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I have similar snap caps and they cycle just fine in my RDB-C. Granted that I have already ran about 200 rounds of XM193 in my rifle at the range without any issue.

One way to determine what could have caused the damage is by slowly let go of the bolt when doing manual cycling. You should not feel any resistance with the bolt movement until the very end of its travel where it locks in to the barrel extension. When I carry out this action too slow, sometimes the bolt will fail to lock properly. Good thing is on RDB when the charging handle is unfolded and is locked into the operating rod, it can also act as forward assist.
I'm going to keep cycling my remaining 6 snap caps and see if they also end up damaged. maybe I need to polish the feed ramps like others have mentioned.
 
I'm going to keep cycling my remaining 6 snap caps and see if they also end up damaged. maybe I need to polish the feed ramps like others have mentioned.
One caveat to bear in mind is if you let the charging handle out slowly it will prevent the extractor from seating on a round most of the time. So have your brass rod from your cleaning kit handy to tap the snap cap out from the barrel end.

Also try decocking (as in dry firing) then taking the 'grip' assembly of the RDB off entirely as if field stripping + taking out the last pin at the rear entirely, putting the mag in, and see how everything's fitting together since a lot of the action happens down there and it's by far the most mechanically complex portion of the firearm. You can manually pull back the hammer and work the mechanism through it's process to make sure there's no obvious binding.
 
Mine seems to be potentially a problem with the magazine latch sorting a bit too low. Opening the lower, I can see that if the magazine is torqued downwards, the top of the bullet will strike the feed ramp at the tip (also damaging it which will affect accuracy). But if it is just 1-2 mm higher, it will instead slide forward avoiding blunting the tip and chamber without anything more than sliding against who notches along the edge of the bullet. In all my failures, there was significant blunting of the tip, so it's likely that a heavier magazine makes the problem worse than a smaller lighter magazine. There is about 2mm of play in my magazine latch so possiblity adding a shim underneath might help improve reliability.

Magazine lifted up
53959

Magazine sagging
53960
 
Opening the lower, I can see that if the magazine is torqued downwards, the top of the bullet will strike the feed ramp at the tip (also damaging it which will affect accuracy). But if it is just 1-2 mm higher, it will instead slide forward avoiding blunting the tip and chamber without anything more than sliding against who notches along the edge of the bullet.
interesting, I had it disassembled recently and I think the same thing may be happening with mine. I didn’t try to move the magazine around though. These feeding issues were with one of those flip magazines, not the stock pmag.

looking at the damage to the snap caps, the “bullet” is definitely getting banged up on the feed ramp.

I’ll take another look and see if it’s actually just the magazine.
 
interesting, I had it disassembled recently and I think the same thing may be happening with mine. I didn’t try to move the magazine around though. These feeding issues were with one of those flip magazines, not the stock pmag.

looking at the damage to the snap caps, the “bullet” is definitely getting banged up on the feed ramp.

I’ll take another look and see if it’s actually just the magazine.
There's a lot of forward and back slack even after I shimmed the magazine latch last night. That's enough to give it a couple degrees of rotation forward and back which actually gives a pretty big variation on whether the bullet strikes the feed ramp. I'm guessing this is to accommodate a wide variety of stanag magazines as it's pretty loose for pmags. I wonder if I can find a leaf spring that can fit in the lower rear of the magazine well to pivot the the magazine up (it might not drop free with this change though, but i'm ok with that). Otherwise, there's always reshaping the feed ramps to create a larger slot so the tips don't blunt. It must do a real number on BTHP rounds.
 
There's a lot of forward and back slack even after I shimmed the magazine latch last night. That's enough to give it a couple degrees of rotation forward and back which actually gives a pretty big variation on whether the bullet strikes the feed ramp. I'm guessing this is to accommodate a wide variety of stanag magazines as it's pretty loose for pmags. I wonder if I can find a leaf spring that can fit in the lower rear of the magazine well to pivot the the magazine up (it might not drop free with this change though, but i'm ok with that). Otherwise, there's always reshaping the feed ramps to create a larger slot so the tips don't blunt. It must do a real number on BTHP rounds.
Well, I both shimmed the magazine latch and I polished the ramps to make the cartridge chamber more smoothly, it also causes less damage to the snap caps when I test with a brand new one (almost no damage now when scraping against the hard corners in the barrel extension). Took it to the range, it worked well over 50 rounds. Looked at the ejected brass and it looks pretty good, no bent rounds. Will try chambering some BTHP rounds to see if there's any tip damage when I have some time.
 
Well, I both shimmed the magazine latch and I polished the ramps to make the cartridge chamber more smoothly, it also causes less damage to the snap caps when I test with a brand new one (almost no damage now when scraping against the hard corners in the barrel extension). Took it to the range, it worked well over 50 rounds. Looked at the ejected brass and it looks pretty good, no bent rounds. Will try chambering some BTHP rounds to see if there's any tip damage when I have some time.
When you say shimmed the magazine latch, can you show a photo of what you mean?
Thanks.
 
