I do not know. That is what the unit was doing when I got there. Who made the initial correlation is likewise a mystery to me.
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Back in 89 i had an H&R A1 in basic. We had the plastic snap in brass deflectors. A few years ago i was in a surplus store on I44 in Missouri when i encountered a bucket of beand new ones. I spent $8 and got me one. I have never used it as my personal A1 has a C7 upper with Brunton bump.
Likely a slightly stiffer extractor spring, or ejector spring...Or lighter....
I have looked at this question myself and found that, a lot of different factors contribute to the patterns of extraction and ejection. I found that most of the patterns are associated with the ejector spring and its wore and care. In production between the three contractors each obtains there ejector springs from different sources thus the tension on the springs are different as long as it ejects the fired case from the rifle. Now pair that up with the build up of crud over time (generally the ejectors are not removed from the bolt for cleaning) ejection patterns will change over time.
when dealing with the M16 in period.... sensible soldiers didn't care HOW it ejected, simply that it fed, fired, extracted, and ejected WITHOUT failing every shot, or leave bits of case in the chamber.
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And just ask any servicemen who had to deal with explosives.. how they felt about having to deal with explosives provided by the cheapest bidder.....
Yes, you must keep them clean. I always liked the old SP1, M16, M16A1 because when I was young and used to handload for my Colt SP1 and home built Rock Island Armory M15A1, rounds were so consistent that the empties would all land in a pile less than a couple feet in diameter to the right rear. Policing brass was/is quick and easy. The built in brass deflector on the C7 on down the list took that away big time! One of my favorite shooters is an old SGW/Olympic Arms lower with an excellent, complete Colt M16A1 upper assembly I bought at Knob Creek many years ago. Light and handy with zero bells and whistles. I like to keep my hand in shooting the MR31 center with the service rifle of my time and iron sights. It'll still do some impressive groups if I do my part.