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Posted: 3/6/2012 5:22:37 PM EDT
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With the square pad ejection port cover, there are 2 different variants.
One has the rounded detent housing with a roll pin through it and the other has an angled detent housing with the roll pin through it. Does anyone know the timeline of when these were used on what models? They were used on 601s, 602s, XM16E1s, 604s, and SP1s. 601s - rounded EPC 602s - rounded EPC (tgus confirms his 602 is rounded), speculation the change to angled could have occurred towards the end of the 602 XM16E1s - angled EPC (My early XM16E1 has the angled one, haven't seen any others to confirm) 604s - angled, possibly both (PrivateSnafu has seen them with rounded housings, may have been field replacements with old stock) SP1s have angled ones out of all the examples I've seen. the one below is 4xx and my 8xx and 3xxx both have the angled housing. If someone has a rounded one, please confirm with serial. So looking at the timelines 601 59-63 602 63 XM16E1 64 604 64 SP1 64 I would venture to guess the rounded ones were used from 59-63/64 and then the transition the angled ones occurred then. This either happened at the end of the 602 line or shortly after the xm16e1/603 was introduced. Anyone else have any info they can add or share? Rounded Detent Housing 601 (pic from willyp) 
602 (pic from RBR) 
Angled Detent Housing early 3 digit SP1 serial 4xx from RMiller 
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Lionel,
I would guess that the rounded covers were also used on the early XM16E1's. I say this for a couple of reasons. First, the first editions of the XM tech manuals show an early XM16E1 that clearly has the rounded, square port cover with the pin retainer through it. Secondly, my recently completed clone has the rounded, square port cover on it–– not that that means anything by itself. However, it came from an early XM16E1 upper assembly with the "M VP" "12" barrel that I purchased. The assembly looked original in all respects. Based on the pictures in the manual and the apparently original upper I purchased, I figure that these covers were used on at least some of the first XM16E1's. |
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Quoted:
Lionel, I would guess that the rounded covers were also used on the early XM16E1's. I say this for a couple of reasons. First, the first editions of the XM tech manuals show an early XM16E1 that clearly has the rounded, square port cover with the pin retainer through it. Secondly, my recently completed clone has the rounded, square port cover on it–– not that that means anything by itself. However, it came from an early XM16E1 upper assembly with the "M VP" "12" barrel that I purchased. The assembly looked original in all respects. Based on the pictures in the manual and the apparently original upper I purchased, I figure that these covers were used on at least some of the first XM16E1's. From what we see again and again, there was not a hard and fast date of transition. Especially for field replacements. If the rounded housings went into the field, it's quite possible they were used past the transition date as field replacements. However using the manuals to date parts isn't very reliable. My 1968 TM 9-1005-249-34 M16 M16A1 M3 Rifle Technical Manual shows the rounded housing and my 71 dated SP1 manual shows the same thing, even has a triangular charging handle. Early SP1s shared a lot of parts w/ their M16 counterparts, they didn't even mark the barrels with the S early on. They had W marked 1/14 barrels, used cast FSBs, dimpled windage screws, carbon steel gas tubes, roll pinned extensions... The parts that were different was a 2 position safety albeit divoted, carrier that didn't have the full m16 tail but definitely more meat than the later SP1s, larger pivot pin, and a hammer w/ the tail removed (you can see the tool marks where it was done). Maybe more differences but those were ones I quickly identified. If we found an early SP1 w/ a rounded housing, that would be a good indication it went into 64 from the factory. Does anyone have a SP1 earlier than the 4xx one and can check? Anyone with a factory 602 or early XM16E1 or 604? |
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cool thanks. that supports my guess. hopefully someone will have additional info that can chime in. so we're pretty confident that all 601s had the rounded housing. tgus's 602 is the only example we have to go off of and it has a rounded housing with his xm16e1 and 604 having the angled one. so one would venture a guess than the angled transition happened sometime btw 63 and 64 but not sure if the angled were found on the 602 or rounded on xm16e1/604 without a solid example showing one or the other to establish an overlap. |
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But there are always these.
I have found several surplus very early 604 uppers that had round latch plate EPCs. No real proof, but there is always that "first in last out" when it comes to parts bins. Just like XM16E1 bolt carriers with full round chrome gas keys. 
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Quoted:
But there are always these. I have found several surplus very early 604 uppers that had round latch plate EPCs. No real proof, but there is always that "first in last out" when it comes to parts bins. Just like XM16E1 bolt carriers with full round chrome gas keys. http://i240.photobucket.com/albums/ff160/nosepiece/veryearly604.jpg http://i240.photobucket.com/albums/ff160/nosepiece/veryearly604epc.jpg interesting. so unless someone can turn up a 602 with the angled housing, we can speculate the angled housing transition occurred early on with the xm16e1s/604s. either that or a sp1 with a rounded housing. |
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i looked any my rounded housing one has a similar wear pattern on the pad suggesting it spent some time banging against a fence every time it was opened.
if the angled one came into play in early xm16e1/604, they must have had a pretty short life span as both models can be found with the rectangular ones. i have a square forge m vp 12 604 upper that has a rectangular. that would explain why they don't come up very often. both the angled versions on my SP1s occur on 64 models. |
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