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Thread: M1 Garand and M1A gas systems

  1. #1
    Super Moderator

    Buckshot's Avatar
    Join Date
    Mar 2005
    So. California

    M1 Garand and M1A gas systems

    .............Help me get a handle on this. I understand the issues with the gas system of the Garand. Port is close to the muzzle, the op rod is long (with a couple slight bends) so is 'Flexy (flexie?) and much easier to have issues. On the other side with the M1A, the gas port is MUCH closer to the chamber (subject to higher pressure) but there isn't much of an op rod to bend.

    So I suspect that the M1A is much more robust as regards pressure, is the M1A suitable for use with most any factory loaded ammo? Would there be a restriction on bullet weight in said ammo? I just got a Springfield Armory 'stainless loaded' model M1A, and just want to check what knowledgeable users might have to say.

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  2. #2
    Boolit Master
    Join Date
    Sep 2016
    Couple things regarding these rifles. Powder selection was always 4895 or a powder with the same burn rate. Port pressure was 8000 +- 2000 psi. usually 168 matcj kings and 173 grain m72 match ammunition for either matches some was marked Match or NM national match. Velocities were about 2600 or thereabouts. The M1A was a different system but gas operated. Powder was 4895 or 4064 as far as I know. Ammo was 150 grain ball, 168 grain for both match and national match. Then they came out with the special ball loading 168 grain and then much later 175 grain match king bullets. What the latter were loaded with I really don't know. And do not know what the acceptable port pressures should be. I believe the 7.62 nato ammunition was loaded at about 55 psi commercial 308 is loaded hotter. The thing that I was always told is load military cases in either M1 Garand and the same for the M1A as the military cases are designed heavier and held to mil spec for that caliber. Load a commercial 308 in a M1A and you may rip the rim off the case. And since I mentioned that commercial is loaded hotter possible damage to the gas system. The 1960's were good years for regular ammo in 30-06 and 7.62 Nato. Quality went downhill in the 70's. Most 7.62 ammo usually were fired in two weapons. Either a machine gun or bolt action or semi auto firearm. Bolt gun fired cases idealy are the ones you should seek out. Semi auto next best and MG cases last resort. Now here it gets confusing, since before 9/11 the US has been outsourcing their 7.62 from Israel IMI or Tzz headstamped good brass.Then you have Santa Barbra I no nothing about this ammo. Think PMC made in Korea makes it for the US but not sure. There also may be others I'm not aware of. In starting with reloading ammo I'd stick with 4895 which you cannot go wrong with. Frank

  3. #3
    Boolit Master

    nicholst55's Avatar
    Join Date
    Jan 2007
    Daegu, Korea
    Not all factory ammo is suitable for the M1A, and for the same reasons as with the M1 Garand - gas port pressure. There are too many variables to go into here, but anything that is called 'light magnum,' or anything indicating that it is loaded to higher pressures or with slower propellant is a No-Go. Although many, many 190 grain match bullets were fired through the M1A over the years, they require alterations to the gas system. Stick to 180 grains or less with your M1A. I would suggest joining one of the firearm-specific forums that are related to the rifle for reloading advice for the M1A, such as (but certainly not limited to)
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  4. #4
    Boolit Master
    Join Date
    Aug 2006
    Also check out the CMP forum. There are a lot of M1A and Garand shooters there.

    Also, with the Garand, an adjustable gas port is available and properly adjusted allows use of any commercial ammo.

  5. #5
    Boolit Master

    Larry Gibson's Avatar
    Join Date
    Jul 2005
    Lake Havasu City, Arizona
    Your M1A has a completely different gas system than the M1 Rifle.

    The M1A gas system is designed to and will cut off gas from the gas port as soon as the gas piston starts moving rearward. The amount of pressure at the gas port required for function is quite low. Using a medium burning powder such a 4895 will give reliable functioning from loads with chamber pressures of 20,000 psi upwards of 62,000 psi.

    M118 Match with the 174 gr FMJ has been loaded with ball powders (WCCs) and extruded powder (mostly IMR4895).
    M118SB was loaded with ball powder (WCC).
    M852 with the 168 MK was loaded with IMR4895.
    M118LR with the 175 MK has been loaded with IMR4064, RL15 and IMR4895.

    M61 AP, M59 and M80 Ball and the M62 Tracer have always been loaded with ball powder (WCC).

