Cartridges: 7.62 NATO Long Range Match Cartridges - Part 1

We are very pleased to bring you this three-part series regarding the development of the 7.62 NATO cartridge for long range match shooting. In a departure from our usual practice, there is significant load data contained in this series. The government arsenals and military teams using and developing these loads had pressure testing equipment, plenty of expendable rifles and plenty of replaceable recruits that they loved a bit less than their mothers did. Just about every load mentioned here other than the basic arsenal match loads will generate excessive pressure and cause damage to your rifle and can certainly injure the shooter. Read and enjoy this for its historical value but please have the common sense to realize that duplicating these loads today with canister grade powders is dangerous and should not be attempted. - GAS -

7.62 NATO Long Range Match Cartridges
By Frederick Salberta
PART 1

This article seeks to briefly detail the 7.62 NATO history as a long-range U.S. military match cartridge and the nominal load details. Adopted in 1955, it was not until well after the 1963 NM M14 rifle adoption that the 7.62 NATO began to compete against the .30-06 as the primary US military long range service rifle match cartridge. With the resurgence of the Palma matches in the late 1960s it began to supersede the .30-06, a status it enjoys to the present day. As recently as 2009, a new variant of the 7.62 NATO intended for long range sniper and match use was adopted; followed in 2010 with a new Army Marksmanship Unit load suitable for 1000 yards match use with the AR-10 NRA legal “service rifle.”

The original 7.62 NATO M80 ball cartridge adopted in 1955 featured a 148 gr. bullet with a muzzle velocity of 2,750 fps measured at 78 feet, which corresponded to an actual muzzle velocity of 2,805 fps out of a 22 inch M14 barrel. This was achieved with a nominal load of 46 grains of WC846 for Winchester-Western loads, or 44.1 grains of IMR 4475 in loads assembled by Remington. The M80 ball cartridge was essentially a mildly improved ballistic match to the old M2 .30-06 ball load of the WWII era. The bullet design was superior to the M2 for long range shooting due to the M80’s boat-tail design. However, M80 was by no means an ideal match cartridge due to the relatively low level of test accuracy: between 3.7″ and 5″ measured mean radius (MR) at 600 yards being the norm. This load would typically go subsonic at around 850 yards, and was not an ideal cartridge for use at distances in excess of 750 yards out of an M14 due to bullet turbulence when making the transition from super-sonic to sub-sonic speeds, an event that occurs at 1117 fps at sea level in standard conditions.

As a comparative reference, the 1962 LC Match M72 cartridge (.30-06) had a 2.1″ MR at 600 yards and was supersonic past the 1000 yard mark; its 173 gr. boat-tail bullet at 2685 fps muzzle velocity retained 1294 fps (1.164 Mach) at that distance. A useful rule of thumb is that total group size tends to be roughly three times the MR; although MR is a more useful measurement it is less frequently used by individual testers as group size is simpler to measure.

Although not well understood by many, at around 100 fps above the speed of sound, there is sufficient turbulence that some bullet designs have their subsonic accuracy seriously compromised when they fall to below this speed. Many well designed bullets fired from an appropriate twist barrel will have minimal transonic perturbations, though there is generally some degradation associated with making the transition from the sonic to subsonic region. Through many years of testing, the US Army has decided that the threshold above which this effect can be ignored is 1226 fps in standard atmosphere conditions. Ideally any load will be above this speed at the terminal target distance.

In the case of the M80 ball and European equivalent loads, maintaining this velocity past 780 yards is difficult when fired from a service rifle with its relatively short barrel. M80 ball does not generally make the transition to subsonic velocities gracefully and it typically shows a fan pattern beyond 800 yards out of US service rifles. The design of the L1A2 and SS77 projectiles (M80 equivalents) when fired out of a 1:12″ twist barrel generally are well behaved in the transitional region, as long as high cross winds are not present (high winds complicate the stability of the bullet as it passes the transonic region). While not the ideal long range cartridge, the L1A2 or equivalent was used by the Commonwealth shooters (UK, Canada, New Zealand, Australia) between 1963 and 1995 as the long range target cartridge out to 1000 yards, only being replaced by a 155 gr. projectile in 1996. Because the US teams had to use this cartridge in the Palma match when that match was held abroad, it was used by the US shooters in Palma competition in 1967, 1969, 1970, 1972, 1974, 1979, 1982, 1985 and last in 1988.

Because this was the cartridge for LR competition in the commonwealth, certain efforts were made to assure that the performance of the normal ball cartridge was superior to its US counterpart. The Canadian version for example, used an extruded powder close to IMR 8208M, which gave more consistent muzzle velocity variations and hence better long range vertical dispersion. Australian projectiles were known for their consistent 0.308″ to 0.3082″ bullet diameter, while Canadian and British projectiles tended to run a bit smaller, between 0.3075″ and 0.3079″. As the 7.62 target rifle prior to 1968 was the Lee Enfield No.4 with a 25.2″ barrel and after 1969 the L39/Envoy variant with a 27.6″ barrel and a very tight bore, the velocities achieved on the British version of the M80 (L1A2) cartridge were a bit higher, on the order of 2,850 fps. As time went along and match barrel lengths approached 29″, muzzle velocities approached 2,900 fps.

