Hey guys,

Finally got my 7.5” AR Pistol out to the range today for some testing and unfortunately we had some problems.
I am running a new Radical Firearms 7.5” Quad-Railed upper with low pro gas block. I opted for the phosphate BCG and standard charging handle (The BCG is M16 profile). I built the lower from a stripped blem. Anderson Mfg. with a PSA MOE LPK. The buffer tube is a KAK Super Sig pistol tube with carbine spring and H2 buffer from KAK. Also running the SIG brace.

We tried a variety of ammunition including Federal XM193, Fiocchi 55grn softpoint, Hornady 55grn. SP, PMC Bronze 55grn ball, and my reloads which are minimum spec/load for .223.

Of all the ammo we tried the 5.56 (M193) worked the best.. I can’t recall if we had malfunctions with it but all the commercial ammo we ran gave us every issue you could imagine.

Specifically the problems we encountered with the commercial ammo were: Failure to feed the next round(most common issue), failure to lock the bolt back on last shot, failure to extract with a fresh round trying to enter the chamber while the spent round was still there, partially ejected FTEs.

Now the 5.56 ammo would work and lock the bolt back – so the gun likes the hotter stuff obviously. On the opposite side of the spectrum my reloads would not cycle the bolt – it would stick open about ¼ of an inch after firing and would take a little force to pull the charging handle back to unlock and eject the round.
To troubleshoot we tried swapping parts from my Stag Model 1. We tried the carbine spring and buffer in place of my H2, as well as the semi auto profile (lighter) bolt carrier. Nothing we tried would make the pistol run 100% with any commercial ammo we tried – reloads are out of the question – these light loads do function just fine in my 16” Stag though, which is why I had so many.

All in all we put about 180-200 rounds through it, 40 being 5.56 which functioned very well if not flawlessly. The rest of the commercial including Hornady didn't work out so well :(

I chronographed most of the ammo and the results were as follows:
M193: 2329FPS
Hornady: 2140FPS
Fiocchi: 2047FPS
Reloads: 1739FPS

I want to get headspace tools to check RF’s work but they are expensive and on back order everywhere. I have ordered the handguard tool to remove my quad rail and take a look at the gas block for alignment issues but I really don’t know what I’m doing here. I also ordered a set of o-rings for the extractor. In the mean time I’m gonna clean the crap out of it and relube it – test again this weekend.

Any Advice? Seems like an undergas issue to me, either that or too heavy a buffer/spring or the gun just isn't worn in yet.

Had a similar problem with my 7.5".  I went to a softer buffer spring, and polished the chamber.  Took care of the problem and the gun has run flawlessly with whatever I put through it since.

I also took my RF upper out this past weekend and has some issues with it. It seemed like it would randomly get a FTE/Double feed like you described where the spent casing wouldn't eject and the next round would try to enter the chamber. It didn't happen all the time though which was odd IMO. My bolt would lock back on the last shot, so I dont think its an undergassing/short stroking issue. I think I have narrowed it down to either being the bolt cycling too quickly (so heavier buffer to slow down cycle rate) or I need to upgrade the extractor spring (it's a Yankee Hill Machine BCG). Either way I'll be watching your thread and seeing what happens and I'll update if I get mine figured out first.

Toss an O Ring in the Extractor Spring; that should fix any FTE and double feeds.  You may also need your gas port opened up a bit.  Are you sure your Gas system is properly set up?  Gas block aligned, etc?

I built my own 7.5" and have had zero problems; it eats anything I feed it.

try a standard carbine buffer

sounds like its not getting enough gas.

have heard radical is cheap so make sure the gas blocks set right and everything else is fitted correctly

This is what I'm leaning towards. I don't have my handguard removal tool yet - The YHM spanner wrench for quad rail forends (it's on order this week). But I can see some carbon/debris around the front of the block - looks like it might possibly be leaking there slightly.

I just hope if I take off my handguard and or the gas block for inspection that I don't open a can of worms - being that I've never done it before and not sure what to look for with the alignment. All I can tell from my very preliminary check is that it looks like there might possibly be a leak around the front of the block.

And yes the upper was super cheap, on sale for $250 minus BCG/CH All told it was $350 + shipping complete.


I just got done cleaning the crap out of the chamber and quickly cleaned the bore up a bit. I got the BCG all clean and oiled with CLP ready for the next range trip. Meanwhile I did the same to my Stag 16" in case I need to swap parts again to troubleshoot.

I had a fail to feed once on a RF 7.5 quad upper. Fixed it by swapping the Carbine buffer for a Spikes ST-T2 buffer. After that it ran fine. Running just a basic BCG. Hope this helps.

