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JWard

Problems with failure to feed AND failure to catch bolt

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Thanks shooterrex. My internal measurement as shown in figure 10 is the top of lower receiver to top of magazine catch which is 0.696”. Yours is 0.6610" for a difference of 0.035" which is closer to the value I expected but not by much. With such a small difference, I still wonder why some of us with lowers that started at 80% have the same feed problem.

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8 minutes ago, JWard said:

 I still wonder why some of us with lowers that started at 80% have the same feed problem.

Because (#1) there's NO standard for the Large Frame AR, like there is the for Small Frame AR (The MILSPEC TDP), and (#2) the mag catch slot on an 80% is already machined for you - by whomever you're buying that 80% lower from - and it's cut in the wrong place.  It's too low.

We have a pretty detailed thread here about this, and it's probably in the 80% section, on this board.  I can't pinpoint and link that exact thread for you, but there's a very, very specific thread here just about this issue. 

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4 minutes ago, 98Z5V said:

...We have a pretty detailed thread here about this, and it's probably in the 80% section, on this board.  I can't pinpoint and link that exact thread for you, but there's a very, very specific thread here just about this issue. 

Is this it?:

CentralNChunter moved his mag catch up 0.040" but I don't know how he arrived at this value or where it started. That makes about four different values of shifting under consideration and I still don't know what would make the optimal mag catch height. I have known that there is no standard for this (perhaps especially among manufacturers of 80% lowers) but it is probable that most reliable lower manufacturers have machined an optimal mag catch height. It is those that I would like to emulate.

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No, there's another thread here somewhere, where we are only measuring bolt catch height on 80%'s, compared to production lowers.  That's not the one.  It's probably not a pinned thread - but I swore I asked for that thread to be pinned, after everything shook out. 

@Lane will probably know exactly which thread it is.  :thumbup:

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@Lane, THAT is the one!  We need to move that to the 80% Section, and pin it in there.  That's a mag-catch height goldmine, right there.  :thumbup:

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3 hours ago, Lane said:

I believe this is what you're looking for? The mag-catch height thread? 

That's the most applicable thread I've seen and I would not have pursued it further simply based on the topic of "measurement poll" in "General Discussion" if you had not brought it to my attention. @Lane, @panel77 , @98Z5V and others gave good info in it. The linked YouTube video by American Made Tactical said the lower is "MIL SPEC". I'd like to hear their clarification of that.

Values in the range of 0.700" to 0.692" (mine) have been reported to have a problem. There were 18 other values/instances reported that were lower and did not have the problem and were in the range of 0.687" to 0.640". I'll have to experiment with height to ensure I don't make the distance too low and cause the bolt to hit the magazine feed lips but I expect the optimal range to be between 0.670" and 0.650".

@Toolndie7 made the same suggestion that I made previously which is to use a depth gauge from the top of the lower to the top of the mag catch internally to get a more precise measurement instead of trying to measure the top of the lower to the top of the mag catch slot externally. That methodology wasn't pursued in that topic so we don't know the values. 

I knew of upper to lower mismatch problems beforehand which is why I bought a supposedly matched set from Daytona Tactical. The problem is that this same 80% lower (and jig) was also sold by American Made Tactical, Thunder Tactical, Thunderguns, Blitzkrieg Tactical, Firearms Unknown, Colfax Tactical (defunct), 5D Tactical, American Weapons Components (previously known as Ares Armor), etc. Several of these were mentioned as having the mag catch problem in the "measurement poll". I'm curious to know who actually made them. Does anyone know this story?

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7 hours ago, JWard said:

I'm curious to know who actually made them. Does anyone know this story?

That's an interesting question; and would likely require a decent amount of leg-work calling around. Some companies might not be forthcoming with that information out of the gate, but it's worth a try moving forward. The best place I can see to start (in this forum at least), is looking for forge marks on the lowers, and also trying to identify any unique cuts in the machining process. There is a good chance that will help sort out at least some of the differences between lowers' manufacturers. I presume there will be some sub-sets that match cut and measurement between different manufacturers. 

I can tell you out of the gate, that Colfax Tactical had one of the most "unique" cuts in terms of FCG pocket position; that may or may not be officially related to the mag-catch issues though. When I was digging for that information (FCG pocket locations), it was rather scarce.  That was considered proprietary information by some companies over the years...

These blueprint images aren't "super" helpful at this point in the conversation, but here are a few semi-related files I had laying around. I don't know off hand what the first image is supposed to be (is that Armalite measurements?); but it has a slant (not radiused) buffer tower... What's interesting there, is that there is a reference to the mag catch listed. The next question is; are there different height mag catch inserts? Are all the mag catch slots the same height; or is that where everything starts to go off the rails? If the official reference point is the bottom of the slot; and some manufacturers use a non-standard height mag catch, it may have compounded the offset issue on those particular lowers.

One other thing that original poll never really addressed; is if there are proprietary upper cuts as well. I never came up with a well defined way to measure that. It could possibly be done with a series of measurements; but my ultimate curiosity was: "does the BCG position vary vertically in different upper receivers". My guess is, the answer to that question is "yes"; and that those should be sorted out. There should be an upper (some uppers) that are compatible with 0.700" measured lowers, and something else that works on those in the 0.640-0.650" range... Maybe time for a new measurement poll to ask the same question about mag-catch height; but also what upper receivers are in use (mated and functional) on those particular builds? 

AR10_Lower_Receiver.jpg

308 Variations.jpg

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Tried to edit that last post; but it looks like I was wrong about Colfax Tactical being the weird one. I'll see what else I can dig up in terms of "strange" 308 lowers. I know I have a few sets of full prints of unknown origin; meaning, I don't know what particular brand or cut lowers they are supposed to match. 

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The 308Lower.pdf file is a slant style; so it may not be directly applicable to all variations... But perhaps it is? It lists the mag-catch slot as being 0.250 in height, and also contains enough references to the vertical position to compare with other files and physical lowers. The other drawings are also slant style; but the quick checks I did all matched each other in terms of those measurements.

One other thing to consider. I measured my slot to be 0.250" (with the coating in the slot). There may be some slight variations in that measurement depending on the finish (sloppy machining tolerances, etc.). My actual mag-catch only measures 0.245" in height which makes complete sense; because it needs clearance to operate. If the mag-catch were in fact 0.250" tall, it would have to be hammered in, would have a serious friction fit; and be completely non-functional.

GIrSagH.jpg

308Lower.pdf AR_10-2.pdf

Edited by Lane

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Since I can't edit the error in my post about the "weird" 80% lower cut I stumbled across; I will link that here. It was labeled "Hell Fire Armory". It's not that those FCG locations are directly related to the mag-catch issues; but they may be indicative of a larger problem in the 80% 308 world. I still don't know enough about the variations between various upper receivers' geometry to know if they were even legitimate modifications; or just oversights in "copying" a design. I've long been curious if those changes were on purpose, to protect in terms of legal liability; perhaps by not being the "same" as the original armalite pattern?

AR10-prints.jpg

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