All, hoping you can give me some advice on how to get this upper working with my RLL. I have a lower that has the modified 3rd burst setup to enable safe, semi, auto. The lower does work fine with my 11" commando upper. Problem is, I am trying to get it to work with another upper. An 8" 9mm. I currently have a .032 paddle in the upper. If I put the gun in auto, hold the trigger, pull the bolt back and then then release it, then release the trigger and pull it, the gun fires. It does the same thing when it is in semi.

I saw it mentioned that I should do a basic function test. To do this I put the gun in auto, held the trigger, and then pulled the bolt back all the way and then gently let it go home (while I held it). The gun did not fire. I then pulled the bolt back a half inch and let it slam forward. The gun still does not fire. I then pull the trigger and it fires.

If I separate the upper from the lower. Then push the bolt into the upper with my thumb till it won't go any further forward, I can slide a .026 feeler gauge in the gap with basically no drag. I can get a .28 feeler gauge in there but there is some drag. A .30 feeler gauge will not fit.

Is there another test I need to do to know what is going on? I'm assuming the paddle I have in there now isn't the right size... Should I go with a smaller one?

Measure the distance from the cut of the carrier to end of the carrier.  If those measurements are different then it will give credibility that the paddle is the wrong size.

Then you can decide to change paddles every time, or recut the carrier to be identical.

Using a 9mm upper, 9mm bolt cut for a LL, modified 3 round burst kit (all Colt I assume), a LL, so many things to go a teeny bit wrong.  When I was doing this, I was about to hang myself.  I did get it working.  Exactly what you are trying to do.

If the hammer's not dropping in auto using the new upper, then the paddle is not getting pushed forward enough to actuate the link. Uppers differ. You may need to start with a new paddle and mate that one to the new upper you wish to use, or switch to a thicker paddle.

Moving one paddle between uppers will probably fatigue the paddle and cause it to break off at the bend more quickly than would otherwise be the case.

I failed to think of the variations in the uppers.

This is a brand new .032 paddle, never used. How do I know what size paddle to use? Do I just try tthe next size up from .032 doing the basic function test outlined above? Or is there a way to tell what size paddle to try based on the gap between the carrier and rear lug of the upper?

Thanks for the advise!

Since this link functioned in the same lower, but with another upper, it's not an issue of clearances in the lower.

If you're using a KNS re-enforcer, the new upper may be too tight for the link to move, requiring the bottom of the takedown pin post to be shaved 1/8" or so. That's the first thing I'd check.

If there's clearance, then the paddle is either too thick for the clearance between the back of the takedown pin post and the front of the ledge on the carrier, meaning it's stopping the carrier and not pivoting to actuate the link, or the back of the post is too far forward, not generating the pivoting leverage needed to actuate the link.

Generally you know the fit is good if the paddle takes on that gentle bend after the first time it's used with a particular upper and carrier combination - meaning it's getting pushed *just enough* to actuate the link without putting undue stress on it. If your paddle, especially a .032" paddle, is not forming that initial bend, then it's probably time to get a thicker paddle. You can simulate this by building up the back of the takedown pin post with some shim stock or tape, and see if that improves function for you, or in fact gets things to function.

The links originally came with a .032" paddle and one that was about .051" - those equate roughly to some gauge of sheet steel they were stamped from, but I don't have any references handy to tell you what they were.

I've always run my 9mm uppers with the link using the thicker paddle, for what it's worth.

The lower is absolutely good to go. It was clearanced by m60Joe. The rear lug of this upper clearanced so there shouldn't be any issues with the KNS enforcer.

I tried out a .040" paddle. It seems like the gun works ok with this paddle. At least it is passing the function tests (holding FA while pulling back charging handle and letting it go forward (no riding it) to see if hammer releases and riding the charging handle forward then pulling back a half inch and letting it slam forward to see if hammer drops). Should I try a .050" paddle too or is it best to go with the thinnest one that works? I do have a .050" paddle. I want to put as little stress as is possible on the link.  

Thanks so much for your information! It is so helpful.

I agree that the thinnest paddle which will work is the best.

If you're not running with a re-enforcer, I recommend you get one - it's cheap insurance compared to a $500 repair job if one or both "ears" get cracked on you.

I am running a protector but thank you for the suggestion. Your advice has been very helpful and appreciated!