Well I took a piece of poly plastic that was roughly 1mm thick with a right angle bend. Put some teflon film lube on it and slid it under the plunger. I felt to lazy too take apart the lower to create a proper shim. With the shim in place, the ejection lever is much tighter, and the mag sits a bit higher. Though I don't know if the shim helps more or the aggressively polished barrel extension.
 

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Well I took a piece of poly plastic that was roughly 1mm thick with a right angle bend. Put some teflon film lube on it and slid it under the plunger. I felt to lazy too take apart the lower to create a proper shim. With the shim in place, the ejection lever is much tighter, and the mag sits a bit higher. Though I don't know if the shim helps more or the aggressively polished barrel extension.
Thanks. Just from the pictures of where the bullet tip hits the feed ramp tilted mag vs not tilted mag, I would guess the feed ramp polishing made the big difference.

Easy to test...just shoot it after removing the shim (if it's not permanently glued).
 
I discovered something today that I hadn't before. The RDB will not extract correctly without a magazine in the well. I ejected the mag thinking I would fire the remaining chambered round to empty the weapon and got a cruel/ casing ripping jam (I in fact had do a disassembly to clear it). I tried the same thing and got the same result (though I was able to clear the jam w/o dissasembly). It makes total sense why this happens: The RDB wants to eject a casing as soon as there's an open path with no resistance. With no magazine in the well the casing simply did a partial ejection but stayed in the way of the bolt as it moved forward into battery. Lesson learned, don't fire the RDB without a magazine in the well. I also experienced a slam fire when I did the MP5, charging handle lever smack-down method of chambering a round. Something I've never had happen in an AR, but always with a bull-pup. I would suggest anyone using an RDB be coached on not chambering a round unless the weapon is pointed at something they wouldn't mind shooting.
 
I discovered something today that I hadn't before. The RDB will not extract correctly without a magazine in the well. I ejected the mag thinking I would fire the remaining chambered round to empty the weapon and got a cruel/ casing ripping jam (I in fact had do a disassembly to clear it). I tried the same thing and got the same result (though I was able to clear the jam w/o dissasembly). It makes total sense why this happens: The RDB wants to eject a casing as soon as there's an open path with no resistance. With no magazine in the well the casing simply did a partial ejection but stayed in the way of the bolt as it moved forward into battery. Lesson learned, don't fire the RDB without a magazine in the well. I also experienced a slam fire when I did the MP5, charging handle lever smack-down method of chambering a round. Something I've never had happen in an AR, but always with a bull-pup. I would suggest anyone using an RDB be coached on not chambering a round unless the weapon is pointed at something they wouldn't mind shooting.
Logically, it's just part of the design that the mag is part of the extraction path. There was a brief discussion about it here: RDB fails to eject with no mag inserted

Thanks for the warning about the slam fire too.
 
Finally took it to the range. Managed to get off about 100 rounds of federal 5.56 with no malfunctions. Shells had a slight dent at the mouth, as the manual says they should, so I didn’t touch the gas.

I switched to some Aguila .223 and started having problems. I’d load a mag, release the bolt and fire. Next trigger pull would just be a click. Manually pulling the bolt back would eject a very dented and unfired round. Removed the mag, checked that the action was clear, and started over.
Same thing, but this time the round got so bent it got stuck in the chamber and I had to disassemble and tap it out from the barrel.

not sure if this was feeding, gas, or both. I have far more of the federal than the Aguila though, so I’ll just keep shooting that since it didn’t yield any issues.

I want to see if there’s a way to shim the magazine latch from the inside. One thing I tried was adding some gaffers tape on the back of the magazine so it’s rotated forward a bit, but I’d like a better solution.

what are you guys using to polish your feed ramps? I’ve never done that before.
 
Finally took it to the range. Managed to get off about 100 rounds of federal 5.56 with no malfunctions. Shells had a slight dent at the mouth, as the manual says they should, so I didn’t touch the gas.

I switched to some Aguila .223 and started having problems. I’d load a mag, release the bolt and fire. Next trigger pull would just be a click. Manually pulling the bolt back would eject a very dented and unfired round. Removed the mag, checked that the action was clear, and started over.
Same thing, but this time the round got so bent it got stuck in the chamber and I had to disassemble and tap it out from the barrel.

not sure if this was feeding, gas, or both. I have far more of the federal than the Aguila though, so I’ll just keep shooting that since it didn’t yield any issues.
That's a classic case of overgas actually on the Aquila. It causes the bolt to travel so fast the magazine spring can't push the next round up in time before it bounces off the return spring, generally gouging the brass super badly and jamming things into the chamber.

I have a literal journal of every ammo type I've used in my RDB with notes of what setting is needed. On my 20" barrel model it varies across roughly an 8-click range so far from 55gr to 72gr depending on brand and velocity and weight, while the 16" and 17" barrel models generally have a much smaller (and higher) range of working gas settings due to only having 1-2" of barrel past the gas port versus the ~4.5 inches my 20" barrel allows.

In effect, yes, over-gas causes feeding issues because the bolt will recoil the near-instant it hits the back of the travel instead of coasting down and accelerating back up like it's intended to, so unless you do something crazy like stuff a 30-round spring in a 10-round mag the magazine can't push the next round up in time.
 
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