If I don't trim the mixed range brass for the Mini 14 is this going to cause a problem? It seems to be running about 1.76-1.77" and 1.75" seems to be the trim length from 2 manuals. Being that the cartridge head spaces on the shoulder how important is this 1.75" length in an auto loader with a 5.56 x 45 NATO chamber? I would like to run untrimmed brass in it.
If the case neck sticks in the throat, you will experience something called high pressure. The case neck will pinch the bullet, and something bad will happen. High pressure creates signs which are not subtle, and from the smoke curling out of your mechanism, you will figure out something is wrong. That is the only reason I can see for brass trimming. It has nothing to do with accuracy, it has everything to do with safety.
Back in the day, a dial caliper was expensive, and a vernier caliper was slow, and took a magnifying glass to read, I decided to skip trimming on once fired brass. The theory was, the brass was new, therefore, it could not grow on its first sizing. Blown primers showed me that my theories were fallacious. I hate it when reality intrudes on a good theory. It turns out, brass grows the most on the first resizing after the first firing. It will also grow a surprising amount if you fire trimmed brass in a different chamber, or use a different sizing die. It may or may not stabilize if you always fire it in the same rifle and size it in the same sizing die. I am pretty certain because of the residual blow back effect, your brass is going to be stretched most unpredictably upon extraction, unlike a bolt rifle, so, I would never recommend skipping trimming.
Size your brass, check the length, and trim everything over length. I used to spend entire Baseball World series trimming 30-06 brass with a manual lathe trimmer. In time, I decided on buying a Gracey, and a Giruard trimmer. I do have an RCBS with a three way trim head. With these faster trimmers I just set them up and trim after every resizing. Trimming is without a doubt one of my least fun reloading activities.
I have not determined any malevolent effects with excessive trimming of a case neck. I suppose there are theoretical bad affects, probably having the same troublesome affect as the rotation of the earth on trajectory, that is, an unmeasurable problem having the same affect as a passionate argument about a meaningless philosophical point. I believe as long as the case neck is long enough to hold the bullet in place, the round will go bang, and you won't notice anything on paper with a trim length 0.020" below the recommended minimum trim length. And that is because, I regularly over trim the first couple of cases, and I have never seen any funnies on paper due to excessive trim length of that magnitude.
