Anodizing doesn't cause unevenness; it's not something applied like paint, it's just a conversion of the existing surface. If it's uneven, it was machined that way.
- Thread starter JS8588
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Anodizing doesn't cause unevenness; it's not something applied like paint, it's just a conversion of the existing surface. If it's uneven, it was machined that way.
So how exactly are we determining that the uppers receiver face is not square to the bolt raceway?
I have a Wilson. Not impressed. The fit to the barrel extension was nothing special. Worse, the fit to the matched lower was very sloppy. Also, the hole for the front take down pin was too small. Rather than cut on the upper I made my pin a little smaller to cope. Lastly, the hole for the safety detent was too small to allow the detent thingy to pass through. That one I had to open up with a drill.
I wouldn't buy another.
I wouldn't buy another.
JP Microfit pins are great for taking out slop between receivers.
I also squared the face of the mk2 upper I posted above. It displayed slight unevenness but nothing I was surprised with. It’s not a billet upper so whatever.
I've been eyeing the BCM MK2 as well.
I built an upper off a BCM m4 and the thermo fit was tight. The barrel is a WOA 18 inch SPR. I followed Padom's advice of also using loctite even with the thermo fit. I haven't had time to really play with relaoding as much as I would like. The rifle shoots IMI 77 grain razorcore lights out. One test load with xbr 8208 shot REALLY well. Had zero vertical dispersion on target on a 5 shot group, SD was 5.7 and ES was 14 fps. More work still needed but all I can say from my personal experience is the BCM upper would be extremely tough to beat, especially for the price.
I built an upper off a BCM m4 and the thermo fit was tight. The barrel is a WOA 18 inch SPR. I followed Padom's advice of also using loctite even with the thermo fit. I haven't had time to really play with relaoding as much as I would like. The rifle shoots IMI 77 grain razorcore lights out. One test load with xbr 8208 shot REALLY well. Had zero vertical dispersion on target on a 5 shot group, SD was 5.7 and ES was 14 fps. More work still needed but all I can say from my personal experience is the BCM upper would be extremely tough to beat, especially for the price.
This is 100% incorrect. BCM MOD4 stripped uppers are 100% thermal fit and require heating up every time to install your barrel. Ive used 100's of them, I have 10+ in the safe right now and I've recommended them to members here for years. Havent had a single one yet that didnt require heating up to install a barrel. It says right on their website they are undersized requiring thermal fitting as well.
BCM® M4 Upper Receiver Assembly (w/ Laser T-Markings)
Fully-assembled upper receiver with forward assist, and ejection port cover, T-markings laser-etched into receiver, M4 feedramps, 1913 rail, made in the USA.
bravocompanyusa.com Last edited:
I’ve pulled a dozen or more of their complete uppers apart and not one was thermal fit. Every barrel pulled right and and most were .002 or so under.
They must have made a switch to the undersize units at some point obviously. The ones they are selling now are themal fit.
Well we are talking about buying stripped uppers here, like the OP asked. IDK about complete BCM uppers
If you like tight fits, try a G rail. Have to lube the barrel nut to get it on.
What is ".002 or so under"? The barrel extension?
We're talking about the receiver itself. If the hole in the receiver for the barrel extension is ".002 under", that makes it a thermal fit. While it is obviously possible to use an undersized barrel extension for a slip fit in those uppers, that is not what we're talking about here.
It should be clear when every other person commenting their BCM uppers were thermal fit that something doesn't jive here.
What the hell is a "G rail"? If you have to lube threads to get a barrel nut on, I definitely don't want one.
Tight threads are for plumbers!
I don't like improperly machined threads, despise them in fact!
I like tight fits where they're appropriate such as:
- receiver - barrel extension
- upper pivot pin bores
- upper pivot pin lugs - lower lug pockets
- lower trigger pin holes
- upper to lower gap with pins installed
You don't lube the threads, although you should be using a good anti seize anytime you put on a barrel for an AR.
The fit between the nut and rail is very tight. Gieselle makes fantastic rails , including the new SOPMOD uppers.
I sent Spikes Tactical an email inquiring about whether or not their AR15 Billet upper is thermal fit as they advertise their AR10 as such. They replied saying it definitely is.
I have a chrome lined Lothar Walther 556 barrel waiting for an upper. Spikes may get my business for that one.
I have a chrome lined Lothar Walther 556 barrel waiting for an upper. Spikes may get my business for that one.
Good to know.
I'd love to hear anyone's experience with Seekins uppers. I really like the way the NX-15 looks but if it's not a tight fit, I'm not interested.
After sending out an email inquiry this morning, Eisenach Arms informed me their side charging uppers are not thermal/press fit. Shame, I was planning on getting one for a 12 inch 6.5 grendel barrel I've got in the parts drawer.
My IRMT upper was one of the loosest fits I've seen. Pretty disappointing. Even the carrier has more play than normal.
