Yeah, the early gas trap Garand troubles gave M.M. Johnson some impetus to get his rifle ready. It was pretty apparent, even to him, that the Army was committed to the Garand but nobody really knew for certain if they could get it all worked out in time for the war we would be in sooner or later. Johnson wanted to have an alternative ready and waiting.
From reading Canfield's book, based largely on Johnon's notes and in-house correspondence, the goal was to provide an option to the Army if needed, and maybe, just maybe, get the Marines to alter their practice of adopting the Army's rifle.
In hopes of selling it as a quick solution in the event of Garand trouble, Johnson did a lot of research into mass producing the rifle. The design was meant to be made quickly on the machinery of the day. He consulted manufacturers of all types about the feasability of manufacturing the rifle and some of these factory owners and reps testified at later Senate hearings. A wheel manufacturer in Detriot said they could make 300-400 per week after a short setup time. An automobile horn making factory said they could enter production within six months and make 1,000 per month.
The reason he didn't go to arms makers was because they were busy with military contracts. That made the production figures even more impressive to me. He also went to makers of knitting needles and to small metal shops.
When he himself had to give in and build them himself, he bought part of an old factory that made machines for manufacturing textiles. Even with no manufacturing experience and using a facility that had never made anything of the kind, he had the first rifle off the line in five months.
BTW- He never really wanted to make the rifles himself and had zero manufacturing experience (he was an attormey by trade, who basically let his fascination for arms get the better of him). What he wanted to do was sell the design to someone else so they could make it. But no gunmaker was buying anything at the time, especially a military rifle right after the Army adopted another design.
The quick-change barrel's main advantage was to shorten the rifle for things like airborne use. The reciprocating barrel design lent itself to quick removal, so it wasn't a far reach to go ahead with the feature. The stock and trigger group come off after pulling one pin, so it can be shortened further, and this was what the ParaMarines did when jumping with the Johnson LMGs.
That removable barrel was one reason the ParaMarines liked the Johnson SAR- it gave them a rifle instead of the SMG they initaially planned to use. They were initially armed with Reising SMGs and Johnson LMGs. They were issued 50-50: one man got a Reising, the next got an LMG, the next a Reising, the next an LMG, and so on. They jumped planning to assemble paired up- a SMG gunner with an LMG gunner. The SMG would cover the LMG gunner while he assembled his Johnson LMG.
Then two things happened: It became obvious the Reisings weren't going to work out; and the LMGs were able to get into action quicker than they anticipated. Someone looked at the Johnson semiauto rifles that were handy and they did some practice jumps using them paired with the LMGs instead of Reisings. The result was that they were hardly slower into action than the Reisings, if at all, and gave them an actual rifle instead of a pistol caliber SMG. Both men were firing full .30 caliber ammunition within seconds of hitting the ground. Besides the tactical advatages, many parts were interchangable as was a lot of the training to operate them.
A lot of people, including the CO of the ParaMarines, Col Merritt Edson, pushed for arming them with Johnson SARs and LMGs. But long story short, the ParaMarines were disbanded before they could hardly get started, and the idea of an all-Johnson unit went with them.
But that got Johnsons in the USMC system, even if they never were officially adopted. They were supposed to destroy the Johnsons, but many didn't. There are photos of Marines carrying Johnsons in many of the Pacific battles, and at least one picture of a Johnson LMG-armed Marine as late as Iwo Jima or Okinawa (can't recall which).