Thermo fit by JP

[montana]

I have done this for years since I started cerakoting my personal rifles. I cerakote the outside of my barrel extension, which tightens the fit to the upper receiver. Add some heat with my butane torch and it slips right in. I have used this same method for removing the bearing cups from aluminum hubs.

https://www.youtube.com/watch?v=-cOlNGxUhsk&feature=youtu.be

[drewthebrave]

Good info. For what it's worth, I've done this with the BCM Blemished upper receivers. They're much less costly than JP uppers, but still have the tighter tolerance so they're good for builds where the budget might be a little tight.

[montana]

Good info. For what it's worth, I've done this with the BCM Blemished upper receivers. They're much less costly than JP uppers, but still have the tighter tolerance so they're good for builds where the budget might be a little tight.

I did not know this about the BCM uppers. Keeping the barrel extension tight and square to the upper receiver is paramount when accuracy is a priority. Some gunsmiths keep different sized barrel extensions for a perfect match to the upper's BCG channel. Some drill and tap screws into the bolt carrier to keep the BCG perfectly aligned with the barrel. When LRRPF52 first recommended lapping the upper receivers, I thought OK and I gave it a shot . It improved my 18" Grendel's accuracy by leaps and bounds. I have learned quite a bit from this forum and it has made my AR building projects a very enjoyable experience.

[drewthebrave]

Same here!! Lots of wiser heads than me here, and I'm doing my best to learn what I can. I'm holed up inside while recovering from knee surgery, so I'm planning to disassemble some of my old uppers, and then reassemble with my newfound knowledge. Lapping, bedding, polishing, etc. Mostly looking for a semi-productive way to spend my time in the coming months.

[montana]

Same here!! Lots of wiser heads than me here, and I'm doing my best to learn what I can. I'm holed up inside while recovering from knee surgery, so I'm planning to disassemble some of my old uppers, and then reassemble with my newfound knowledge. Lapping, bedding, polishing, etc. Mostly looking for a semi-productive way to spend my time in the coming months.

That is quite the coincidence. I'm dealing with an old knee injury, looking at a replacement knee and have been doing the same with my AR rifles. Nothing like a knee injury to make up for much needed time for AR improvements and build knowledge. I've had 6 knee surgeries since the late 80's and none of them were fun. My sympathies!

[SightedIn]

[montana]

I do it a little different. I Cerakote the outside barrel extension with H type Cerakote and cure. If the OD is too much it is easier to remove. I have become pretty good at judging just the right amount of Cerakote to apply. This has always created an over sized barrel extension to upper barrel channel. I then place the barrel into the freezer, and let set over night. I then Cerakote the lapped receiver, but just flash cure since I usually add stencils and then finish the Cerokote job. The barrel channel was previously Cerakoted with air dry micro-slick at least a week prier, so the inside is taped off with heat tape when Cerakoting with H product. I then cure the Cerakote for 1 hour at 300 degrees. When cured, I add green loctite to the cold barrel and slip into the upper. After the upper cools,"but not all the way" I do the three barrel nut tightening and loosening procedure before tightening to spec. I micro-slick my bolt carriers and upper receiver BCG channels and have had excellent results. I use an adjustable gas block and a JP Silent Captured Recoil spring assembly. For my Grendels, I use one tungsten weight and for my 5.56 rifles I use the standard weights. The weights are easily changed as are the springs if needed. My favorite trigger is the American Gold for competition, but for an over all AR rifle the LaRue has become my go to trigger. It is the best value for the buck I have seen in AR triggers. I love to set my standard AR or someone elses on the shooting table and let them shoot mulitple close targets with a timer. I then let them try my tuned AR and watch their faces. It is hard to explain the difference, "especially with a 5.56 rifle" but they are believers after they shoot a standard AR next to one of my tuned AR's.

[snarkscarbine]

JPs method works like a charm. I did it on a garbage PSA upper/barrel I had sitting around along with a 0.002" shim to create a 0.001" interference fit. Haven't shot it yet, but the barrel slid right in while it was hot, and was like 1 piece with the upper as soon as it cooled.

[Djgrendel]

Good rule of thumb is .001" of press fit per inch of diameter. So a 1" bore would be served with a 1.001" bushing. You can get away with a bit more, but usually .001 per inch is plenty. I use liquid nitrogen to freeze the barrels. It will shrink the extension .004 under size in about 3 minutes. The thermal press fit will happen in less than a minute.

[LRRPF52]

JP told me about this over the phone back in 2007 when I was asking him about doing 300 SAUM in the AR10, as well as his thoughts on Quick Barrel Change uppers.

We had several lengthy conversations at Blue Steel Ranch in 2016 when I was RO'ing for Steel Safari.

His story is pretty crazy how he got into all this.