In addition to recording a series of internal and external measurements, I visually inspect every barrel that comes into my hands from end to end, inside and out. Special attention is paid to the crown, feedramps, gas port, muzzle and bore. The crown is checked under high magnification for burrs, gouges or other imperfections that can cause projectiles to leave the barrel inconsistently, diminishing potential accuracy in the process. A magnifying glass or jeweler’s loupe and a good light will go far to help you see how this critical transition from bore to muzzle face has been cut. Most barrels have some form of recessed crown that are cut at varying angles, depending on the maker and barrel type. My experience is that the degree of crown is far less important than how cleanly cut and uniform it is.
Damage to a new barrel’s crown can be caused at any point along the way between production and installation and this is certainly not unique to the AR. Crown damage on any rifled barrel should be carefully evaluated to ensure it is not going to cause problems during firing. If the barrel arrived with an already-damaged crown, I would contact its source in hopes that they will exchange it for a new one before troubling yourself to fix it.
Flawed or damaged crowns can be cleaned up through the use of either hand crowning tools or a machine lathe. If you are only dealing with a single barrel, the most cost-effective route to repair is through a gunsmith. However, if you lack a lathe but are regularly dealing with banged up crowns, investing in a good hand crowning tool will pay for itself in the long run.
After checking the crown and if the muzzle is threaded, I always “chase” the threads with a properly-sized die to ensure they are fully cut and cleaned up. Just behind those threads, the shoulder is the next area that I check. If you plan to use a sound suppressor, this shoulder should be square with the bore and threads to ensure proper suppressor alignment.
This is true for both direct-thread suppressors and those that use QD mount-style muzzle brakes and flash hiders. If your muzzle device seats on the face of the muzzle instead of the shoulder, the muzzle face will need to be checked instead. I seldom see damage in these areas, but occasionally a barrel comes from the factory with a flaw that would prevent a muzzle device from seating squarely. I use a lathe to slowly spin barrels while I check runout [with gauges] on shoulders or muzzle faces, then remedy any flaws at that time. Again, a gunsmith or machinist can perform this check in just a few minutes and even if it needs to be re-faced, it is a simple operation.
Whether or not you have this check performed, I strongly recommend that at a minimum, you use an alignment rod to verify that your suppressor and bore line up correctly before live firing the system.
Sliding back along the barrel a few inches, we should find a gas port that is drilled at top-dead-center of the barrel when mounted in a receiver. Most manufacturers drill their ports once the barrel extension has been installed so that the index pin, which nests in the upper receiver’s index notch, actually performs its namesake function. Nonetheless, once in every hundred or so barrels, I find one that has the gas port and index pin out of alignment. If the misalignment is more than a couple degrees, there is a pretty fair chance that a properly timed gas block will result in reduced/restricted gas flow, impeding operation and increasing wear of the gas block itself. Unfortunately, there aren’t any great options to repair this problem. If you lack the correct tooling to install and remove a barrel extension, this manufacturing error is best left to a gunsmith or (preferably) the manufacturer to remedy.
A simpler and far more common problem is having an undersized gas port. A decade or so ago, barrel makers tended to oversize gas ports, especially on service-grade barrels. The prevailing wisdom being that it was better to have too much gas and keep the gun running rough and dirty than too little gas preventing it from running at all. It was left up to gas block makers and gun builders to figure out how to modulate the excess gas, especially when using a sound suppressor.
The situation is nearly the opposite today, as both match and rack-grade barrels are frequently sold with ports that are either undersized or only marginally functional as-is. Therefore, gun builders oftentimes must open ports according to the configuration of the gun and its intended use. Opening a barrel’s gas port is not difficult, but knowing how far to open it is a critical part of the process. I measure every gas port that comes across my bench and cross-reference it with a database of the 1000+ barrels I have previously measured to ensure that the size is appropriate for the length, caliber, gas system, gas block type, planned suppressor use and bolt carrier/buffer system weights.
Well, he’s a better mechanic that I am, he has a world of experience, and he has tools that I don’t.
