How do I convert a carbine lower, to run a rifle upper?

[ACES&8S]

Got a rifle upper for an AR Ruger which originaly has a 16" barrel.
The new upper is a 20" rifle in 350 Legend and jams with any load due to the
lower being a carbine set up, rookie guessing here.
The lower buffer has no markings on it and I have never had to change
a buffer to make an upper work, but it seems to be going that way.
What can I change in the reciever to make it run the 350 Legend?
Buffer?
Forgive my ignorance, Edit= got a 350 L mag on it

 

[bigmike45]

Aces&*s,
I have swapped many rifle uppers onto a carbine lowers without any changes, and without any functioning problems. If I were you I would reach out to Ruger and explain the situation and see if they can give you some guidance. I have never swapped out to a .350 legend, but have done so for a .300 blackout rifle upper on a .223 carbine lower. One thing I have read about the caliber is that you must use magazines for that caliber, per several websites. One is Palmetto State Armory.

 

[Blue68f100]

Jams?? Meaning your having a feeding problem.....

I would look at the magazine first. Hand cycle using dummy rounds or remove FP. determine how and where the jam is? Of you pull back on the charging handle to back the bolt up will if chamber?

Once chambered will the BCG lock back on the last round?

I've swapped a lot of uppers with no problems. Carbine length gas system work just fine as long as you have a min of 5" of barrel after the port.

 

[mcb]

In theory it should not matter. I would look for issues with the 350 Legend upper first before worrying about the lower. This assumes the lower runs well with other uppers.

 

[ACES&8S]

I should have mentioned the fellow I am doing this for got the 350 L mag which was the first thing
we changed to before even trying to load for it.
Was quite obvious it wasn't going to run with a 556 mag.

 

[ACES&8S]

Got a Rem lower coming today which runs rifle uppers, will try it with the 350L on top.

 

[mcb]

What kind of jams are you having. The more specific you can be with what the malfunctions are the more likely we can help you look for the problem.

 

[ACES&8S]

What kind of jams are you having.

First shot ok but the upcoming round always at a 45 angle or even almost going in but
didn't make the last push.
I never had any problems with upper like this, but it is designated as a rifle upper, which
throws me, because I have never seen that.

 

[mcb]

First shot ok but the upcoming round always at a 45 angle or even almost going in but
didn't make the last push.
I never had any problems with upper like this, but it is designated as a rifle upper, which
throws me, because I have never seen that.

Is the bolt behind the case still pushing on the case head or on top of the case digging into the side of the case when the jam happens?

One thing to try is load the rifle and put an empty magazine in the gun and fire it. Does the bolt lock back or not. This is a good indication if the bolt is fully cycling.

 

[ACES&8S]

Is the bolt behind the case still pushing on the case head or on top of the case digging into the side of the case when the jam happens?

One thing to try is load the rifle and put an empty magazine in the gun and fire it. Does the bolt lock back or not. This is a good indication if the bolt is fully cycling.

The bolt mostly jams into the side of the case, only -one- time it was pushing against the case head.
The bolt does lock back with an empty 350L mag.

 

[bfoosh006]

FWIW ... I see people praising the Wilson Combat 350L Lancer mags... As in the best thing since sliced bread. https://shopwilsoncombat.com/MAGAZINE-AR-15-350-LEGEND-15-ROUND-POLYMER-LANCER/productinfo/TR%2D350LMAG15/

And what brand upper ?
How many total'ish rounds fired through it ?
I am assuming a carbine length gas system ?
What ammo have you tried ?

So far it sounds like not quite enough gas to push the BCG fully back , so it can catch the cases rim.

Is the mag a good fit ? , or is it wobbly when inserted ( front to back or side to side )

The upper has been cleaned of the OEM shipping oils ? and relubed properly ? Chamber included

You might look for any obvious gas leakage around the gas key, gas block / gas tube.

 

[edwardware]

The new upper is a 20" rifle in 350 Legend and jams with any load due to the
lower being a carbine set up. . .

I think not. Every upper I've ever tried on any lower just runs, unless some knucklehead had been fooling with buffers and springs trying to reduce recoil.

 

[BigBlue 94]

Did you buy or build the upper?

I ask because receiver and barrel feed ramps should match. If the receiver has M4 feed ramps but the barrel doesn't, you run into feed problems.

Does it load a round at all? If the first round doesn't chamber, it has nothing to do with buffer or gas system. That would be an issue with the BCG, barrel extension, and upper receiver, and maybe magazine.

 

[FL-NC]

These are your most likely choices: Magazine issue (if you tried several, eliminate) buffer issue- buffer is not the right weight- or gas issue- gas key on bolt carrier not tight or bad compression rings on bolt head, or gas flow impeded by bad/improperly installed gas block

 

[Varminterror]

The new upper is a 20" rifle in 350 Legend and jams with any load due to the lower being a carbine set up

The gas system length in the upper vs. the buffer arrangement of the lower has nothing to do with the jamming.