Few specific true match loading of this cartridge were produced; the Commonwealth approach being to designate a series of the first production from cartridge lines after a die refit as “green spot”, suitable for match use, if the test accuracy merited it. When the lines were refitted, the first few hundred thousand cartridges loaded gave superior accuracy. By this method very consistent ammunition was available, though in general the best accuracy one could expect with British ammunition was MR of 2.9″ to 3.3″ at 600 yards, which corresponded to an expected 10 shot group at that distance of just under 2 MOA. Some lots of Canadian and Australian ammunition grouped better, though few lots would provide 15 shot groups of consistently better than 1.5 to 1.7 MOA past 600 yards, even in the best match rifles optimized for the L1A2 and equivalent Commonwealth ball of the era. Some lots used at the club level were far worse.

In the United States with the adoption of the M14 as a national match rifle in 1963, there arose a need for a longer range version of the 7.62 NATO, which would allow for greater accuracy up to 1000 yards. M80 ball proved to not be suitable due to the bullet construction. The US chose to adopt a new cartridge for sniper and long range match use. The obvious solution was to place a 174 grain M72 projectile on top of a suitable quantity of WC846 powder. This was first tested in 1963 and designated as XM118 cartridge. In testing during 1963, it became apparent that the accuracy of this load was less than desired, which resulted in a change of the powder to IMR4895, the same powder as used by the M72 .30-06 match cartridge adopted in 1957. Target group results over 500 yards were much better with the IMR powder.

The final M118 7.62 NATO cartridge, adopted in 1964, utilized a 174gr FMJ Lake City projectile over approximately 42 grains of IMR4895 in a Lake City case and primer. This load gave a nominal velocity of 2,550 fps at 78 ft, which correlates to approximately 2,600 fps at the muzzle of the 22 inch M14 barrel. This load showed better accuracy then the best lots of M80 ball, with considerably less wind drift. Such a load will be supersonic to just over 990 yards under adverse conditions. Under ideal conditions (high temp, low humidity and low atmospheric pressure) the load was supersonic to 1000 yards when fired out of an M14. In either case this bullet, if from a good lot, handled the transition from supersonic to subsonic flight well in either 1:12″ twist or 1:10″ twist barrels, even under high wind conditions.

Between 1964 and 1967 when the National Matches were receiving full federal support, the National Match M118 lots were considered some of the very best long range loads available. This was due to the high care used in assembly and selection of bullets out of the many lots manufactured for match and sniper use. Bullets for the national match yearly production were selected from those that showed superior accuracy during normal acceptance test firing. In addition the cases were specially selected, either by being from a single case line, or from multiple machines that had recently had the forming dies replaced. Those lots intended for national match use were headstamped “NM”, while those lots fielded for regular matches and sniper use were headstamped “Match”. The 1966 and 1967 NM loads showed 600 yard MR of 1.76″ and 1.73″ respectively, which corresponds to expected equivalent 1 MOA 10-shot groups at 600 yards. The 1967 NM ammunition was so good it was utilized for the 1966 and 1971 US Palma matches, by which time most of this excellent ammunition was expended.

After the final fully government supported National Matches of 1967, no further NM lots of M118 were produced. Because the cartridge was now only loaded to the nominal match specification (MR of under 3.5″ at 600 yards), the powder was changed back to WC846 after 1968, between lots # 12074 and 12078. The nominal load for the M118 loaded with WC 846 was 44.0 grains. In addition the cases were no longer specially selected; cases used from 1968 through 1979 were taken from the regular production line, the only difference being the lack of a primer crimp and the different headstamp. After 1968 the quality of the lots gradually dipped close to the 3.5″ MR at 600 yard requirement, which would result in expected 10 shot groups of between 9.5″ and 11″ at 600 yards.

Such ammunition was not particularly suitable for use on the post 1966 10X target at 600 yards with its 2 MOA 10 ring. Service teams could make suitable across the course ammunition by substituting Sierra 168 gr. HPBT bullets for the 174 gr. arsenal bullet over the standard M118 powder charge. This load would effectively cut the groups in half, although because of the 13 degree boat tail bullet on the Sierra 168 gr. bullet, such a load was not suitable for use past 800 yards. All of the service teams sought additional long range advantages. The result was the development of a series of 1000 yard service team hand loads in the post 1967 era.

Copyright 2012, F.R. Salberta, used by permission.

Part 2 of this series will pick up with the development of the service team loaded cartridges.

CLICK HERE FOR PART 2

We are very pleased to bring you this three-part series regarding the development of the 7.62 NATO cartridge for long range match shooting. In a departure from our usual practice, there is significant load data contained in this series. The government arsenals and military teams using and developing these loads had pressure testing equipment, plenty of expendable rifles and plenty of replaceable recruits that they loved a bit less than their mothers did. Just about every load mentioned here other than the basic arsenal match loads will generate excessive pressure and cause damage to your rifle and can certainly injure the shooter. Read and enjoy this for its historical value but please have the common sense to realize that duplicating these loads today with canister grade powders is dangerous and should not be attempted. - GAS -
 

All contents Copyright 2012 The Rifleman's Journal