JWard

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?

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.

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.

Thanks shooterrex. My external measurement as shown in figure 11 is the top of lower receiver to top of magazine catch hole which is 0.692”. Yours is 0.6640" for a difference of 0.028" which isn't as much as I expected based on drawings which is 0.049". A friend with a lower that started at 80% also had the same problem as I do and fixed his by shifting his mag catch up 0.050". Does your caliper also have a depth gauge that can get the internal depth to the mag catch as in figure 10? That might give a more consistent measurement between yours and mine.

No need to answer that last question. I'd delete it if I could. I was curious about how other magazines behaved in the lower so I measured them to simply verify reasonable consistency. The max difference is 0.030".The real problem is where the mag catch hole is in the lower. I do need to know the correct distance of the top of the mag catch to the top of the lower as in figure 10 by using a depth gauge. If someone can answer that, I'd appreciate it.

Would anyone be willing to measure and report their heights from the top of the lower to the top of a cartridge case head? I suspect yours will be higher than mine. I'd like to get measurements with lowers that did not start as an 80% because there have been reports of 80% lowers having the same problem as mine. I used a digital caliper depth gauge as shown in the picture. The table shows the measurements of several magazines and the left or right cartridge. It is tight getting the depth gauge to the lower so the caliper may need to be slightly rotated but not in a direction that will affect the height measurement.

With the mag catch removed, a mag can be inserted so far into the mag well that the follower moves upward beyond the point where it can lift the bolt catch. I don't know if this is an indication of another problem being that the bolt catch is too far left. The mag doesn't need to come up so far as to cause this condition but the bolt catch tooth begins at the center of the follower then teeters toward the edge until it drops off. I sent an email to Daytona Tactical support telling that the magazine catch hole is 0.049” too low compared to others I've seen drawings of and am awaiting their response.

What do I have to do?

Maybe some day, I can delete the cruft.

I'd call it a workaround.

I can't even edit my own posts.

The plunger hole was too shallow but even after deepening it, the bolt catch still isn't pushed up high enough because the magazine follower is too low. I can manually push the bolt catch tab in to raise the catch higher. The plunger hole on this 80% lower was supposed to have already been finished. It was initially 0.323” deep. The plunger and compressed spring are 0.490”. I deepened the hole to 0.480”. Could the magazine be too low? If so, it would cause both symptoms. Fig. 10: Top of lower receiver to top of magazine catch = 0.696” Fig. 11: Top of lower receiver to top of magazine catch hole = 0.692” What measurements do you have on your lower?

The plunger hole was too shallow but even after deepening it, the catch still isn't pushed up high enough because the magazine follower is too low. I can manually push the catch tab in to raise the catch higher. The plunger hole on this 80% lower was supposed to have already been finished. It was initially 0.323” deep. The plunger and compressed spring are 0.490”. I deepened the hole to 0.480”. Could the magazine be too low? If so, it would cause both symptoms. What measurements do you have on your lower? Fig. 10: Top of lower receiver to top of magazine release = 0.696” Fig. 11: Top of lower receiver to top of magazine release hole = 0.692”

Correction to original post: ...If the solution to the feed problem involves a modification to the upper or lower, I’ll test the bolt catch and take another measurement as shown in figure 6 (not figure 5). I wish I could edit the original post to correct this.

Thanks for the reply. I'll have to get measurements later. I didn't include that aspect because the bolt easily clears the bolt catch (figure 4) so I didn't want to throw in a red herring and confuse the issue. This clearance is partially determined by the use of a Luth-AR .308 rifle buffer tube, spring, and weight [https://www.luth-ar.com/product/buffer-tube-spring-assembly-kit-308/].

I have a DPMS pattern LR-308 under construction that has a failure to feed and fails to catch the bolt on an empty magazine. I don’t have another LR-308 with which to compare my measurements so I don’t know which dimensions are problematic. Because both symptoms exist, it seems that the components within each receiver may be too far from each other which leads me to suspect that the interface between the upper or lower is too thick. The main concern is the failure to feed. The bolt passes over the cartridge instead of stripping it from the magazine. The bolt partially strips a round but doesn’t chamber it and just drags over the top of it. The bolt catch problem can be solved by drilling the plunger hole deeper but I’d first like to solve the feed problem. If the solution to the feed problem involves a modification to the upper or lower, I’ll test the bolt catch and take another measurement as shown in figure 5. I don’t readily know of a solution to the feed problem but I don’t want to alter magazines to compensate for a receiver flaw. I started construction with the lower at 80%. The upper, lower, and jig are from Daytona Tactical d.b.a. Elite Aluminum Products although I don’t know who made them. These same parts are available from other vendors. Other applicable parts: Bolt catch is from a TacFire .308 AR Lower Parts Kit (not for an AR-15) BCG: United Defense LR-308 Magazine: Magpul PMAG 20 LR/SR GEN M3 Figures: Fig. 1: Right round Fig. 2: Left round Fig. 3: Magazine feed lips height = 0.446” Fig. 4: Clearance to bolt catch is adequate when bolt is fully retracted Fig. 5: Bolt catch does not catch Fig. 6: Bolt catch fully extended = 0.359” Fig. 7: Bolt catch fully extended, plunger removed = 0.443” Fig. 8: Bolt Carrier to upper receiver = 0.283” Fig. 9: Upper and lower receivers Can anyone offer insight to the cause of or solution to these problems? What corresponding dimensions do you have?

I have a Daytona Tactical (was 80%) lower that is too wide to accommodate an ambidextrous selector switch. The selector will clamp down on the sides of the LR308 lower when the screw is tightened but it fits an AR15 lower fine. Of the dozens of ambidextrous selectors I've seen, all say they are one-size-fits-all AR15 and LR308. The external width of the AR15 is 0.885" and the LR308 is 0.911". I haven't been able to find a specification for the width in question so I don't know if I should be looking for a selector that is wider or if my lower is out-of-spec (if common) considering that all selectors should supposedly fit. What do you think?