If you learn how to shim the barrel extension, any upper can be a thermal fit. The process is a little bit fiddly trying to hold the shim in place, but not too bad and it can give you the press fit we’re looking for. Or you can make it a precise slip fit and Loctite the barrel in as some gunsmiths do. The point is it gives you the options to make the fit as loose or tight as you want. I prefer just using a BCM thermal fit upper, but shims do give the same effect.
My M4e1 upper was very tight on my Faxon pencil barrel and a Colt barrel. Needed a mallet to coax either barrel in there.
That’s my biggest complaint over forged vs billet machined. Forged aren’t nearly as consistent as machined. But I also wouldn’t be trying to build a precision rifle off a forged receiver group. A 2 MOA fighting rifle I’d do it all day erday.
Was yours the threaded M4E1 or the pseudo-monolithic version?
It's not "forged vs machined", it's just about quality. That inner hole is machined on every upper, whether it started out as a forging or a piece of billet stock. High quality forged uppers are consistent too, and low end billet uppers are less so.
Correct.
Every AR pattern forging looks like this when fresh:
Still alotta machining done to finalize everything. I'd definitely take a billet that was forged then machined over a casting that was machined.
Pro tip: Anyone with a square ⏹ forging mark on their upper/lower ... it was born at Brass Aluminum Forging Enterprises LLC
Yes.
There is no hard/fast rule that can be identified by manufacturer since there is variance in anodizing.
Manufacturers can select uppers and extensions to mate together how they want.
I had this conversation several times with JP and he said anodizing thickness variation is the main headache to deal with. He’s been thermo-fitting uppers and barrels for at least 15 years. First time we talked about it was in 2006 maybe.
Last time was in 2016 down at Blue Steel Ranch.
There is always a set of manufacturing variances in parts. You can hold yourself down to extremely tight machining dims for the ID of the tunnel, but anodizing will add a wider range in surface thickness.
Then there is the extension, and a lot of extensions get warped a little bit depending on when they do the heat-treat and feed ramps in their sequence.
So yeah, there is no, “Here’s the list of companies......"
It all comes down to the 2 parts and how they fit. You will need several part samples until you find the right combination, or you have the extension made to not fit your upper without heating of the upper and cooling of the barrel.
I get that this is technically correct, but BCM, Precision Firearms, Mega/Zev, & Lantac side chargers are ALWAYS thermal fit. Even JP advertises their stripped uppers as such.
My last zev was so tight (giggity) I had to have the barrel extension turned down just a hair so I could get the thermal fit to work without having to basically turn the upper red.
It depends on the barrel extension. BCM are absolutely on the undersized/minimum end of the spectrum and MOST of the time require thermo-fit, but not 100% of the time. I've had 3 different barrels the past 2 years that had undersized extensions that slid right in on BCM uppers.
Bcm upper do have a slightly reduced barrel extension opening on thier upper recievers. All 6 of the uppers i have from them have had to be hit with heat in order to install the barrel. In fact BCM acrually inserts a small typed/copied note explicitly stating this.
Attachments
BCM uppers have been my go-to for a while. They've been thermal fit for every barrel I've used (LaRue, Faxon, PSA, Anderson Mfg, Wilson Combat, Alexander Arms) EXCEPT one. That exception was a Bergara barrel from PRI that was as tight as could be without actually requiring heat to open up the BCM receiver. There was no wiggle room once it got seated, but I added a bit of Loctite to make sure there was no play whatsoever.
On a related note, the Wilson Combat barrel I used must have been oversized, because it was a thermal fit into a standard Aero Precision upper receiver. That same upper had play with every other barrel I could test-fit, so it just goes to show that there's variance on both the receiver and the barrel extension. I guess if you can't find a thermal fit upper, you could try a Wilson Combat barrel?
On a related note, the Wilson Combat barrel I used must have been oversized, because it was a thermal fit into a standard Aero Precision upper receiver. That same upper had play with every other barrel I could test-fit, so it just goes to show that there's variance on both the receiver and the barrel extension. I guess if you can't find a thermal fit upper, you could try a Wilson Combat barrel?
onto a Expo Arms 10.3" barrel. 
I guess that explains why that one BCM upper was so damn tight...
Uppers are fairly cheap and it's super easy to bed one. If it's a high end barrel, I'll just loctite it in there permanently with green 620 (I think it's 620) after I clean up the face. I can salvage small parts off the upper, the handguard, maybe the gas block if I wanna keep it, and then the upper gets trashed with the barrel.
I like the VLTOR MUR1 or a good forged upper. The MUR1 is pretty solid and they tend to mate up well with LMT, KAC and Noveske lowers.
Uppers are fairly cheap and it's super easy to bed one. If it's a high end barrel, I'll just loctite it in there permanently with green 620 (I think it's 620) after I clean up the face. I can salvage small parts off the upper, the handguard, maybe the gas block if I wanna keep it, and then the upper gets trashed with the barrel.
