Firearm User Network

Ultimately the trials were won by the Garand, with the G30M placing third in total malfunctions and broken parts. This had involved 37 different tests and more than 12,000 rounds through each rifle. The Garand had 1,480 total malfunctions and 49 parts broken, replaced, or repaired. The Johnson had 1,547 and 72 respectively, and the G30M 2,864 and 97 (roughly double the number of problems as the Garand).

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USMC Rifle Test 1940

This table shows (assuming 12,000 rounds as the denominator, which is close enough because our purpose here is comparison) that as reliable as those rifles were for their day, they were pretty buggy by today’s standards. Looking at the percentages really makes the data pop.

Assuming a “malfunction” equals a stoppage, we’ll label those percentage of stoppages and we’ll label the parts breakages as “failures.”

This was good reliability in the 1940s!

Compare this to the “unreliable” M4. In the worst M4 test ever, the notorious and outlying 2007 Extreme Dust Test (here and here), ten M4s fired 6,000 rounds per rifle with 1.4% stoppages.

And this number was over 4x the number of failures in an earlier iteration of the same test, a result the Army Research Lab has never replicated or explained. Now we can’t compare the 1940 and 2007 tests directly, however, the data indicates the M4 is nearly ten times more reliable than the M1. Even if you don’t like these numbers, any apples-to-apples test would show the M4 is significantly more reliable.

Also, the AR-15/AR-10/M16/M4 is not direct impingement, it is an internal piston design, as formally described in the patent.
http://www.google.com/patents/US2951424

Reliability Under Harsh Conditions