 

[RevolvingGarbage]

Just wanted to reinforce that it’s not simply the carbine lower causing the issues. I have only ever run my standard PSA carbine stocked lower with the 20” flat top upper I have and it’s never had any kind of failure in several hundred rounds.

This being said, something about the interaction between the gas system, the bolt, the buffer assembly, and the magazine isn’t jiving and it’s just going to take some experimentation to find where the problem specifically lies.

 

[ACES&8S]

FOUND IT !
It is the Mag!
I would never have found it if I hadn't loaded the mag with one type ammo then had access to another
receiver so I went to change the ammo. = 5 round mag Duramag for the 350 Legend.
When I pushed the first round out, the front of the second one raised up and an instant later the rear
came up. So I found the mag has some kind of drag - friction - in the back of it. Haven't broken a mag down
in years so I will have to find video or look thru my notes.
When I find something like this which should have been so easy to see, it makes me doubt
my sense of observation or problem solving.
My mag test was to charge the mag till empty, but I did it slowly, which gave the rounds time to
slip up into proper position, I didn't watch the rounds or I would have caught them delaying.
Hope this is all it is. I have thought many things seem like, only to have no confirmation.
My friend, that owns this thing, needs to get another mag, to confirm.

 

[ACES&8S]

Is the mag a good fit ? , or is it wobbly when inserted ( front to back or side to side )

The mag does fit with no movement, really fooled myself with a lack of observation while
charging the rounds thru it without looking at the ammo.

 

[N555]

Going forward you probably don't want to run a midlength gas system on a rifle length barrel with supersonic ammo and the standard 3oz buffer.
The standard 3oz carbine buffer might be fine with subsonics.

 

[ACES&8S]

Did you buy or build the upper?

No I didn't buy it.
It is a low end upper, Bear Creek.
I had a run with one of them myself a while back, sent it back, wouldn't have another for myself, doing this one for a friend.

 

[sarduy]

Got a rifle upper for an AR Ruger which originaly has a 16" barrel.
The new upper is a 20" rifle in 350 Legend and jams with any load due to the
lower being a carbine set up, rookie guessing here.
The lower buffer has no markings on it and I have never had to change
a buffer to make an upper work, but it seems to be going that way.
What can I change in the reciever to make it run the 350 Legend?
Buffer?
Forgive my ignorance, Edit= got a 350 L mag on it

I ran into this issue not toonlong ago when i build my friend his 18” rifle length, he put it on a Carbine lower and it jammed every other round, we changed it to a A2 Rifle lower and problem solved. In theory it should work, but that’s not always the case with all the different calibers and chamber pressures.

if you want to run the carbine lower try changing the buffer weight to a H2 or H3

 

[ACES&8S]

f you want to run the carbine lower try changing the buffer weight to a H2 or H3

I am starting to learn this stuff again, been gone from AR builds a long time.
This makes 2 Bear Creek problems this month for me, both 20" barrels, and they
plainly say on their advertisement it is a rifle system.
This rifle isn't mine, it is a friends. And he has been thru a lot lately so I
volunteered to help before knowing it was a 20" Bear Creek on a carbine lower.
My head hurts!!!!

 

[Skylerbone]

The gas system length in the upper vs. the buffer arrangement of the lower has nothing to do with the jamming.

Refer to the above.


Now...350 Legend rifles using AR style magazines have, in my experience, been an issue. A work friend, fresh off a 20+ year career as a Marine who worked at the range and has built hundreds of rifles had issues with his first 350 build. That was solved with a modified magazine (Duramag), one of several I shortened feed lips on for a Ruger American that takes AR style magazines.

Same issue, nose up jam, late release; the solution being shortened lips and a touch off the front edge for good measure.

Unmodified


Modified (left), unmodified (right)


Now able to clear the front of the magazine.


Areas modified. Lips shortened ~1/16” Quick note, run some 400 grit on the underside of the feed lips if you notice any scratches on your brass. My mags felt like razor blades under there.

 

[Bartojc]

I'd say Skylerbone is right on with his recommendations. I had similar problems as OP with a 450 BM. I opened the feed lips on the mags and polished with 600 grit as mentioned and it runs 100% now. Skylerbone's pictures tell the story and my experience as well.

-Jeff

 

[ACES&8S]

Modified (left), unmodified (right)

That is great info, I was wondering if I could adjust the mag and make it
run the ammo.
My buddy, that owns the rifle has ordered another mfg mag and we will try it as soon as it gets
here. Even if it works or fails I am going to go with your info, to see if if I can make it work.
Been looking at the way the rounds seat in the (Duramag) and can see why they wont feed.
Thanks a lot. + Saved your text and pics for us to go by.