THERMAL FIT
Some AR15 upper receivers require the barrel and upper receiver to be thermally fitted together. The upper receiver is designed to be very slightly undersize, to ensure a very tight fit between the receiver and the barrel extension. Thus eliminating the potential of free space between the two and potentially improving accuracy and long term durability.
Thermal fitting is easily accomplished with the right tools and patience. .
We recommend using a heat gun or propane torch. A hair dryer can work as well in a pinch but may require more time. Another option, which allows more control of the process is a small cone style bearing heater, such as this product from McMaster Carr https://www.mcmaster.com/6519K11/ These allow you to set a maximum temperature and work quickly and efficiently.
Ideally, a barrel vise would be used to hold the barrel but those aren’t common on many work benches. A good, inexpensive option is the ATLIN magnetic soft jaws from Amazon. For 12 to 20 dollars, it adds a lot of versatility.This guide will help those of you without a barrel vise.
The goal is to evenly heat the front of the receiver to help it expand. Be careful not to apply too much heat, especially in one small area of the barrel seat extension, as it can substantially weaken the anodizing on the receiver. Especially with the “Bright Dip” type of anodizing.
When using a heat gun or torch, begin by evenly heating your receiver. Paying a tiny bit of extra attention to the upper portion where the receiver is thickest. It is best to constantly rotate the receiver as heat is applied. It may be best to secure the heat gun in a vise and hold the receiver rotating it so heat is evenly applied and to lessen the heat you feel in the hand holding the receiver. A leather “driving” type glove isn’t a bad idea.It is also advisable to use a non-contact thermometer to make sure you don’t get the receiver too hot. 175 degrees fahrenheit is generally plenty
When using a cone type bearing heater, set the temperature on the thermostat to 175 F, place the threaded bore of the receiver on the cone of the heater and wait for the beep. Again, use a non- contact thermometer to confirm the receiver is up to temperature over the full depth of the barrel seat extension.
Once the receiver is hot, very quickly install the barrel. The aluminum will cool down very quickly so you have to move fast. Move quickly and decisively. Find a solid surface and place a non-marring barrier between the receiver and the bench. A microfiber cloth, thick piece of leather or a small piece of UHMW works very well for this. Set the receiver upright and quickly insert the barrel into the receiver. The barrel should slide right in WITHOUT excessive force. A light coat of Slip 2000 on the BARREL EXTENSION, may ease the process some. DO NOT try to hammer the barrel into the receiver.
It is imperative that you do this very quickly. The receiver will begin to cool immediately and seconds are very valuable. If it does not go in, try heating it up again and repeat the process. DO NOT try to wiggle the barrel in or out. It is EXTREMELY important to insert the barrel in straight and in line with the bore of the receiver.
It can also be helpful to place the barrel in your freezer overnight to allow the barrel extension to shrink slightly.
Sometimes, the barrel will go most of the way in but not all the way. If you can get several rotations of the barrel nut, you can tighten it down to get the barrel extension the rest of the way in. Make sure to apply aeroshell and only tighten a little at a time. Backing the barrel nut off periodically to make sure the barrel index pin is going in straight to the notch in the upper receiver. This is CRITICAL.
Once it is fully seated, you can continue with the process of assembling the upper.
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