In preparation for this year's squirrel-mageddon trip I decided to change the barrel on my squirrel killing AR-15. Last year I used a Rainier Arms Ultra Match 18" barrel, which proved to be pretty accurate, but which also has a 1:8 twist. That's a great twist rate for 55 gr and longer bullets, but it's slightly aggressive for the short 40 gr V-MAX bullets I use on ground squirrels. I thought that by getting a barrel with a 1:12" twist I could get a little more accuracy.
Fortunately White Oak had one last 20", 1:12 twist varmint barrel in stock when I called them, and about a week later the UPS man was at my door with a shiny new barrel for me. Their "varmint" contour is quite thick—0.936" in diameter at the gas port. The included gas block brings the diameter up to 1.5", which is larger than the inside diameter (ID) of the BCM KMR rail I was using. So, changing the barrel meant I needed to change the handguard as well.
(This is a problem that folks with suppressors have, too, if they want their suppressors to fit under the handguards. Most handguards try to be as thin as possible, with 1.2" being a pretty common ID. Most suppressors are 1.5" in diameter, though, and only a few brands of handguards will accommodate something that large.)
I thought that ALG's handguard would fit the bill, having an ID of 1.55", but when that arrived it turned out that there are splines inside the handguard that reduce the ID to less than 1.5". Boo! I had to return the ALG handguard and find something larger—and in stock, since I quickly needed to work up a new load with the new barrel before handloading 1,000 rounds for the trip.
I found some folks discussing Aero Precision's rails on some of the forums, and fortunately their rails have a 1.8" ID, they were in stock, and Aero Precision could ship one to me before the weekend. So once again I waited impatiently for the mail to arrive with a rail that would fit over my new barrel.
The Aero Precision rails are designed to mate with their "enhanced" AR-15 upper, which has a non-standard trunnion protruding from the front. The rail attaches with eight screws to the trunnion, and the barrel nut is surrounded—but not touched—by the trunnion. It seems like a pretty good system if you don't mind non-standard parts and are sure you'll like the handguards Aero Precision makes. If you don't have an Aero Precision enhanced upper, however, you need to use a special BAR barrel nut which acts like both a barrel nut and the trunnion on the enhanced upper.
The BAR nut, though, is a compromise and is my least favorite aspect of Aero Precision's rail. Like an old school barrel nut, you need to time the nut so that the gas tube can pass through one of the holes drilled through it. However, unlike an old school barrel nut, there aren't holes evenly spaced around the circumference. Instead, there are only four sets of three holes. This means you need to use shims (included, fortunately) to get the nut to index correctly. This also means that you have to be absolutely precise in your timing of the nut as any deviance will result in a top rail that is slightly rotated in comparison with the upper receiver's rail.
Even worse, because the gas tube holes aren't evenly distributed around the circumference of the nut, you can't use many typical AR armorer's wrenches. I have a nice one with five pins, but only three of the pins line up with the gas tube holes in the nut. So, I had to buy another (cheaper) armorer's wrench and Dremel off several of the pins so that the remaining pins would match gas tube holes in the BAR nut. (Apparently Tapco makes a three-pin wrench that the folks at Aero Precision use, but I couldn't find one locally and couldn't wait for one to arrive by mail order.)
Aside from the non-standard, finicky BAR nut, the rail itself seemed to have two of the eight mounting holes drilled slightly out of spec—they didn't line up perfectly with the holes in the BAR nut, so it took some finesse to sink those two screws. (Of course, it could be the nut that's out of spec instead of the rail, but I don't have a good way of determining that for sure.) And, eight screws? Really? Eight? Most other rails only need two, and I don't see an advantage to having to sink eight screws instead.
So, overall I'm not impressed with the BAR nut mounting system, but at least the rail was large enough to fit over my new barrel so that I could do my load development over the weekend. I'll see how robust it is over the next few weeks, and I'll probably contact Aero Precision to ask about the alignment problem with the mounting holes. I just don't have time right now to deal with returning parts.