Thursday, March 19, 2009

Praxis: Will loaded USGI magazine springs "take a set"?

Over at the M14 side of what used to be, John Wagner asks a question. It is one that I have been pestered unto death to answer by many newbies, so I present it here.

I know you can safely store AR15 mags full without ruining the springs. Is this also true of M14 USGI mags? I'd like to keep a couple of them full for unexpected SHTF needs, but not if it's going to destroy the springs.

"Different", who has his own M14 website here, answered as follows.

The government Springfield Armory did a study on long term storage of magazines: Springfield Armory. SA-TR11-2643 Evaluation of Pretreatment Processes And Long-Term Storage On Magazine Spring For The M14, 7.62MM, Rifle. Springfield, MA: February 01, 1966.

From M14 Rifle History and Development Third Edition:

"The question often arises as to whether or not a compression spring will take a permanent set if compressed to the minimum length and held over time, e.g., a fully loaded magazine or the bolt is held open. Coil springs can be designed to compress to solid height or length without taking a permanent set. The solid height of a compression spring is the length of the spring when fully compressed. If a permanent set is not desired, the spring material and diameter is chosen so that the torsional stress when compressed solid does not exceed approximately 40 % of the material minimum tensile strength. The minimum tensile strength, or yield strength, will vary with the diameter of the wire, e.g., 231,000 to 399,000 psi for ASTM A228 music wire. A permanent set occurs when the compression spring is compressed beyond its elastic limit and does not return to the original length. This results in a shorter free length but more significantly, lower spring force.

M14 magazine springs were tested and inspected to ensure they would not take a permanent set. Magazine springs selected for inspection were compressed to a height of 11/16 ", essentially compressed solid height, three times then examined for compliance with USGI drawing C7267078. This included meeting the free length requirement of 13 " – 2 ". A permanent set in a USGI M14 rifle compression spring is not formed when compressed to the minimum length and left indefinitely."


"Springfield Armory performed an evaluation of M14 magazine springs in the 1960s. The effects of long-term storage and repeated cycling of magazine spring were studied. The specified load (spring force) for the USGI M14 magazine spring is 5.5 pounds + 0.75 pounds at a compressed length of 5.5 " (unloaded magazine). However, the Springfield Armory tests found that the M14 magazine spring will perform satisfactorily at a load as low as 4.5 pounds force at 5.5 " length.

In the Springfield Armory study, three USGI M14 magazine springs were placed into a vertical shaper for a gymnastication test. Each spring was cycled at a rate of 116 strokes per minute. The spring force was measured after so many cycles, 5, 55, 655, 1,655, etc. After 6,655 cycles, the spring force at 5.5 " long were 6.1, 6.1 and 5.75 pounds each. The load, or force, of each spring was checked again after 10,000 cycles. The results were 5.1, 5.75 and 3.9 pounds at 5.5 " length. One spring measured 5.25 pounds at 5.5 " length even after 12,000 cycles. The springs were found badly distorted at 10,000, 12,000 and 14,751 cycles, respectively.

In another part of the spring evaluation, ten magazines were stored loaded for five years. After the first week in storage, the magazine spring force was found to range from 5.1 to 5.6 pounds at 5.5 " length. After five years of loaded storage, the same ten magazines were test fired with six loadings (120 rounds per magazine). The magazines were then disassembled and the spring force measured. The results were 4.6 to 4.75 pounds for length of 5.5 ". There was no malfunction of any magazine.

Unloaded magazines were tested as well for the effects of long-term storage. Ten magazines were stored unloaded for five years. After the first week in storage, the force for each spring ranged from 8.3 to 8.8 pounds at 5.5 ". After five years, the same ten magazines were loaded and fired six times each (120 rounds per magazine). The force for each spring was then measured. The results ranged from 5.0 to 5.25 pounds at 5.5 " length. There was no malfunction of any magazine."
Different's M1A Site
Salus Populi Suprema Lex Esto

OK, are we clear? The short answer is "NO!"


Texasred said...

Thanks for the info.

We were told in the Army to only load 18 rounds in our 20 round mags. I don't know if this was an old wives tale passed down from sargent to sargent or if it was true but I did as I was told. I just purchased a new M1A NM and 15 20 round mags. All mags are now loaded and in the SHTF bag.

Anonymous said...

The 18 rounds in a 20 round mag, or 28 in a 30, is to ensure that mags will be inserted completely into the M16. However, I haven't found that to be a problem, contrary to what folks like James Yeager says.