Ok guys,

The YHM Handguard wrench came in today so I took a look at my gas block. First thing I noticed is that I barely had to touch the handguard retainer ring with the wrench to get it to loosen up. When I got to the gas block much to my horror it was about the same... The screws were loose! Once I took off the two 3/32nd allen head screws on the gas block I could see that only one dimple was drilled on the underside of the barrel but two screws were used with this gas block type - is this something that will need to be addressed?

I captured some pictures - maybe someone here more knowledgable can tell me what I'm seeing here. I don't know how to check gas port diameter - all I can tell is it looks visually correct and was unobscured by burrs or flakes or brass/etc.

http://i.imgur.com/dzjOvxr.jpg?1
http://i.imgur.com/NweVfgj.jpg?1
http://i.imgur.com/o7V3keR.jpg?1

I just got the same upper. Mine ran fine until I got a stuck case after about 100 rounds. My handguard screws weren't very tight, I will have to check the others.

What you're referring to as a "dimple" was not drilled on the barrel. That is what happens to the barrel coating when a set screw is properly tightened. To me it looks like they only properly tightened one of the set screws on that gas block. Then your gas block probably came loose and partially blocked the port, which would account for most of the malfunctions it sounds like you were having.

Just realign the gas block and apply some blue lock-tite to those set screws and really tighten them down. Let that set and it should fix your problems.

(Disclaimer: I am not a gunsmith. Just what I think happened. Others please chime in.)

I am having the exact same issues with my RF 7.5" upper. I ordered the BCM extractor upgrade hoping that would fix the issue but I haven't got the chance to test it out. If that doesn't fix it than it's on to a heavier buffer. And if that doesn't fix it then I'll be waiting for your results.

Is this the wrench you purchased? I just have a basic armorers tool so it looks like something else I have to pony up some cash for.

ETA - from the last picture it does look like that is the wrench you ordered.

Yep that's the exact wrench. Should work for taking the barrel off as well but I noticed you would have to take off the flash hider first - to get the gas block completely off - so that the tool will clear all the way back to the barrel nut. Then you need a new crush washer to reattach the FH.

While I had my handguard off I cleaned the threads with alcohol and then applied blue loctite and torqued the allen screws down to 25 inch pounds. Hopefully after doing this and a good cleaning of the entire gun/bolt/bore/chamber and relube with a generous application of CLP I will get better results... Otherwise I'll be looking at the JP adjustable gas blocks now that I know how easy it is to take off the stock one. I wanted to try to tighten down the barrel since everything was sort of loose on mine but I couldn't reach it without disassembling more than I wanted to. Will just have to hope RF at least did that ONE thing right.

Going to the range this Sunday to test it out again. Will report back with results.

If your system doesn't have the gas to push the mass, it's easier to reduce the mass. Use a regular carbine buffer (like another suggested earlier and you ignored).

Also, one dimple is standard. Gas blocks are different lengths, so two dimples are impractical due to different distances needed between them.

I appreciate your input and we did try the carbine buffer and semi auto carrier in our troubleshooting - nothing would cycle the .223 commercial ammo reliably. The gun was initially not super clean and had some factory residue. We lubed it in the field with CLP but I had put a coat of grease on it at home beforehand which we wiped off. After taking off my handguard which was slightly loose and finding that the gas block screws were also loose - I think a gas leak was the problem. I know the blue loctite isn't meant to withstand heat but I did torque it down to 25 inch pounds which is hopefully enough - if not I can always re-tighten it now that I have the handguard tool.

To quote my original post:

"To troubleshoot we tried swapping parts from my Stag Model 1. We tried the carbine spring and buffer in place of my H2, as well as the semi auto profile (lighter) bolt carrier. Nothing we tried would make the pistol run 100% with any commercial ammo we tried – reloads are out of the question – these light loads do function just fine in my 16” Stag though, which is why I had so many. "

TLDR – The winning combination was a semi auto bolt carrier and a carbine buffer – the rest is the details of the range day, the 300+ rounds we fired including 100 5.56 rounds and the problems we had with the supplied full auto/M16 carrier that came with the Radical Firearms upper.

- Started with M16 carrier + H2 Buffer – Clean Gun and chamber, lubed very well with CLP.
- Shot 5 single rounds of 5.56 with M3 pmag and locked open on each shot.
- We then tried small strings of 2-5 rounds and these would sometimes lock back on empty and sometimes not… We then fired 80 rounds total of 5.56 before trying something else – firing was fine it just would only lock the bolt back occasionally.
- We then tried some commercial .223 – PMC Bronze and were surprised to see that it ran and mostly extracted just fine, however it would almost never hold open on the last.
- We then swapped the buffer for carbine buffer and it ran better, even locking back the bolt on last shot most of the time.
- Lastly we swapped in my Stag Arms semi-auto bolt carrier along with the carbine buffer… The gun would then cycle anything and everything including my low powered reloads, some wolf .223 62 grain, PMC and of course 5.56.

