The Strength of the Gibbs Lee-Enfield Summit .45-70 Rifle
By Ron Card
The WW2 and post WW2 Lee-Enfield Mk 4 - No 1 (and No. 2) receivers used by the Gibbs Rifle Company in making their fine Summit carbine in caliber 45-70, are sufficiently strong enough to allow the use of the hottest .45-70 loads listed in the popular reloading manuals. These are loads reserved for strong single shot and bolt action rifles. Let's examine the historical facts and the figures.
SAAMI specs for the maximum average pressure (MAP) of the .303 British round is 45,000 cup. Today's commercial .303 ammo is purposely kept below the permissible MAP due to the large number of surplus SMLEs in the hands of shooters, many of which are of questionable condition. By comparison, SAAMI specs for the MAP of the .308 Winchester (7.62mm NATO) round is 52,000 cup.
In the early 1950's the UK, in a preliminary effort to use the then new 7.62 NATO cartridge, reworked a number of WW II No. 4 SMLEs to accept the round. An aside, as NATO solidified its cartridge standardization requirements, the Brits adopted the FAL, L1-A1 rifle. The 7.62 NATO SMLEs were reconfigured and designated L39A1 for competition shooting and L42A1 for sniping. After considerable use, no problems were reported as to receiver strength or safety.
In the 1960's, the Indian government, desiring a police rifle that used the same 7.62 NATO round as its newly adopted FAL military rifle, used existing arsenal tooling to manufacture No. 1-MK III SMLEs with improved metallurgy and chambered for the NATO round. The No. 4 receiver is somewhat stronger than the No. 1, so the Indians increased the strength of the steel used in their No. 1-Mk III's to compensate. Historically, these SMLEs were the final evolution in a long and colorful line of Lee-Enfield rifles. These law enforcement rifles, made from 1962 to 1967, were designated 2A and 2A1 rifles. Again, there were no reports of receiver strength problems with the Indian SMLE in 7.62 NATO.
At present, Hornady offers a .303 British Light Magnum load that ballistically duplicates the .308 Winchester round. Hornady does not exclude SMLE rifles from use with this cartridge.
Three American cartridge companies, Cor-Bon, Buffalo Bore and Garrett, all load very hot, commercially available .45-70 ammo for big game use. These companies limit cartridge pressure to 35,000 cup, due mainly to fall safely within the Marlin lever gun's receiver strength limitation, which is around 44,000 cup. One cannot expect a lever gun to be as strong as, say, a Model 98 Mauser action or a modern falling block rifle.
SAAMI MAP specs for the .45-70 are set at modest 28,000 cup due to the number of ancient Trapdoor Springfield .45-70 rifles in circulation. Ammo manufacturers like Remington, Winchester and Federal simply don't want lawsuits, so standard commercial .45-70 factory ammunition is actually loaded to about 21,000 cup.
However, the 45-70 can realize its true potential with careful handloading. The round can be loaded to pressures in excess of 40,000 cup, but strictly for use in strong bolt-action and falling block rifles.
With all of this in mind, the Gibbs Lee-Enfield based Summit rifle should have adequate strength to handle any .45-70 round authorized by the major reloading manuals for use in strong rifles. Again, the hesitation on the part of Gibbs and a smattering of gun writers to recommend the use of high-pressure loads in the Summit is the cautious legal approach. They don't come right out and say not to use the hot stuff, but rather to exercise caution. Get the picture?
Now, for the good news! When I get a new rifle--like my Gibbs Summit--I start wondering how to improve on it and make it more powerful, if possible. Of interest is the fact that the Summit uses a reworked, removable, proprietary magazine very similar to the original SMLE .303/.308 magazine. The opening dimension is 3.1". Note that a typical commercial .45-70 cartridge factory loaded with a 300 grain JHP bullet actually measures about 2.6" long. When a cartridge of this length is inserted in the Gibbs magazine, there is empty space in front of the bullet. .303 British cartridges have a maximum cartridge overall length of 3.075". Thus, there is available in the Gibbs mag nearly a half inch of potential additional cartridge space beyond the normal length of a factory loaded .45-70, and to my way of thinking, this "free, but wasted" space screams to be used!
There is enough room in the Summit magazine for a cartridge of 3" to maybe 3.05" in total length. How can this be put to good use? Enter the .45-90 WCF of yore. The .45-90 is dimensionally the same as the .45-70, only stretched an additional .3", to a 2.4" case length instead of the 2.105" case length of the .45-70. More powder, more power. Voila! This idea is similar to the several, dual-use cartridge/gun combinations, like the .45 Colt to .454 Casull, etc.
All that is needed for the .45-70 to .45-90 conversion is a chamber conversion reamer. Most of the chamber depth is already there, so a hand-reamer can get the job done, but this is best left to a competent gunsmith for safety's sake. The .45-90 Gibbs conversion well achieves its true maximum potential, and has a significant ballistic edge over the .45-70 when using short bullets like the common 300 grain JHP's or the Hornady Interlock 350 grain RN.
Depending on bullet and powder selection, this can translate to a whopping 200 to 300 fps and 500 to 600 ft. lbs. advantage, without excessive pressure, over the hottest .45-70 loads. That is literally nipping at the .458 Win. Magnums heels.
Long bullets, such as the 350 grain and heavier spitzer form Barnes X-Bullets, some Barnes Solids and Barnes Originals, and various 500 grain bullets, can be seated to a COL of 3" to 3.05" in regular .45-70 cases with plenty of bullet shank still left inside of the case. With such bullets the .45-90 case offers no increase in powder capacity. The beauty of the conversion to .45-90 is that .45-70s can also safely be used.
With the Summit, we have an affordable ($385.00), and quite handy, sport-utility carbine that would be equally at home in the Alaskan wilderness to the African plains. Lever-guns, look out! Here comes the Summit!
Copyright 2003, 2016 by Ron Card and/or chuckhawks.com. All rights reserved.