5.56x45mm case as the basis. In short, from the neck and up the cartridge is exactly like the 7.62x51mm round. The bullet can be seated to the same depth and all the chamber tricks for the 308/7.62x51 will apply. Going for another 2.5mm of case length doesn't do much if anything because you are still constrained by the 57.4mm overall length of the 5.56x45 cartridge if you what the round to function in traditionally 5.56mm platforms. At best you'll buy yourself slightly better support for the bullet will is not exactly necessary.

With pressures of 62,300 psi to work with the 7.62x37.5mm cartridge will drive a 125 grain Ballistic Tip at about 2500 fps (20") or a 210 grain Berger VLD at about 1500~1550 fps. 1 in 7 twist will probably work well for the heavier bullets. This will probably over stabilize the light ones somewhat, but not to the point where a Vmax blows up mid-flight.

Realistically, a typical military load for the 7.62x37.5mm will be the 127 grain M993 bullet currently used in the black tip 7.62x51 rounds at ~2300 fps, a 150 Grain M80 Ball at ~2000 fps or a 168 gr matchking HPBT at ~1900 fps. from a 14.5" carbine. This is basically 7.62x39 velocity and energy. For suppressed applications Berger 210 Grain VLD or 220 Grain Matchking driven to about 1500 fps will work pretty well if you don't mind a little sonic crack. Or you can go long and heavy with a 240 grain Matchking at 1025 fps to stay safely under the sound barrier. That's only about 150 fps better than a .45ACP 240 grain load, but with Ballistic Coefficients almost touching 0.70 which is as good as it gets.


Edited by dwightlooi (12/22/10 12:24 AM)

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