In the never ending pursuit of weeding out fliers, lotting accurate rimfire ammo, and finding ways of "upping" the performance of typically non-match type ammos, two suggestions are usually brought to the forefront: lot ammo according to weight and lot by rim thickness. The idea of weight lotting is that the same measurement of components used to make a round can be lotted by weight, therefore putting "like" rounds together for better consistancy. Since rimfire ammo headspaces on the rim as it rests around the outer chamber, the idea is that the varying thickness of the rim changes the headspacing / overall length, and that "like" rim thicknesses give better performance. But which one produces the best results? I set out today to test weighing vs. rim thickness at 50 and 100 yards as conditions were really calm. As I started to shoot at 100 the wind picked up to 10 mph, and started to mess with the groups, so I'll have to do a 100 yard test later (part 2) when the weather will have less effect on the results. I figured I'd start out at 50 yards using 3 types of ammo I had handy: Federal bulk, CCI Blazer, and Aguila Match Rifle. I had 30 rounds of Wolf MT, but even though I didn't shoot them, I measured them anyway, both weight and rim thickness. The Wolf produced hardly any variance in either catagory, which could be another reason why it's so accurate in so many rifles. First I weighed out lots of each ammo, to the tenth of a grain. Second, I seperated each ammo into lots by rim thickness, using a homemade rim thickness gauge (http://www.longrangehunting.com/forums/f33/homemade-rim-thickness-gauge-55016/). To ensure that the results of the rim thickness measurements were unbiased by similar weights, the rounds that I used to in the thickness lots were 1 round from each weight lot with the same rim thickness, then put together in an lot of 5. The odds of having 2 or more similar weights in the rim thickness lots at 0. All the ammos produced many weight lots, but fewer lots of rim thickness, which I thought was interesting and convienant if the rim thickness lots shot better than the weight lots (more rounds with less seperation). The rounds seperated by rim thickness had varying weights within them. But will that work? Federal bulk - 10 major weight lots with 3 lots holding the most amount of rounds. 5 major rim thickness lots with 2 lots holding the most amount of rounds. CCI Blazer - 8 major weight lots with 2 lots holding the most amount of rounds. 5 major rim thickness lots with 2 lots holding the most rounds. Aguila Match Rifle - 9 major weight lots with 3 lots holding the most amount of rounds. 6 rim thickness lots with 3 lots holding holding the most amount of rounds. Wolf MT - (measured but not shot, only 30 rounds on hand) 4 weight lots with 1 lot holding the most rounds. 3 thickness lots with 1 lot holding the most rounds. The ammo was shot from a Savage MKII BV, bipod and rear bag, 1lb trigger, 16x on the scope. 47 deg., low humidity, sunny, no wind to mention at the time. I shot a baseline group of 5 rounds consisting of 1 round of varying weight and rim thickness to ensure the best weight variance I could to simulate a "out of the box, untouched" result. The Federal bulk "assorted" weight group produced a 1.75" group. 3 five round groups of same weight lotted ammo were shot and produced tighter groups, a 1.6" average. Still not the best, but it is cheap bulk ammo, and an improvement none the less. 4 five round rim thickness lotted groups were then shot, with an average of a 1.35" group. A better improvement lotting with rim thickness over weight. The CCI Blazer "assorted" weight group produced a just over an inch group. 3 five round groups of same weight lotted ammo were shot and produced tighter groups of and average of just under an inch. 4 five round rim thickness lotted groups were then shot, with an average of a .71" group. Most of the rounds in these groups were touching each other (clover leafing) and very impressive for an economical ammo. The Aguila Match Rifle "assorted" weight group produced a little over a .75" group. 3 five round groups of same weight lotted ammo were shot and produced slightly tighter groups averaging .66". 4 five round rim thickness lotted groups were shot, with an equal average of a .66" group. Both 1 weight and 1 rim thickness measured lot produced a 5 round .5" group. Seems that the rim thickness lots produced the smallest groups after all, and within those groups, the holes seemed to be have more clover leaf than the weighed lots. A few more trends showed that certain rim thicknesses shot better groups than others (same as weight lotting) and higher / lower points of impact depending on thickness (same as weight lotting). Another trend was the thicker the rim, the higher the point of impact - not by much, but enough to notice. Perhaps more primer filling in more space? I'll have to chrony different thicknesses at a later date, as I didn't anticipate needing it today. Another thing that struck me as odd is that the Aguila had the most amount of weight and rim thickness lots, yet historically shoots the best for me out of the three ammos fired at 50, 100, and 200 yards. Perhaps it's the standard velocity being more consistant than the high velocity through my rifle? CONCLUSION - both measuring weight and rim thickness did improve groups from just shooting from the box, and both are viable ways of improving accurracy. As mentioned before, I wanted to shoot the same course of fire at 100 yards, but the wind picked up enough to give inconclusive results at 100. I believe that at 100 yards the advantages of mesuring either weight or rim thickness would be more revealing to which method produces the best results. I'm not abandoning all my weight lots - yet - until I can see the difference at 100 yards, but from what today has shown me (at least at 50 yards), rim thickness lotting seems to have a bit of advantage over weight lotting.