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JWard

Problems with failure to feed AND failure to catch bolt

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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?

Fig 2, left round.JPG

Fig 1, right round.JPG

Fig 3, magazine feed lips.JPG

Fig 4, bolt fully retracted.jpg

Fig 5, bolt catch does not catch.JPG

Fig 6, bolt catch extended.JPG

Fig 7, bolt catch extended, no plunger.JPG

Fig 8, BCG.JPG

Fig 9, upper and lower.JPG

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2 minutes ago, JWard said:

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?

Fig 2, left round.JPG

Fig 1, right round.JPG

Fig 3, magazine feed lips.JPG

Fig 4, bolt fully retracted.jpg

Fig 5, bolt catch does not catch.JPG

Fig 6, bolt catch extended.JPG

Fig 7, bolt catch extended, no plunger.JPG

Fig 8, BCG.JPG

Fig 9, upper and lower.JPG

As weird as this will sound, you need to measure the length of your recoil buffer, and the internal depth of your buffer tube and post it here in addition to the excellent info already provided.

 

Really appreciate your thorough pictures provided, that's an excellent way to provide data on your issues.

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57 minutes ago, Matt.Cross said:

As weird as this will sound, you need to measure the length of your recoil buffer, and the internal depth of your buffer tube and post it here in addition to the excellent info already provided.

 

Really appreciate your thorough pictures provided, that's an excellent way to provide data on your issues.

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/].

Edited by JWard

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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.

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17 minutes ago, JWard said:

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.

 

The edit feature will become available to you with just a few more posts on the forum. It's a precaution against folks who like to cause trouble immediately after joining up.

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Not up on the 80% stuff myself but we have a thread with lots of different brand bolt catches and the dimensions that may help on that end, please add to it if you have something not listed there. :thumbup:

 

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8 hours ago, JWard said:

Fig. 6: Bolt catch fully extended = 0.359”

Fig. 7: Bolt catch fully extended, plunger removed = 0.443”

 

 

 

Fig 6, bolt catch extended.JPG

Fig 7, bolt catch extended, no plunger.JPG

 

These two pics right here, and the height difference.  It's not the bolt catch, itself, obviously.  What's stopping your bolt catch travel, when you have that plunger and spring in there?  Is it the length of the plunger?  Depth of the hole?  Coil bind on the spring?  Measure the depth of the hole, first.  Then collapse that spring ON the plunger, to coil bind, and measure the OAL of both (with spring in coil bind).  That should tell you what's going on.  Either the hole gets deeper, or a couple coils come off the spring, or the tail on the plunger gets shortened.   That one issue, right there, because in the previous pics, that bolt catch isn't sticking up enough to reliably get that bolt face every time. 

Excellent work with the pics and details.  :thumbup:

Edited by 98Z5V

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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”

Fig 10, mag release inside.JPG

Fig 11, mag release outside.JPG

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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?

Fig 10, mag catch inside.JPG

Fig 11, mag catch outside.JPG

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22 hours ago, Matt.Cross said:

 

The edit feature will become available to you with just a few more posts on the forum. It's a precaution against folks who like to cause trouble immediately after joining up.

What do I have to do? 

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Again, kids running with scissors did this to us. If you have something major to change a quick PM to a mod will get it fixed for ya. I don’t know what the limit is before you earn the “edit” privileges. For a while you only had so long to make the edit and then never again. 

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6 minutes ago, JWard said:

What do I have to do? 

 At the moment just keep posting as usual, the edit feature will become available. Also, we'll be happy to correct or delete anything if you point it out to us in the meantime.

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5 hours ago, JWard said:

 

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?

Fig 10, mag catch inside.JPG

Fig 11, mag catch outside.JPG

Indeed, if could be too low, and we have a HELL of a thread here on that very issue.  I'm pretty sure it's in the 80% Section.  I thought about a low mag catch (the machining of it) very first, based on your info, but I wanted to know about that mag catch's internal parts first. 

I'm sure someone here can find that thread on the mag catch heights.  It's a whole lot of details.

Before that would be a solution - you need tocheck how much further into the upper receiver that your magazine will go.  You'll need to clay the upper, just like you'd do if you were checking an engine for piston-to-valve clearance.

Edited by 98Z5V

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10 hours ago, 98Z5V said:

Indeed, if could be too low, and we have a HELL of a thread here on that very issue.  I'm pretty sure it's in the 80% Section.  I thought about a low mag catch (the machining of it) very first, based on your info, but I wanted to know about that mag catch's internal parts first. 

I'm sure someone here can find that thread on the mag catch heights.  It's a whole lot of details.

Before that would be a solution - you need tocheck how much further into the upper receiver that your magazine will go.  You'll need to clay the upper, just like you'd do if you were checking an engine for piston-to-valve clearance.

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.

Fig 12.JPG

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11 hours ago, JWard said:

 the bolt catch tooth begins at the center of the follower then teeters toward the edge until it drops off.

I had that issue with a couple Lancer mags, ended up replacing that bolt catch when it broke with a Sig 716 bolt catch and now the one Lancer mag I saved that had the issue works in that rifle.

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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.

cartridge to lower top.JPG

table.PNG

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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.

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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.

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