I like the VLTOR MUR1 or a good forged upper. The MUR1 is pretty solid and they tend to mate up well with LMT, KAC and Noveske lowers.
This is bad info on a couple different points.
1 - Bedding a barrel extension to the receiver with Loctite is good, but not as good as a thermo-fit (with or without shimming). Metal flexes, and having that receiver stretched around the barrel extension is a more solid attachment than bedding.
2 - A bedded or thermofit upper doesn’t need to be trashed with a barrel; you can remove them without too much trouble. A little heat and a soft mallet does the trick either way. I’ve done more than a couple and it’s not a big deal to drive the bare out with a reaction rod.
A 1" wooden dowel has always done the trick for me. Deforms the wood but I'd rather it be the sacrificial part of the equation anyways.
Yeah I expect that’d work fine too.
BCM - M4 blem I bought earlier in this thread is now together.
The thermal fit does make blending the feed ramps a PITA, but I'll take that trade off vs shimming the extension with .001 SS shim stock.
1" Aluminum Bar is what I used for removal, but Wooden dowel would also be GTG.
I know this is a thermal fit upper thread, however it's worth mentioning that I don't build any upper without fitting an Accushim.
I hate loose fitting rattletrap AR's & fuck those little Red plastic Acuwedge's.
BCM - M4 Blem .224 Valkyrie 18" Craddock criterion 7T rifle +1" gas is showing some promise with factory FGMM 80.5 Berger's.
Shot using a Gen 1 Ckye pod, rear bag, & trigger is set @ 3lb.
FGMM 80.5 Berger's
Cheapo American Eagle 75gr TMJ
The thermal fit does make blending the feed ramps a PITA, but I'll take that trade off vs shimming the extension with .001 SS shim stock.
1" Aluminum Bar is what I used for removal, but Wooden dowel would also be GTG.
I know this is a thermal fit upper thread, however it's worth mentioning that I don't build any upper without fitting an Accushim.
I hate loose fitting rattletrap AR's & fuck those little Red plastic Acuwedge's.
BCM - M4 Blem .224 Valkyrie 18" Craddock criterion 7T rifle +1" gas is showing some promise with factory FGMM 80.5 Berger's.
Shot using a Gen 1 Ckye pod, rear bag, & trigger is set @ 3lb.
FGMM 80.5 Berger's
Cheapo American Eagle 75gr TMJ
Iron city rifle works, very nice tight fitting billet receiver sets very reasonably priced.
....just my personal experience, YMMV...
...back when stripped uppers were going for cheap in 3-paks, I picked up a couple "just bcuz". Whenever I bought barrels, I would test fit them in the uppers until I found the tightest fit and use that upper/barrel combo for that particular build. I'd do this even if I was replacing a barrel, that upper may or may not be used with the new barrel. IMHO and experience, the barrel extension in use by the barrel can vary, even within the same brand of barrel depending on date of manufacture and the supplier the barrel maker used at the time.
...having "extra" stripped uppers makes it easier for me to do the barrel matching. I've had some uppers have a loose fit with one barrel, but take some "coaxing" to seat a different barrel.
...BTW, I have a mix of 1st's and "blems"....never found a difference in functionality, ever, let alone WTF was the "blem"...
...back when stripped uppers were going for cheap in 3-paks, I picked up a couple "just bcuz". Whenever I bought barrels, I would test fit them in the uppers until I found the tightest fit and use that upper/barrel combo for that particular build. I'd do this even if I was replacing a barrel, that upper may or may not be used with the new barrel. IMHO and experience, the barrel extension in use by the barrel can vary, even within the same brand of barrel depending on date of manufacture and the supplier the barrel maker used at the time.
...having "extra" stripped uppers makes it easier for me to do the barrel matching. I've had some uppers have a loose fit with one barrel, but take some "coaxing" to seat a different barrel.
...BTW, I have a mix of 1st's and "blems"....never found a difference in functionality, ever, let alone WTF was the "blem"...
Very much a thermal fit upper.
bkingsfirearms.com
BKF M4 MOD-0 Stripped Upper Receiver – Black (Light T-Marks)
The BKF M4 MOD-0 Stripped Upper Receiver is a step up from our regular BKF-15 stripped uppers. The M4 MOD-0 uppers are held to tighter tolerances, Dry film lubed and tight fitting when paired with our M4 MOD-0 Lowers. Forged…
bkingsfirearms.com Very much a thermal fit upper.
BKF M4 MOD-0 Stripped Upper Receiver – Black (Light T-Marks)
The BKF M4 MOD-0 Stripped Upper Receiver is a step up from our regular BKF-15 stripped uppers. The M4 MOD-0 uppers are held to tighter tolerances, Dry film lubed and tight fitting when paired with our M4 MOD-0 Lowers. Forged…
Thanks for the heads up. Got one inbound.
I like the BCM uppers, but they've often been out of stock. If these are as good, they'll be a nice alternative.
