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Posted: 1/28/2026 12:38:22 PM EDT
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I just put a Criterion in to a BCM upper yesterday, and wowza was that a tight fit. I've built between 2 and 3 dozen uppers now, and I've encountered a few tight ones, but that was really something. It took all the tricks in the book -- barrel in freezer, heat gun to upper, etc. -- to get that barrel in, and even then, that last half inch was no picnic. I've had a several especially tight ones with both BCM uppers and some other tight-throated uppers, but this seemed like the tightest barrel I've encountered. What's the tightest barrel/upper fit you've encountered when building? |
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Personally I haven’t encountered many because I avoid buying thermal fit uppers and barrels with barrel extensions that are known for tight fits. The tightest fit I’ve come across was between an RRA A2 upper and a Windham Weaponry 20” A2 barrel. I had to take a heat gun to the upper to get it to go together, but it really wasn’t all that bad and didn’t require much heat. |
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Originally Posted By bluedog82: For those who might not know, BCMs are indeed a thermal fit and require a torch or heat gun to fit. True that. There are other brands of uppers that are thermal fit, too -- B King's M4 uppers, for example, IIRC. It's the result of cutting the inner diameter on the very narrow end of specs to intentionally create a more secure fit for the barrel. Even with a thermal fit upper, it is possible to get barrels that aren't tight if their barrel extensions are near or below the low end of the barrel extension diameter spec. It can be a pro or a con, depending on if you want your barrel fit that tight and if you are set up for the work. And the work, including the torch/heat gun, chilling barrel in a freezer and even pounding using a mallet (with a technique and type that won't damage the barrel), are to be expected with a thermal fit upper. BCM's uppers come with a statement saying to heat them with a heat gun on low or a hair dryer. Some especially tight combinations require a heat gun on high or a torch to heat the upper enough that the barrel will fit. My curiosity on what combinations are the tightest fits isn't meant to be a judgement on if thermal fits uppers are good or a comment on product quality, either. Whether you want to avoid it or get it, these combinations are of interest... and can change over time. |
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Originally Posted By Rkangel777: Too many variables. The tightest receivers I have used are BCM. Even then every single barrel I have used fits differently, even those from the same manufacturer. If you are looking for the tightest fit, it will be trial and error. Nope, not too many variables, as this isn't a quest to find the tightest out there. Just curious what combinations individuals have personally encountered that were the most challenging. Ya know, technical but laid back conversation about building ARs. |
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Have had a couple that some 320 or 400 grit paper was needed to sand down the anodizing a half thousandth or so and reduce the uppers internal diameter a bit to get the extension to slide in. I don't buy into the thermal fit crazy since you don't even know if the extension has any torqued value clamping it to the upper. How do you determine that the barrel extension is actually clamped with some torque value against an upper if there is some force needed to move one part closer in contact to the other when there it a thermal fit resistance to overcome added to the tightening torque? |
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