So I got some parts from PSA and I'm thinking some things are going to need to be swapped. I just wasn't thinking.



The lower parts kit I got with Magpul collapsible may not function with the upper, as I forgot that it's a rifle-length gas system (18" barrel). I suspect this may pose a problem because for grins I stuck the upper on another PSA lower half of my fathers that was set up for carbine use and the new upper would never pick up the next round from the magazine, regardless of the magazine used. His same lower worked fine with its dedicated 16" mid-length upper. So I popped the upper off Dad's 20" Colt H-Bar and tried my upper on that A2-style lower. Problem solved, so it was a gas issue.

So if it turns out that my intended lower as pictured won't work with the rifle-length gas system, what should I try to make it run? I'd really like to stick with the Mag-Pul STR stock, to keep adjustability in LOP. Can I just try different buffers first? Can I keep my stock and change out the tube, spring, and buffer, or will I need more parts there? I'm not well versed here. Thanks.....

As a side note, I did find that I really like the little 2.5x Leupold on there. Good eye relief and field of view. Was a fun optic off-hand but still offered some precision.

Your upper should work perfectly fine on a "carbine lower". I've had multiple 20" and 18" barrels with rifle length gas systems that work fine on carbine lowers.


A couple things to check:

Lube. Your BCG looks dry in the picture. Take some oil (CLP, Mobil 1, M-PRO 7, etc....) and liberally coat the bolt carrier group inside and out. Also apply oil to the inside of your upper receiver, charging handle, recoil spring, and a very light amount on your trigger group.

Gas block alignment. Make sure the gas port on your barrel lines up with the one on your gas block. It's pretty common for people to mess it up.

Buffer weight: A heavier buffer requires more gas to cycle. If you are having short stroking issues, a lighter buffer may be needed. Try yours first, it may be fine.

Gas Key. On a new gun this is rarely a problem but double check that the gas key screws are not loose and that they are staked i place.

Put the lower together and shoot it.
Do what WE said, I wouldn't trust PSA to get it all right.
Check your buffer. Is there anything stamped on the face? If not, it's a carbine weight and nearly guaranteed to run.

Thanks guys.

I did lube up the bolt after the photo was taken, but will bear that in mind and be more generous. I'll also use one of the recommended oils listed above. If the new lower won't run that way, I'll look into the gas port issue next. I wasn't sure if there were any inherent problems with the combo I was looking to assemble.

Gas port alignment or internal gas leak of the bolt piston rings

18 inch Rifle gas can cause issues as there is not much dwell time after the bullet passes the port. Some things that will help:

Use full powered 5.56 ammo.

Use light buffer and light carrier (semi auto).

Make sure you have a good fitting gas block/gas tube.

Polish the hammer cocking cam and race on your bolt carrier. reducing friction will help.

Lightly buff the sharp edges on the gas rings. The bolt with the rings should move smoothly and without much effort in the carrier. Drag from sharp rings and a rough piston bore will steal a lot of energy from the system.

The Vltor A-5 system will allow you to use the better spring rate of the rifle length spring. Adds expense but the A-5 will make a bit of a difference in smoothing out the recoil impulse.

Mil spec for the 18 inch RLGS port is 0.105 for Crane built Mk 12s. I like 0.110 better as there is not much lee way for cold weather or dirty guns at 0.105.

Were you holding your mouth right.............

All pertinent & valid suggestions (for any gun) & he should surely do it to his gun, but he hasn't even put "his" lower together yet to try "his" upper on it.........he put it on another carbine buffered lower & had the problem.

JPro, let us know what happens when you get it together........just so we know.

MM

you said it "may" not work? I guess you need to try it first, Not sure what buffer is in your dad's lower that did not work, is it H or H1? If so you should start with a plain carbine buffer.

You need to check to see if the gas block is properly aligned, this involves some blowing....I use a compressor and one of those plastic adapters you use on pool toys inserted into the barrel, plug the chamber... If you don't have this set up check with at the local DNC headquarters and have them recommend someone to blow it for you..

I was sure trying. Made it better that my brother was standing behind me watching and asked if I'd bought one of those new "single shot California ARs". There's one in every family.

As several have pointed out, I haven't shot it on my lower yet, as I just got all the parts right before Christmas. Since I knew we'd be shooting at my dad's place, I brought the upper along on Saturday. The PSA lower of dad's should be mechanically the same as the one I intend to use, as it was bought from PSA only last year as a complete MOE lower. So I figured I'd give it a go just to get to shoot my new Christmas stuff. When it didn't work out like I'd planned, I thought maybe I'd committed some kind of well-known no-no in combining that upper with that lower. Again, the world of AR rifles is not well-known to me.

This thread has offered a lot of insight, and while it may shoot great on my lower, I at least know what to look at if it doesn't. I just wouldn't have expected what happened. The 18" rifle system does seem to be a relatively uncommon setup these days, but it sounded like a good idea to me. I'll likely get to put everything together this weekend and give it another go.

18" barrel with rifle length gas is not at all uncommon. It's probably one of the smoothest shooting setups you can get. Unless you have a problem, your setup will work fine. Don't start polishing, drilling, or grinding on anything. Again, there is nothing wrong with your setup on paper. Let us know how it shoots.

I have the same PSA upper in your picture, got it as a present. PSA did fairly well on installing the parts, but the gas block was not on straight. I'd check that first.

I had heck with a rifle gassed rig a while back and after trying all sorts of tricks, I ended up fixing it by opening the port in the gas tube. The bbl port was properly sized and everything else checked out, so I decided to open the tube port as it was a cheap part to mod. I can't remember the dia off hand, but it was a great deal smaller than the port in the block, so I used a jig and opened the port in the tube to approx. the ID of the tube. It worked and that rig will even function well with low powered 223 ammo now.

Good to know! Will keep that in mind.

So I went ahead and lined up the gas block a bit better, I assembled the lower with carbine buffer and spring (thanks youtube), lubed up everything well with CLP and shot it yesterday. It fed perfectly with 5.56 M193 55gr Ball, but I'm getting light primer strikes about every third round. What's the remedy there? It's a standard PSA trigger group and the pull is actually not awful. It's lighter than I've seen on other mil-spec triggers. Do I need a more powerful hammer spring? The ammo I shot the first day was W-W 223 plinking stuff and I may not have had the harder primer cups of the M193 ammo. I had no failures to fire with that ammo that day, just the feeding issues. Now I'm feeding fine, but having firing issues.

Had a problem with an ACT trigger assembled with the included purple 4.5# hammer spring. Failure to fire twice in 27 rounds yesterday using 193 Ball. Light primer strikes. Second strike fired both. Have the also included full power hammer spring in hand but not for long. The lighter spring is coming out.

Bolt bounce? Buffer/spring?

Well, damn. At least I'm learning.....


Looked at an extra PSA trigger group I had that was laying around (pulled out of another complete lower a while back) and it appears I put the hammer spring on backwards when doing my lower build yesterday. Setting up the new hammer like the other one I'd stored away produces a much stouter hammer strike. Was much harder to force into place as well. Will shoot it again today and see what's up. Bet it is "Problem Solved : Operator Error".....

That would do it!

And now the trigger pull sucks, much like other factory AR's I've had.......go figure.... (grin)

Sucks is better than "don't friggin' work" though.....