I have been spending my day pondering all things "variable" in my reloading world. Nice to have a holiday weekend with no other plans. I have the Hornady Lock-N-Load OAL Gauge. I like it very much as it allows me to measure bullet/cartridge combo out to the lands and then back off the lands accordingly. In other posts, there are mentions that there are little problems here and there. One in particular is that the machined "dummy" cartridge that Hornady sells may be shorter than your sized brass. This particularly applies to folks that neck size, or size only 1-2 thou off the shoulder to reduce working of the brass. It has been written that you should measure the "dummy" and your own sized brass and then "correct", by adding the difference. Assuming the dummy is shorter than your own brass. Now, if you fail to do this correction, assuming you brass is LONGER than the dummy, what will happen is that your final cartridge will have a datum/shoulder to ogive (what we really care about anyhow) that is SHORTER than what you expect. So if you are trying to seat exactly to the lands, and your brass is 5 thou longer than the Hornady dummy, you will end up 5 thou off the lands. Now, why I feel the need to explain this , is because I decided I should measure the Dummy "just to see" -- and I discovered that my brass is SHORTER than the Hornady Dummy. That means that if I were trying to seat to the lands, I would have ended up jamming the bullet into the lands. This whole discussion serves to underscore the problem with trying to seat "to the lands". Its got a pile of variables that can add up to variation and possible dangerous pressures, or at a minimum, a stuck bullet while hunting. If you are using the Hornady Lock-N-Load OAL Gauge, you need to be aware that the difference between your Dummy and your own resized brass needs to be accounted for if you are trying to seat to the rifling. As to why my brass is SHORTER than the Dummy -- I do not know. It is 2 thou under my fired brass when measured with the Headspace Gauge. When I put the Hornady Dummy into the rifle, the bolt closes with a little resistance, indicating to me that the headspace of the Dummy is long for my gun.... I measured a couple unfired Rem factory loads. They are 2 thou less than my Hornady Dummy as well. I find the Hornady OAL Gauge very easy to use, and I've been a proponent of its use. But this puts a tarnish on its reputation, in my mind..... They really should have used case that was a little shorter, in my opinion, rather than trying to apparently go for "average" As other threads note -- what really matters is the datum to ogive distance, at least in cartridges that headspace off the shoulder. Without correction, this can vary. Hornady does not talk about that correction in their instructions.