Weird upper problem

AR/M4 AR Technical Discussion
1

Figure I would post this here instead of the other place so I wouldnt get moron responses.

I have a RRA upper, one of the newer ones with RRA engraved on the top front. When I place it on a lower, it has what I guess is a binding problem. Without the lower in the bolt moves freely, but when I place it on a lower it has a very hard time pulling back. Also it is very loose on the upper wobling around. This isnt a problem if it is very lubed, but after around 100 shots it just gives up and doesnt have the power to push rounds into the chamber, as it kinda grinds along.

I have tried it on other lowers, and I have tried other uppers on the lower I normally use it on (They work fine)

I am assuming this lower is just out of spec. I am leaning towards just replacing the upper. What you guys think is the problem?

2

Is the bolt binding in the carrier and causing the carrier to stop traveling forward or is the carrier binding in the receiver? Pictures would also be helpful.

3

Is it a new/newer RRA lower? Some newer RRA lowers have been out of spec, so that might be a problem.

4

Carrier is binding in upper. Bolt moves freely.

Carrier only binds when attached to lower.

Pictures of it wont really show anything, it looks 100% normal. And actually works when overly lubed. But after 100 rds or so it slows up and cant push a round into the chamber. Its not the buffer spring either, as other uppers work fine on it.

5

As already stated, it is really hard to tell a lot without seeing it, but it sounds like the bolt bore and receiver extensions tube are not lined up on axis or face to face.

Simple test, with the upper on a lower, do not push the rear pin in… kinda hold things together enough to start pulling the bolt to the rear but do not squeeze the two receivers together – let the rear of the upper receiver “float” if you know what I mean?

If things feel better like that, then it is most likely the rear pin holes are off… we can go from here if this does in fact make a difference.

6

Tried that, didnt really make a difference.

The pullback is just slightly rougher than usual. Problem is, once lube gets burned away, it cant push the rounds in.

I do believe that the problem is the reciever is not lining up with the extension though.

7

Was this purchased from RRA assembled or was it assembled by someone else? If it was assembled by RRA you may want to contact them directly.

8

assembled by me.

9

Just to make sure I got this right… when the upper is not mounted ot a lower, the bolt assembly moves back and forth freely (charging handle in place)?

When it is mounted to a lower, it drags and will not go fully forward at times?

A few things to look at…

Magazine lips dragging on the bottom of the bolt - easy to eliminate this, try it without a magazine.

Bolt carrier binding in the receiver extension tube

Bolt carrier binding on the hammer

Bolt carrier binding on the bolt catch

These three are a little harder to narrow down without being able to see the gun, but you should be able to see from wear marks or “drying” if it looks like one of these might be the problem.

10

If your carrier is binding inside the receiver with over 100 rounds you should have enough wear to actually see.

When the upper is put into the lower, is it difficult to hand fit the upper and lower together? Are the pins difficult to push through?

11

Yes, bolt moves freely charing handle in place when it is not attached to lower.

I believe its binding in the reciever ext, as the problem occurs when trying to load rounds, it simply doesnt have the power to push them in.

I wouldnt think its anything in the lower such as hammer or bolt catch, as the lower works fine with all 3 other uppers.

There is wear, but it appears to be the normal charging handle wear at the top of the reciever, I dont really see anything out of place.

Its easy to get the upper and lower together but, the pins are very hard to get into place. Also there is a very very large amount of woble.

12

Check the length of your action (buffer) spring to make sure it’s up to spec.

Carbine 10 1/16" to 11 1/4"

Rifle 11 3/4" to 13 1/2"

It’s hard to diagnose without actually seeing the rifle/carbine.

13

As I said, the lower is fine.

I am just going to go ahead and assume that the upper reciever is out of spec, get a new one, and transplant the barrel etc onto it.

14

Apparently you have found the problem.

15

Ace,

troubleshooting is an art, dont discount the lower. Especially the buffer tube, buffer and spring. this is the area of the lower that has the most “interaction” with the bolt, and could be the source of your problem…

16

I had a bad bolt catch haunt me for awhile, between different uppers. I even thought one was short stroking, replaced the catch and it works with any upper now. This was on a PWA lower, because it worked with one upper, it took me damn near forever to figure out the problem.The catch was not dropping down all the way, and drug the bolt carrier.Later, W.B.:cool:

17

I believe RRA lowers have a high shelf in the rear of the lower reciever. You mentioned that the pins were hard to push in, is the rear lug of the upper reciever bottoming out on the shelf of the lower? This could possibly raise the angle of the upper enough that the bolt carrier is binding in the reciever extention.

18

I have actually tried it on another RRA lower, and it wouldnt even let the pins in. On the other lower it looks like the rear pin hole is too far forward on the lower.

19

Sounds like the pin holes ended up in the wrong spot… these things happen.

The good news is that if it says RRA on it, I know all you have to do is send it back and they will take care of it – that thing about putting your name on stuff you know.

20

I had a problem with a BM upper fitting on a RRA lower. I had to grind down the rear pin point on the upper, and take a little off the back of the upper to get it to fit.

All the parts could be ‘in spec’, and still not work together if they are each at opposite ends of the tolerance spectrum.