    As mentioned you'll want to stay away from the "Light Magnum" or "Extended Range" type of cartridges and not use slower burning powders at 7,62 or 308W level MAPs. The reason is not because the action or gas system can not handle the pressures (within .308W MAP level) but because the time/pressure curve is too slow for proper functioning. With a proper time/pressure curve the chamber pressure will drop allowing the case to contract and release it's grip on the chamber walls as the bolt begins opening. Too slow a time/pressure curve and the case is still gripping the chamber when primary extraction starts. that can cause damage to the oprod cam surface, bend or break the oprod, damage the bolt or cause the extractor to pop out.

    M118SB and M118LR are loaded to 61,000 to 64,000 psi ( have measured the pressures not guestimated) but have a time/pressure curve still suitable for the M14/M1A gas system. That is the ammo that has been used in the M14 SDMR, M24 and M40 sniper rifles the last 15+ years.

    Since 1972 I have used H4895, AA2230 and H335 powders with quite successful loads in M14s and M1As with jacketed bullets weighing 100 to 180 gr. With cast bullets I favor 4895 (any flavor) with a Dacron filler under 164 to 210 gr at 1850 to 2400 fps (depending on barrel twist).
    Larry Gibson

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  6. #6
    Boolit Master

    Join Date
    Mar 2013
    Northwest Ohio
    The garands system the gas pushes the full stroke of the op rod and dosnt bleed off till the rifle is ready to cycle forward. The M14 M1a system get a short push from port into piston which Hits the op and and travels with it about .400 then inertia does the rest. Gas is bleed off much faster in this system Garand is a long stroke gas system and M14 M1a is short stroke. The M14 M1a also has a on off valve as part of the gas system for heavy rounds or high pressure applications.

  7. #7
    Boolit Master

    shdwlkr's Avatar
    Join Date
    May 2006
    moved to Idaho
    from springfield armory owners manual
    The M1A is designed and built to specifications to shoot standard factory .308 made to SAAMI specifications or 7.62x51 NATO ammunition.
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  8. #8
    Boolit Master

    NuJudge's Avatar
    Join Date
    May 2005
    SE Michigan
    Also be advised that the M1A gas piston and plug need regular reaming to remove carbon fouling. I bought my M1A in 1978, and the National Guard competitors that taught me to shoot Service Rifle, taught me to use an eyedropper and put several drops of Hoppe's #9 in the gas cylinder's vent, but that is not enough. Unscrewing the gas plug puts enough force on the gas cylinder to loosen it on the barrel, if you don't use a gas cylinder wrench that fits over the gas cylinder. The commercially available reamers for the plug and piston are just drill bits in a holder. There is a writer by the name of Zediker who has written a lot on loading for and shooting the M1A, and some of his work is available on-line for free:

  9. #9
    Boolit Master
    Join Date
    Jan 2010
    Space Coast, FL
    Anti-seize nicely placed on the threads of gas plug/screw threads on the M1A or M1 will make ones life more enjoyable. The external wrenches for both cylinders help remove all stress from spline or pin. I would imagine Mr. Buckshot could make as nice or better than commercial wrench.

  10. #10
    Boolit Bub
    Join Date
    Jun 2011
    Surrounded in Central California
    One can also do a M25 Gas Cylinder Vent mod to an M1A/M14 to improve reliability and gas cylinder life. Nothing but a .0625" hole drilled near the bottom end of the gas cylinder. Allows the system to "pump" out carbon and soot.
    I've done this to all of mine(4).
    Last edited by Sailormilan2; 08-08-2018 at 09:16 AM.

  11. #11
    Boolit Master

    Join Date
    Mar 2013
    Northwest Ohio
    Zedicker was a national level competitor with the M1A. He had a lot of good advice and Ideas,

    A lot vented the gas plugs with a .060 hole. Some got fancy and drilled a .070 thru hole and a blind 10-32 tapped hole inside. then drilled holes from .030 - .065 in .005 increments allowing the rifles gas system to be tuned. Several made ported plugs for the garand or found the early plugs with the valve in them.

    There are 2 holes in the plug than need cleaned the one in the main body is a letter o drill (Close to 5/16) and the smaller on in the stem ( I don't remember the size now). As the carbon builds up the rifles zeros can change and functioning suffer. The gas cylinder wrench is a big plus when doing the cleaning. I made my own from a piece of 3/8" thick aluminum. The + wrench for the garands plug is a plus also.

    With the appropriate GI tool each rifle can be maintained easily. They even fit in the buttstock trap holes. The M14 tool can be used as a screw driver, chamber brush ( this brush can usually pull a case head separation ), Change extractor ejector and or firing pin. There was one for the M14 M1A and the garand.

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check