You are right Rich. I've done some research now, and thought it would be a good idea to report back. I'm no gunsmith (and I welcome corrections, and/or if anyone can confirm my results) but I've looked over two different reamer prints and here's what I've come up with: the freebore for a 7mmRM optimized to shoot 160+ grain bullets should be in the neighborhood of .295" (edit: this is WRONG. See page three of this thread and you will see the correct value is .268") This should result in a COAL under 3.700" with all but the 195 grain 7mm bullets, keep most of the bullet out of the case to maximize volume, and still keep plenty of the bullet body in contact with the neck. For those who want more details, see below.
But first this: I find it hard to believe that nobody else (especially on this forum!!) has thought of optimizing the 7mmRM for heavy bullets. So my question to you all is this: Does anyone here have a 7mmRM reamer with a freebore in that neighborhood that they would be willing to let me use (or rent) to chamber my barrel?
My dummy round has a 180 grain ELD-M bullet seated with the boat-tail junction of the bullet just above the shoulder-to-neck junction of my brass. According to my Sinclair International OAL comparator (which I trust slightly more than my Hornady comparator) the base-to-ogive length of my dummy round is 2.818". (Since comparators differ, for further reference, COAL is right about 3.675") Looking at a 7mmRM reamer print with a .114 throat, the lands start at 2.534". Looking at a reamer print with a .179 throat the lands start at 2.699" (EDIT: this is where I went wrong originally. I was looking at the wrong number on the reamer print. Again, see page 3 of this thread for the correct calculations). Therefore, assuming my bullet comparator is reasonably accurate, a reamer that would fit my dummy round would have a freebore in the neighborhood of .295".
I should also add that I seated several other bullets to the same OAL, using the same comparator, just to see how they would fit in the neck of the case at that OAL. My goal is to throat not for a particular bullet, but for heavy 7mm bullets in general. A 160g Accubond, which has a much shorter boat-tail, seated with the boat-tail junction right at the neck-shoulder junction of the brass, meaning fully in the neck but extending not-at-all into the body of the case. Perfect! A 175g Accubond Long Range seated with the bullet body contacting about 2/3 of the neck, and the (loooong) boat-tail extending beyond the neck into the shoulder area of the casing. Again, pretty perfect, in the sense that it's still making plenty of contact with the neck but not extending into the case much at all. (I wish I had some 162g ELD-Xs to test, but I'm all out of those.) My conclusion is that as long as you don't want to shoot bullets lighter than 160 grains, then an OAL of 2.818" (= freebore of .295") is pretty ideal for heavy 7mm bullets.
As mentioned above I welcome corrections and/or confirmation of my numbers, for the good of all.
Last edited:
