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Discussion Starter #1 (Edited)
From Day One my OD Green RDB (Z0SXX) has dropped USGI mags after the first round fired with the weapon fitted with its original mag catch. Not a one of many USGI 20 rd mags has ever dropped, but every one of maybe 16 30 rd Okay or Colt 30s (same manufacturer and CAGE code) that I tried did self-drop after one round, each time.

The second mag catch (first replacement Kel Tec sent) showed a definite improvement, retaining maybe 60% of 30 rd USGI mags attempted. The third mag catch (2nd replacment) was a step backward, retaining maybe one in three 30 rd USGI 30s. Clearly the mag catch fix wasn't robust for my RDB/USGI 30 rd mag combination.

I took a close look at the mag catch penetration in the 30 rd mag walls in comparison with that of the USGI 20 rd mags and the 20 rd Gen 3 PMag included with the RDB. Here's what the penetration on an Okay-made 30 looks like, viewed from the bottom upward past the stop dimple:



And looking from the mag's spine toward the front:



I could insert pretty much any of the couple hundr . . . let's say many . . . 30 rd GI mags that I have, all OKays or Colts, into the RDB mag well and I'd get a faint, kinda wimpy "click". Grasping the mag body I could rattle it fore-and-aft or sideways and most would release from the mag well. The USGI 20 rd mags WOULD. NOT. RELEASE. Also, when the USGI 20s or the 20 rd PMag were inserted in the RDB mag well, they latched in with an impressively audible "CLICK," far louder than the "click" made by the 30s.

I concluded that the mag catch was not getting a good "bite" on the top edge of the mag catch penetration in the USGI 30s. I did not do any dimensional studies, but rather took a direct experimental approach and selected a ratty USGI 30 from inventory that was showing copious gold anodizing through substantially worn gray dry film lubricant coating and performed a "field expedient modification." What's one of a couple hundr . . . many USGI 30s, anyway?

This is the tool used:



This is how it was used:



In the above photo, the slotted screwdriver tip is inserted into the mag catch penetration in the 30 rd mag sidewall, with the flat of the screwdriver bit resting on the mag's stop dimple just below the penetration. I pried up with sufficient force to feel some "give" and visually detect the top edge of the mag catch penetration bending outward just slightly; the next photo shows a modified mag looking from the bottom past the stop dimple toward the top edge of the mag catch penetration:



Modified mag, from the rear forward:



Note the slight "flare" upward of the top edge of the mag catch penetration in the photo above.

The modified mag latched into the RDB mag well with a loud "CLICK" identical to that of the USGI 20s and the PMag 20. I wiggled the modified mag six ways to Sunday continuously for over a minute and it would not release. ANY other unmodified USGI 30 would release from the mag well within 10 seconds of vigorous wiggling.

I then modified four more USGI 30s, loaded three of them with M193 and two with Tula 223 and went to the range. NOT ONE modified USGI mag dropped in 150 total rds fired, and despite four or five removals and reinsertions of each modified USGI 30 during firing; doing so every 6 or 7 rds.

So after a total of 30 to 35 insertion/removal/reinsertion actions with a total of five modified USGI 30 rd mags and NOT ONE RDB self-drop, I came back and modified 40 of a couple hundr . . . lots more Okay / Colt USGI 30s as shown above. Each one latched in with the now-normal loud "CLICK" and would not wiggle free with vigorous shaking. I had one unmodified USGI 30 to use as a "failure standard" and would insert it in the RDB every four or five modified mags and the unmodified mag would release readily with a couple seconds of shaking. Again NONE of the modded mags would do so.

I should add that the mag retention testing was done using the original mag catch that came with the RDB, the one that had the worst retention of all three catches I have (I didn't send two of them back yet because I told my contact at Kel Tec that I would be doing some testing).

After modifying and "wiggle testing" the modded mags for retention, I ID'd the modded mags with a green paint dot below the stop dimple so as to differentiate modified from the non-modified mags which will soon undergo the process:



The now failure-free RDB wearing a securely-latched modified USGI 30 rd mag and a Gen 1 Bushnell Trophy 1x32 red/green T-dot optic:



Note: I tried the first modified mag also in two of my AR-15s and my SU-16 and it latched and unlatched fine, and fired ammo without issue in any of the guns.

Note also: There may be some who feel that modifying the mags is the wrong way to go. That's your opinion, and while I could also argue the same point, the objective evidence told me that with three mag catches, the RDB was still not robustly repaired such that USGI 30s would be 100% retained from full to empty. Yes, my "fix" does deform the USGI mag, and does depress the stop dimple just a tad, but the modified mags latch in tight, do not overtravel, work in other rifles accepting STANAG magazines, and best of all no longer self-unlatch. Don't like the "fix"? Then don't perform it. YMMV, but it might help some RDB owners who are heavily vested in USGI 30s as I am (I own just three polymer mags, the 20 rd PMag that came with the RDB, and the two 10 rd poly mags that came with the SU-16.).

Good luck!

Noah
 

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This a excellent study. Thanks for doing it.
I took my release off, put a GI mag in and I see a real issue.
The area of the mag catch that is supposed to insert into the magazine and lock onto the hole is physically longer than the hole. But that is not the case on a P-mag. The catch hole on the P-mag and the area of the mag catch are a perfect match. Also, the P-mag has an extended upper hole lip molded in giving more contact area.
See my comments here:
https://www.thektog.org/threads/rdb-magazine-retention-issue.268043/
so yes, it would be necessary to either alter the magazine or the catch.
This pic is a GI mag with white paint around the catch hole inserted in a RDB:

Hole length=.478
Here is a P-mag:

Hole length=.558
Use the letters to reference the length if the holes. The P-mag is longer.

Insert the catch in the holes...
P-mag:

Lots of room.

GI mag:
Its a tight fit. If you actually get this.

But you are probably getting this:

Because the hole length is short, the catch only gets partial purchase.
This is why KT gives you P-mags with the RDB.
 

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Discussion Starter #3
Scott:

The 30 rd GI mags looked mostly like this prior to modification:



Following the mod, the purchase of the top edge of the catch slot looked like this:



(Thank you for your excellent photos!)

I've since taken 10 of 30 modified 30rd GI mags, loaded them with M193 (South African) and spent some quality range time dialing in the optic settings and gas system. There were ZERO mag self-drops. Just for funs I "did the other half of the experiment" -- I had two full unmodified GI mags and snapped them into place, and they both self-dumped with the first shot. Switching back to a modified 30 rd mag, no problem. I'm going to work my way through the balance of the GI 30 rd mags in inventory and mod them all.

Thanks for the comments!

Noah
 
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