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Why did I purchase a Hornady headspace comparator? Let me explain.
As a shooter that also reloads his own ammo, I did it for a couple of reasons. First, it allows me to save some money because I can buy the reloading components and produce my own cartridges for less than it costs for factory loads at a sporting goods store, gun shop, or some big-box store. Second, it allows me to load cartridges that are custom built to perform the best they can in my firearm. My 9mm and .45ACP reloads, made on my Dillon 650 press, pretty much support the first reason I mentioned. When it comes to building precision rifle ammo, the second reason is what it is all about if I'm expecting sub-MOA accuracy.
With the recent purchase of my 6.5 Grendel Overwatch AR upper, I needed to raise my reloading to the next level. You don't find Grendel brass laying around at the local shooting range. Lapua brass runs about $80 per hundred so maximizing the number reloads I can get from a piece of brass helps take a little bit of the sting out of the price. My Overwatch is more than capable of producing very tight groups and I wanted to exploit that feature for my varmint hunting loads as well as heavier loads for deer hunting. By adjusting my full length resizing die specifically for the headspace measurement of my rifle, I'll reduce the amount of work the brass experiences which boosts the number of reloads per brass. It is also a step in the right direction for building higher accuracy ammo. A headspace comparator is just the thing to help in obtaining the headspace measurement for setting up my resizing die. There are other methods but I decided to use this one.
Here is the Hornady Lock-n-Load Headspace Comparator Kit. It does NOT come with the vernier calipers.....those were purchased back in the '70s when I first started reloading. If you haven't obtained a vernier caliper by now and you are already reloading, you really need to resolve that issue.
The kit comes with an adaptor body (red in the above pic) and 5 bushings. A bushing is attached to the body using the included hex key. The entire assembly is slipped onto the jaw of the calipers and secured in place with the brass thumb screw.
The hole in the end of the bushing contacts a spot on cartridge's shoulder which is referred to as the datum line. In the above pic, a piece of Laupua brass that I had loaded and fired in my Grendel has been inserted into the bushing. The caliper jaws have been closed onto the head of the case and the caliper's dial has been set to zero. This is the headspace that exists in my rifle. I checked this using several pieces of fired brass and found consistent readings each time, indicating I had a good measurement for my rifle's headspace. It makes no difference what the actual length is but just that I have the dial set to zero using the fired brass.
At this point, the case is removed from the headspace comparator and inserted into the resizing die (with proper lube, etc.). For a semi-auto rifle, like my 6.5 Grendel, the die is adjusted in the press so that the shoulder is pushed back (during the resizing stroke) somewhere between .003" and .005". In other words, the distance from the case head to the datum line on the case will be reduced by .003"~.005". A case used in a bolt action rifle would probably be set back about .001"~003".
It is important to set the shoulder back the proper amount. As such, check the brass after resizing it to see if the necessary set back has been achieved. If not set back far enough, screw the die into the press just a bit further and resize again. The bolt may not properly close when the cartridge is going into battery if the shoulder of the case is not pushed back far enough. Proper set back helps reduce the possibility of a slam fire which is possible in an AR rifle since the firing pin floats in the bolt. If the shoulder is set back too far, such as .010", the brass is being unnecessarily worked and this can certainly result in a reduced number of reloads.
In the above pic, I had inserted a round of Hornady factory ammo into the comparator and then checked it, having already zeroed it on a piece of fired brass. As you can see, the headspace on this unfired case is just over .011" shorter than the fired brass I zeroed the caliper on. What this means is that this piece of brass will stretch (from the case head to the shoulder datum line) .011" when I pull the trigger. That is more than needed (in my rifle) but there is nothing I can do about it this first time it is being fired.
In the above pic, I had inserted an unfired piece of Lapua brass into the comparator. As you can see, it is even shorter than the unfired Hornady cartridge, this one being a generous measurement of .017". The Lapua brass will get stretched even further than the Hornady brass when it is loaded and subsequently fired.
So you may be wondering why the factory brass appears to be far "shorter" than it needs to be? Or maybe it is that the chamber on my 6.5 Grendel is just a bit too deep? Brass and cartridge manufacturers make their headspace to minimum SAAMI spec and this ensures their brass will fit in a chamber that is at minimum headspace. At the same time, if Alexander Arms, the manufacturer of my upper, wants to make sure almost any cartridge will fit, they might be inclined to bore their chamber just a bit long. Who knows? I don't, but I do know what my head space measurement is and I know how much I have to bump the shoulder back to ensure proper chambering in my rifle and maximum brass life. Take the time to measure your rifle (every chamber is different and so is the headspace) and get your full length resizing die set up correctly too.