Thedaddycat,
Proving that clever minds think alike, what you have done is create your own version of the
Sinclair chamber length gage. Their picture shows three sizes together, either to confuse folks or hint that you should buy one for every chambering you own, I suppose, but you can buy one for just under $7 plus S&H for 6.5 mm or any of a number of other calibers. I bet you could call Sinclair (now owned and operated out of Brownells) and learn what diameters they use and learn how yours compares.
I think another aid you might want for reference is a
chamber drawing for 6.5×55 rather than a cartridge drawing. One is attached. This should give you the worst case for how much neck to trim on your test case.
The way SAAMI does dimensions is a little confusing, as they don’t use the familiar plus and minus expression of tolerance. Instead they list the critical limiting dimension then a tolerance away from it. For the cartridge, this means linear dimensions are stated as maximums with a minus tolerance only, while chamber linear dimensions are stated as minimums with a plus tolerance only.
Radii are handled differently. Since an outside corner on the cartridge case corresponds to an inside corner in the chamber, and vice versa, they always pair one outside corner with one inside corner. The outside corners must have a minimum and maximum radius, but the inside corners may be left sharp, so they have only a maximum and no minimum
The diameter tolerance on the 6.5×55 case neck used by SAAMI is the same one they use for most rifle cartridges and that is a maximum over a seated bullet with a tolerance of -0.008”. For this cartridge that maximum is 0.2972”. The bullet has a maximum of 0.2638”, with a -0.003 tolerance. So, for a case neck to lay over top of a bullet within specs, it must not exceed 0.2972” over a maximum diameter bullet, nor be less than 0.2892” over a minimum diameter bullet. The first criterion requires neck wall thickness no greater than 0.0167”, while the second requires it be no thinner than 0.0142”. Since manufacturers mostly hold bullets to maximum these days, I’ve noticed a lot of brass favors the small end of that range. Note that since bullets are seldom undersize, you normally have about 0.015” you can turn off the outside before you undershoot the OD tolerance. That’s enough to remove neck wall runout from the majority of brass if you are inclined to take the trouble.
As to trim length, this is a little complicated. The 6.5×55 cartridge headspaces on its shoulder, meaning the shoulder is what stops the case mouth and bullet from going too far forward into the chamber. The idea behind trimming is to prevent neck contact with the mouth of the freebore and to control bullet ogive distance from the lands. Yet, if you pushed the shoulder too far back (see exaggerated drawing below), you’ll see you can have two cases trimmed to the same length and one performs this function properly while the other does not. All it takes is for the shoulder to be pushed too far back to mess it up.
In the real world, the throat jam depicted above isn’t so severe because the case rim is captured by the extractor hook, and that stops it from going that far foward (headspaces the cartridge on the extractor hook).
The standard sizing and trimming strategy is to use sizing dies that correctly limit shoulder setback, then trimming to achieve the same end. This means you are maintaining two interdependent tolerances, and 0.005” variance in the length of the neck coming off the shoulder isn’t too uncommon to encounter using the standard approach. It is better if all cases have the same load history and are always trimmed together. Most chamber designs give the neck ten thousandths or more extra room beyond case maximum for the above reasons. In 6.5×55, the base to case mouth dimension is 2.165” -.020”, while the chamber breech length to the freebore is 2.1772 +0.015”. So the minimum extra chamber neck length is 0.0122”, and the maximum is 0.0472”. Since suggested trim-to length is in the middle of the case length specification range, an average case is 2.155” long, and since the middle of the range for the chamber neck is +0.0075, it averages 2.1847”. From those numbers the average extra neck length in a chamber is 0.0297”, or about 30 thousandths in this cartridge. You only need to maintain the minimum, though, and can adjust your trim length to your chamber, accordingly.
A variation on the standard method that increases precision is to use the Wilson trimmer with Sinclair's optional micrometer adjustment. Measure each sized case to the shoulder datum line with a case gauge, and when one is a thousandth long, you turn the micrometer back a thousandths, and vice versa and so on with whatever shoulder lengths you have. Slow going if you don't sort the cases by shoulder length first so you can run each length as a batch without stopping to adjust, but possible to do.
Another control strategy is to neck size only, where it's practical. Neck growth occurs mainly when the expanded diameter and stretched shoulder position of a fired case are narrowed and pushed back, respectively. During resizing the extra metal needs somewhere to go, and the neck is the open end it flows toward. If you neck size only and you aren’t pushing the stretched parts back anyplace, the case doesn’t grow much. For example, Lee claims using their collet die for neck sizing, trimming is mainly eliminated. The only problem with this is that even neck sized cases eventually get tight and need to be sized and trimmed and started over. But this will cut down a good bit on trimming duty.
Still another strategy is to use a trimmer that registers on the case shoulder and fixes the neck length from there, regardless of where sizing has located the shoulder. The least expensive of this type I am aware of is the Possum Hollow trimmer. Next is the WFT. At the other end of the cost spectrum are the motorized Giraud and Gracey trimmers. These have the advantage their cutters chamfer and de-burr at the same time as they trim, so you're done in one operation. The only failing Ive notice with that kind of cutter is that if a neck wall is not uniformly thick, because the outside surface locates the case, the length and outside de-burring will be right, but the chamfer will be off center. I don’t have any evidence this measurably affect shooting precision; I just know you’ll notice it happens. I use the Giraud and ignore it, though I admit I sort out cases with uniform necks for long range precision loads anyway, so I'm not giving it a chance to be seen if it could be.
So, when do you need to trim? Well, if you know your rifle's chamber length, you can walk up to the minimum distance from the end of the chamber neck for sure. Since necks don’t change much in length during firing, you could go further. Some old timers ignored the whole issue and didn’t trim at all, instead letting the force of closing the bolt on a long neck push the shoulder back a little. Apparently you can get away with that in at least some guns. I wouldn’t want to try it in one with a steep angle into the freebore, like the .30-30, though. Indeed, I think it’s wiser not to introduce that variable at all because it might affect bullet alignment, so I keep trimming to a constant and usually conventional average length. If I had a very long chamber and was having trouble getting a short bullet located as close to the lands as I wanted, then I'd go for all the length I could. It might take several rounds for the case to get out there, I suppose, and I'd have to eat a little inconsistency until it did.
That reminds me to mention a lot of service rifle match shooters used to trim necks back about 15-20 thousandths below SAAMI minimum. The idea was, since case life in the M14/M1A was often only about five loadings, they would never have to trim again before retiring the cases. They didn't seem to have accuracy issues doing this, and you know the cases didn't grow uniformly. So major precision is likely a lot of unnecessary fuss for anything but benchrest shooting. But of course, the only way to be sure it doesn't affect precision in your particular gun is to try it and see.
Nick