I wanted to make the m16 carrier work because damn it – it came with the new upper and it SHOULD work with any ammo I feed it… Well with the carbine buffer we had some limited success.
For the last shots of the day I was able to get 15 rounds of Wolf .223 off without a hitch and it even locked the bolt back – the low powered reloads however would still not successfully load the next round – the first round would eject but it wasn’t enough to force to chamber the next round or lock open on last. I'm still unsure if it will function 100% with the auto carrier and the carbine buffer with .223 ammo - no doubt it will run 5.56 with this combination all day long.

The problem is with the M16 bolt carrier. It’s a little too heavy to run low powered reloads and doesn’t always work 100% with .223 either – although it was much better this time around than last week. BUT there is another issue with the carrier besides JUST the weight… I put this m16 carrier in my Stag to see if the rifle with the longer carbine length gas system and 16” barrel would cycle this carrier with PMC or my reloads – the answer was no – not at all. In the Stag the carrier was hanging up so much on the rounds in the magazine that it would NEVER go fully home unless you wailed on the forward assist or just chambered the LAST round of the mag.

With more than one round in the mag the bolt would get stuck about halfway to ¾ of the way home – whether you slapped the bolt release or just pulled and released the charging handle – made no difference. You can feel and see the resistance the bolt is putting on the rounds in the magazine – it is rubbing the rounds below the one being chambered. I have no experience with M16 carriers so I don’t know if this is a problem with the Stag gun not being mil spec/made for FA carrier or what but I noticed some additional resistance in the Radical Firearms upper too and this is the cause of ALL of our malfunctions with the gun – other than the gas block being loose after the first outing of course!

Bottom line is - use a carbine buffer, make sure the chamber is super clean and possibly try throwing a semi-auto carrier in and you have a 100% functioning gun. I just wish I could get it to work with the M16 carrier... Maybe it need polished - or sent in to Radical?

- The gun now has about ~500 rounds through it - 200 being 5.56 - although not every single one being on the FA carrier - 85%ish? We figure it's pretty well broken in at this point...?

Thoughts?


EDIT: There's more to the story and troubleshooting but I'm trying to keep the post from being overly long - We did try several magazines as well just as we did the first outing. HK Polymer clear, Pmag gen2 windowed, pmag gen 3, lancer L5 original translucent, pmag gen 2 20 round, etc. Definitely NOT a magazine issue.

EDIT 2: OK I definitely found the issue to be the bolt carrier - the FA carrier is rubbing on the feed lips of the polymer mags like pmag gen3 etc. It slides right over GI aluminum mags without really touching them much. The source of the resistance is the bolt is grinding on my mags feed lips - anything that isn't a GI spec mag. Do M16 carriers only reliably function with GI Mags?

I should have known this was the issue all along, I noticed from the get go that seating mags like the gen3 pmag and the HK translucent require a good SLAP even empty to seat into the mag well - whereas the GIs just "pop" in with little force like you would expect.

I'd send the BCG back to Radical and see if they will swap it out.

Ok so I got the problem narrowed down for sure now. It has to do with the BCG but I'm not certain it’s entirely the BCGs fault as there are many factors here like the fit of upper to BCG and my choice of lower receiver and LPK.

In this case I went with an Anderson Mfg. blem lower which actually works out ok.

Putting the M16 carrier from the Radical upper into my Stag rifle was bad news – just about any mag besides USGI aluminum would drag on the bolt carrier.

In the Stag: (for troubleshooting the BCG part)
I took a brand new gen 3 PMag and tried cycling just the bolt across the mag and there was a TON of resistance. The mag also had to be seated by a hard slap to the bottom (empty). I decided to further troubleshoot and see if the follower or the feed lips were causing the drag by taking out the mag spring and follower. The BCG still drags across the FEED LIPS of these mags. HK Mags – same story. GI mags GOOD TO GO in the Stag.

Now with the Radical upper and the M16 BCG:
Tthere is a slight resistance but its not nearly as bad as the fit in the Stag. Some mags like HK and Pmag Gen3 require a firm slap to seat empty against the BCG – this is the first indication that there is a fitment problem. Next cycling the bolt by hand I can notice a slight rubbing on the feed lips but it’s only like 10% of how bad it hangs up in the Stag upper.

Looks like my options are to run GI mags with this BCG, Use my Stag’s BCG which works flawless in the Radical Upper – OR take my chances with a new BCG like a Fail Zero.

I sort of want to keep the M16 carrier around for parts and buy a new BCG – only want to spend like $125. Is there anything better than the Fail Zero? Maybe Radical would split the difference if I return the auto carrier..