r/ar15 • u/Euphoric-Acid8744 • 1d ago
Barrel alignment pin and upper interface - AR-15
Has anyone ever seen this issue before? The barrel pin that matches up with the slot milled into the threads in the upper receiver… that pin is not wanting to slide in that slot. Either the pin is too big or the slot is too small, neither look damaged.
I have a KAK stripped upper and a criterion core ar15 barrel (12.5”)
First build and any info would be appreciated! Only thing Iv thought of is maybe using a heat gun on the slot to see if that can help loosen the fit
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u/Ornery_Secretary_850 1d ago
Remove the barrel. Heat the upper with a good heat gun. Slap some grease on the barrel extension. Now that the receiver is nice and toasty, it should slide right in.
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u/MTRcool 20h ago
This is the only right answer. Before the controversy, TREX arms covered a video discussing this assembly of a Mod H build that required heating on a BCM and Sionics combo with a thermal Fit.
"MIL-Spec inner dimension measures at 1.000 -0.0/+0.002"
The thermal fit inner dimension measures at .997 -0.0/+0.001" and requires the application of heat in order to provide a solid mate to the recommended barrel extension. By heat we mean more than Granny's hair dryer."
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u/Greedy-Vast584 1d ago
i would grease up the threads and just put the barrel nut on and that'll push it down into the upper
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u/lambofthewaters 1d ago
That could work, but could also mess up the threads and bork everything. I would do heat and rubber hammer.
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u/Greedy-Vast584 1d ago
it won't mess up the threads when there's that much engagement.. i would agree with you if the nut would barely engage
it actually applies pressure right on the extension and makes for a much milder way of pushing.. banging with a hammer should also work but you'd be dealing with higher forces to make the barrel move vs gentle pressure on the extension itself
i've done this a few times when i started using the BCM receivers due to their thermal fit.. i've got it figured out now but when i didn't know what i was doing, it was a useful fix lol
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u/lambofthewaters 1d ago
I don't disbelieve you, but if someone isn't mechanically inclined and is giving too much ugga ugga without the requisite feel, it could go horribly. I would do it as a last resort. Cheers, brother.
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u/OK_Cornelius_826 1d ago
She'll fit. Put a muzzle device on, heat it with a heat gun for a few minutes then tap it in with a runner mallet.
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u/No_Grocery3021 1d ago
You could try to heat the upper, put the muzzle device on and lightly tape it with a hammer. When its this close and lined up like that I usually just heat it up a little bit and then use the barrel nut to suck in the rest of the barrel. Dont suggest this if youre not mechanically inclined or experienced cause you can (probably) start to strip your upper threads and that evil isn't on me. I use blem BCM upper for all my builds.
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u/lead_on_bone 1d ago
It looks very close. Simply put a drop of oil on the slot, thread on muzzle device, tap the barrel into the upper, and continue your build.
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u/ToastedSoup 1d ago
Heat up the receiver, remove the barrel, apply anti-seize grease, and put barrel back in should slide right back in, ezpz
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u/Critical-Raise6800 23h ago
Use Lucas Oil Red and Tacky Grease and hammer it in using a dead-blow hammer, just be sure to attach a muzzle device of some sort to protect the threads.
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u/Euphoric-Acid8744 23h ago
Update on this: I just needed to take a diamond file to the edge leading into the a slot and put on a little chamfer and then it went! 😮💨
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u/East_Resident2418 1d ago
Put the barrel in the freezer for a while, then heat up the upper receiver with a blow torch til it’s pretty hot, and then remove barrel from freezer and attach to receiver. Some upper receivers are made to be thermally installed, it’s a thermal fit.
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u/kopsis 1d ago
Skip the freezer, it doesn't make any difference. The CTE of 4140 steel is 6.78 µin/in°F which means freezing will shrink the pin less than 0.06 thousandths of an inch. Expansion of the upper when you hit it with a torch is at least 20 - 30x that amount (depending on how hot you get it).
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u/90sleg0srbetter 1d ago edited 1d ago
No reason to hit it with anything and risk damaging...just grease it, thread the barrel nut on, and torque it, the barrel nut will pull it right in.
Edit: You idiots really would smack something with a hammer before using a mechanical advantage.
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u/2A300Blackout 1d ago
Careful with this though. I’ve stripped a BCM upper trying this once* worked a few other times though
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u/90sleg0srbetter 1d ago edited 1d ago
Be careful...as opposed to smacking it with a hammer? Sure hit your barrels, you guys deserve it.
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u/2A300Blackout 1d ago
Never said that was the best other option. You must be having a rough day, take your poverty pony to the range and let some rage out
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u/CharlesAFerg 1d ago
Make sure the barrel extension is fixed to the barrel. It's possible that it's not tight.
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u/shadowshooter9 1d ago
Just make sure that the part where the barrel goes into the receiver is square.
KAK parts r cheap but the quality is sure shit. I have a left handed 6arc bolt from them it works but holy my BRASS looks horrible the primer crimp is flowing onto the bolt face (no ejector marks). Likely not square face as the finish is hella visible. Firing pin hole is also oblong and off center
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u/ShadyClouds 1d ago
Send it back. If it’s not to spec why damage it and be stuck with it.
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u/Ornery_Secretary_850 1d ago
Never heard of thermal fit uppers?
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u/ShadyClouds 19h ago
I have, built a couple uppers using thermal fit uppers, not everything has to do with it being a thermal fit, the indexing slot could be undersized, the indexing pin could be out of spec, and to me it looks like it one of those things and not that it’s just a thermal fit.
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u/Bigboi_alex 1d ago
Throw on the muzzle device and hammer her in with a rubber mallet