Based on what I understand, I think the most probable explanation is that your doubles are due to bump firing.
Both the M1a and Garand kick and if you do not have a firm hold on the things they will move enough that your trigger finger will bump the trigger, even though you did not expect to.
I always shoot my standing rounds with one round in the chamber and thus have never had the opportunity to see if the rifle would double. Standing offhand is not the most braced position, as you are trying to your utmost to precisely pull the trigger at the right time without disturbing your sight picture.
As for slung positions. With Garands/M1a’s you must always ensure that the sling is tight. You should have to put your hand on the buttplate and push the rifle forward as you move it into your shoulder. Lesser rifles can be held loosely but these full power service rifles have to be slung tightly or they will move during recoil and a bump fire is possible.
Most unintentional bump fires with 30 caliber rifles occur on the bench. Shooters have these things on sandbags and a rest, loose in the shoulder, the rifle recoils back, goes forward, and the trigger finger bumps the trigger.
I do not consider this safe at all as you could loose control of your weapon and the hammer is following the bolt down. The only thing preventing an out of battery slamfire is the hammer nose and bolt notch interlock. Garand designed a mechanical interlock to prevent the hammer from touching the firing pin until the firing pin has cleared the bridge notch. The hammer nose will rotate the bolt to an inbattery position before the hammer can touch the firing pin.
Hammer nose notch to left of firing pin.
But the firing pin is able to go fully forward well before the lugs are fully engaged, the pencil mark shows just when the firing pin clears the receiver bridge notch.
Safety devices can fail, mechanical interlocks are not necessarily 100% positive , I would not trust this interlock time and again from to prevent an out of battery slamfire and if the rifle only goes off in battery, there is the very real possibility that the lugs are not fully supported.
My advice for reloading for Garands/M1a’s is to
1. Full length resize in a small base die
2. Trim cases
3. Clean primer pockets, ream to depth
4. Prime all cases by hand, verify that all primers are below the case head, and use the least sensitive primers you can find.
5. Use IMR4895/AA2495/H4895 powders.
6. Seat the bullets to magazine depth, no longer than 3.3” inches for the 30-06, no longer than 2.8 for the 308, shorter is fine.
The M1a has a free floating firing pin, like this M1 carbine. The M1 Carbine, M1 Garand and M1a have a firing pin retraction cam, often called the "receiver bridge". This cam pulls the firing pin back during extraction. It also has limited utility as a safety device, but is easily defeated by tight or long rounds. If you sized your round smaller than the chamber the odds would will be in your favor to have an in battery slamfire.
Due to the longer and heavier firing pin in the Garand, it has always had more inbattery and out of battery slamfires than the M14/M1a. The primer sensitivity specifications were the same, logic, based on physics, provides the understanding that given KE = M * V**2 that the greater the mass, the greater the kinetic impact energy on the primer. I have not weighed the M1 carbine firing pin, but it may be the heaviest of them all. Examining M1 carbine primer specifications, the all fire limits were inches above the large rifle primers, so carbine primers are even less sensitive than the primers used in Garands/M14's.
Only at final cam down is there any firing pin retraction and the firing pin is never fully retracted behind the bolt face. For almost three inches of travel the firing pin is totally free floating and tapping the heck out of the primer. Static pictures give a false impression of a smooth and linear movement. Based on reports of people who have seen high speed camera images, the actual movement is not smooth and there is bouncing and back and forth oscillations. I believe the dynamics of firing pin movement to be chaotic and given the right conditions, it will rebound hard off the primer before bolt lock. Given a sensitive enough primer you will have primer ignition.
This is a M1 Carbine firing pin retraction cam.
The M1 Garand/M1a firing pin retraction cam are functionally identical, just the carbine is easier to visually understand.
This is a M1 Garand receiver and the firing pin is fully forward and just touching the firing pin retraction cam. As you can see there is only thousand's of an inch of forward movement left in bolt cam down and yet the firing pin is out about 0.064" of the bolt face.
This is the location where out of battery slamfires occur.
If the bolt has to stop here to crunch fit a long case or a fat case that firing pin is rebounding off the back of the primer at its highest velocity and the lugs are not engaged.
That is why it is important to small base size cases used in these rifles and to set up the dies with a case gage and size to gage minimum. (Assuming you don’t know the headspace of your chamber, if you do, always be 0.002 to 0.003” less) You want the bolt to close without resistance. This will reduce the risk of an out of battery slamfire.
There are some who say small base dies are not needed in these rifles, the Gunwriter Mike Venturino has been one. For years he has been saying in print that only standard sizing dies are need. But in the July 2012 issue of Guns Magazine, he is testing an M1a and a AR10 and his reloads are too tight. I find it humorous to read of him beating the bolts open with scrap lumber. Ha, Ha.
If you attempt to small base size with a spray on lube you will stick the case in the die. I recommend RCBS water soluble or Imperial Sizing wax. These are excellent lubes.
For these rifles it is safety critical to ensure that all primers are below the case head. Reaming primer pockets to depth is a good idea, seat the primers by hand, and verify that all of the primers are below the primer pocket. There is a chance that a cocked primer, with the anvil firmly seated on something, will cause a primer initiated slamfire. One poster swaged his primer pockets, which shaved brass donuts into the pocket. He left the donuts in the pocket, which resulted in high primers, and his AR10 slamfired in battery. Clean those pockets! A high primer can cause a slamfire but only if the anvil is firmly seated. It actually turns out that high primers are the most common cause of misfires because the primer won't fire unless the anvil is seated and is pushed up into the primer cake.
http://www.shootingtimes.com/2011/01...motaip_200909/ However, given a shallow pocket it is theoretically possible that high primers could slamfire, given debris in the pocket, you can get a slamfire.
Mr Faatz stuffed an extra anvil in his primer pockets as he was unable to get unsupported high primers to ignite. Reference A.
Just examine the back of the ammunition you have and see if there are high or cocked primers.
It is also safety critical to use the least sensitive primer around because these rifles will slamfire in battery or worse, out of battery, given a sensitive enough primer.
Federal primers are the most sensitive primer on the market and the most "slamfiring" primer in M1a’s and Garands. I have lots of web accounts of slamfires with Federal primers. Don’t use them. I recommend CCI #34's and Tula7.62 primers as they considered "Mil Spec" primers. Which means they are less sensitive than commercial primers. The military chose the characteristics of its primers after careful analysis of test data. The military spec primers are the appropriate primers to use in these mechanisms because they reduce, but do not eliminate, the risk of an slamfire. These primers are insensitive enough that the risk of a slamfire is in the magnitude of 1:10 million and yet the mechanism has enough energy to reliably ignite these primers at -40F. The chance of slamfires go up the more sensitive the primer.
When firing single shot, always load from the magazine. Do not put a round in the chamber and drop the bolt. Lots of inbattery slamfires, and a few out of battery, have happened because of this. You want to slow the bolt down. When rounds feed from the clip the friction between cartridges slows the bolt a bit. Firing pin initiated slamfires are all about primer sensitivity, you want to reduce the kinetic energy of the firing pin as it rebounds off the primer.